J. Phys. Chem. C 2008, 112, 123-130
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Effect of Electrode Pt Loading on the Oxygen Reduction Reaction Evaluated by Rotating Disk Electrode and Its Implication on the Reaction Kinetics Yu-Hung Shih,† Guggilla Vidya Sagar,‡ and Shawn D. Lin*,†,‡ Department of Chemical Engineering and Material Science, and Fuel Cell Center, Yuan Ze UniVersity, 135 Yuan-tung Road, Chung-Li, 320 Taiwan, ROC ReceiVed: March 6, 2007; In Final Form: August 21, 2007
The oxygen reduction reaction (ORR) activity was evaluated over film electrodes prepared from different Pt catalysts using rotating-disk electrode (RDE) and rotating-ring-disk electrode (RRDE) systems. The ORR activity as a function of Pt loading using the same catalyst was examined first, and it is concluded to be strongly influenced by mass transfer limitation. Two types of mass transfer limitation were identified. One is the external diffusion as described by the rotating-speed-dependent Levich equation. The other one is the internal diffusion within the electrode in which the electrode thickness is the governing parameter. The electrode thickness is closely related to the catalyst loading of the electrodes. After corrections of these mass transfer limitations, all the tested Pt catalysts under the conditions of this study show similar specific ORR activity with a Tafel slope near -120 mV/dec This result suggests that the ORR over Pt catalysts is a structureinsensitive reaction and the overall electrode performance can be limited by the mass transfer.
1. Introduction Polymer electrolyte fuel cells (PEFCs) demonstrate great promise as future energy sources, as they convert chemical energy to electrical energy with a significantly greater efficiency than in standard combustion processes. The PEFCs are considered to be a very promising type of device for applications as a power supply for automotive, portable, or stationary systems.1-7 However, the high cost of noble metals used as the catalyst (e.g., Pt) poses a significant challenge in the commercial viability of PEFCs. Currently, both the anode and cathode electrocatalysts in the PEMFCs are composed mainly of Pt nanoparticles distributed on a high-surface-area carbon black. While H2 oxidation is kinetically rapid at the Pt anode, a significant voltage loss occurs at the Pt cathode catalysts. Therefore, extensive effort is currently underway to develop a highperformance and low-cost cathode catalyst. On the other hand, the increase of the utilization efficiency of Pt in order to reduce the amount of Pt used in PEFCs has been one of the major concerns during the past decade.8 ORR activity has been extensively studied over Pt catalysts.9-14 The effect of Pt particle size on ORR activity using Pt catalysts with different Pt loading on the carbon support is also the subject of extensive investigation by many researchers.5,15-22 According to the literature,15,18,23,24 the specific ORR activity steeply decreases with a decrease of the size of the Pt particles. A maximum activity per unit mass of Pt was frequently mentioned at a Pt average particle size around 3 nm. Kongkanand22 reported a maximum specific ORR activity at 5 nm size monolayer Pt islands. Takasu et al.15 attributed the particle size effect in ORR to a stronger interaction of oxygen with smaller Pt particles, that is, an electronic effect. However, a completely different explanation for the observed particle size effect was proposed by Watanabe et al.25 According to their so-called territory theory, * Corresponding author. Tel: +886-3-4638800, ext 2554. Fax: +8863-4559373. E-mail:
[email protected]. † Department of Chemical Engineering and Material Science. ‡ Fuel Cell Center.
oxygen diffusion to the Pt surface can be influenced by nearby Pt particles; consequently, not all of the Pt surface area is usable for the reaction. The particle size effect is thereafter not truly dependent on the crystallite size but rather on the interparticle distances, and the specific activity was reported to be constant at a critical distance of about 18 nm and above. This hypothesis was disproved by Gioradano et al.,23 by Gamez et al.,16 and by Higuchi et al.26 who found no evidence that the interparticle distance plays any role in the observed activity of the ORR. However, the description of the territory theory implies the presence of the transfer limitation in ORR electrodes. Higuchi et al.26 reported that the Pt particle distribution on carbon black would not affect the specific ORR activity. Instead, the ORR activity can be affected by the diffusion effect induced from a thick catalyst layer. All these results suggest that the measured ORR activity can be subjected to significant influence from the mass transfer limitation. As mentioned above, it is still not unequivocally resolved if the ORR activity over Pt surfaces is particle-size-dependent and/ or particle-distribution-dependent. Using the terminology in catalysis, the ORR may be a structure-sensitive reaction. As the specific activity was reported lower with decreased Pt size, it suggests that a flat (100) plane can favor the reaction.5,15 However, single-crystal data indicate that the ORR activity follows the trend Pt(111) < Pt(100) < Pt(110),12-14 although the type of electrolyte may change the activity sequence of (111) and (100).27 When higher-index Pt surfaces are examined,28 the (111) terrace with the (100) step showed an increased activity. These results from ideal Pt surfaces suggest a higher ORR activity on a rugged surface and are contradictory to the results5 proposed from the particle size effect studies. The controversial interpretation of ORR activity over Pt surfaces caused the strategy for catalyst development to be very ambiguous. This study intends to resolve the reasons for this controversy. At first screening of the electrocatalysts, planar electrodes with well-defined characteristics (surface area, surface and bulk compositions, and crystal structure) have often been examined in aqueous electrolyte solutions. Use of the rotating-disk
10.1021/jp071807h CCC: $40.75 © 2008 American Chemical Society Published on Web 12/13/2007
124 J. Phys. Chem. C, Vol. 112, No. 1, 2008 electrode (RDE) has a great advantage in eliminating a masstransfer problem and evaluating kinetically controlled ORR activity.29-33 The rotating-ring-disk electrode (RRDE) provides additional important information for ORR of the byproduct H2O2, which may oxidize the polymer electrolyte membrane used in PEFCs. However, the ORR kinetic performance was studied by RDE and RRDE frequently and indicates the transition of the Tafel slope from -60 to -120 mV/dec.13,17,32 Some studies reported different Tafel dependencies,34 and the transition in the Tafel slope is usually interpreted as being due to a change in the cathodic charge-transfer coefficient.12,32,35-37 In a recent report, Neyerlin et al.38 interpretated and suggested that the transitions in the Tafel slope are an experimental artifact based on mass transfer losses. However, no direct evidence was provided. In order to unequivocally identify the ORR activity over Pt catalysts, we propose that the true kinetic activity of the catalyst must be verified via examining the effect of catalyst loading using RDE or RRDE. Thereafter, the dependence of the ORR activity on particle size or particle distribution can be identified and the Tafel slope of the catalyst can be analyzed. In this study we use different Pt catalysts to investigate the effect of electrode Pt loading, and the results suggest that the ORR over Pt may be a structure-insensitive reaction after the diffusion effects are accounted for. 2. Experimental Section The Pt catalysts tested in the present study include 10% Pt/C (ETEK), several house-prepared Pt/C (Vulcan XC 72 R carbon), Pt black (Johnson Matthey), and Pt nanoparticles prepared according to a method reported earlier.39,44 The Pt particle sizes of the catalysts were determined by transmission electron microscopy (TEM) on JEOL JEM-2000EXII operated at 100 kV. The film electrode was prepared typically by pipetting 20 µL of aqueous Pt catalyst suspension onto a clean glassy carbon (GC) rotating-disc electrode (RDE, Pine Instrument) or the GC disc of a rotating-ring-disc electrode (RRDE, Pine Instrument). The cleanness of the GC before Pt loading is monitored by cyclic voltammetry (CV). After the evaporation of the water at room temperature, 20 µL of 0.05 wt % Nafion solution (diluted from 5 wt % DuPont solution) was pipetted onto the glassy carbon to attach the catalyst particles to the GC substrate. The Nafion loading is about 0.1 µm, within the range where Nafion resulted diffusion limitation is considered to be negligible.26,29 The two-step preparation method is used to ensure good electric contact between catalyst particles and with the substrate. This is important especially, when the Pt loading is low, for example, in the range of this study. The electrode Pt loading was varied by diluting the prepared Pt suspension with deionized water while the Nafion loading was kept constant. Because the Pt suspension covered area varies from 60 to 90% of the total GC area from one experiment to another, the electrode Pt loading was reported as the total Pt loading (in µg Pt) over the φ5mm GC (geometrical surface area ) 0.196 cm2). A one-compartment glass cell was used in the RDE electrochemical experiments. A large-area platinum foil served as the counter electrode, and a saturated calomel electrode (SCE) system was used as the reference electrode; however, all reported potentials are given with respect to the reversible hydrogen electrode (RHE). All electrochemical measurement was carried out at room temperature using 1 M H2SO4 as the electrolyte, a RDE system (Pine Instrument, AFMSRX), and a bipotentiostat (CH Instrument, 700A). After preparation, the film electrode
Shih et al.
Figure 1. The Pt active area of the film electrode prepared from a commercial 10% Pt/C catalyst as a function of electrode Pt loading. The inset shows a typical cyclic voltammogram, and the shaded area indicates the hydrogen desorption area.
was immersed in electrolyte solutions with N2 (Sanfu, 99.995%) purge and cycled several times between 0.05 and 1.2 VRHE until steady-state voltammetry was reached. Stable cyclic voltammetry was used to characterize the platinum active surface area by calculating the charge of hydrogen desorption in the potential range from 0.05 to 0.4 VRHE, and assuming that hydrogen is adsorbed only on Pt sites with 210 µC corresponding to 1 cm2 of exposed Pt area.21 The Pt active area before and after ORR activity measurements was found almost identical, which suggests no change in Pt nanoparticles during the measurement. The ORR currents were recorded under constant O2 (Sanfu, 99.98%) purge at disk rotating speeds from 400 to 2500 rpm. The recorded ORR currents were corrected by subtracting the background currents, which were measured under N2 at identical potential scan settings but without rotation.43 All the ORR data reported in this study were collected at a scan rate of 1 mV/s. We verified that the ORR data collected at 10 mV/s are within experimental error of that collected at 1 mV/s. The RRDE (rotating-ring-disk electrode) analyses were also used to provide information about H2O2 formation (eq 2), compared to the direct reduction route (eq 1). Experimentally, the ring potential is set at 1.2 VRHE to oxidize the escaped intermediate from the disk electrode via eq 3.32 The H2O2 selectivity can be calculated from the disk current (ID) and the ring current (IR) according to eq 4:
O2 + 4H+ + 4e f 2H2O
(1)
O2 + 2H+ + 2e f H2O2
(2)
H2O2 + 2H+ + 2e f 2H2O
(3)
SH2O2 ) rO2fH2O2/(rO2fH2O2 + rO2fH2O)
(4)
where rO2fH2O2 ) IRing/n/2, and rO2fH2O ) IDisk/4. The ring collection efficiency, n, provided by the electrode manufacturer (as 0.39) is used without further calibration. 3. Results Figure 1 shows the calculated Pt active area of the film electrodes prepared from the 10% Pt/C commercial catalysts as a function of catalyst loading. A linear relation is found, and
Electrode Pt Loading and Oxygen Reduction Reaction
J. Phys. Chem. C, Vol. 112, No. 1, 2008 125
Figure 2. Measured ORR currents of a 10% Pt/C commercial catalyst at different Pt loading (a,b) and different electrode rotation speed (c,d). The CV scan rate is 0.001 V/s.
it indicates that the accessible Pt active area per mass of loaded Pt is not changed by changing the catalyst loading. Similar results were found in all the catalysts we tested in this study. The effects of the electrode rotating speed and Pt loading on the ORR current over 10% Pt/C commercial catalyst are shown in Figure 2. The recorded ORR current obviously increases as a function of both the electrode rotating speed and the Pt loading. This is consistent with the general understanding that the ORR is under mixed control, where both mass transfer and catalytic kinetics effects are presented. Although limiting currents can be observed in Figure 2, it indicates that the limiting currents differ with different catalyst loading. After being converted to limiting currents per unit mass, they still differ with different catalyst loading. This suggests that the recorded limiting currents from the RDE cannot represent the kinetic activity of the catalyst. When the data in Figure 2 are plotted in Levich format, as shown in Figure 3, rotating-speed-independent ORR currents are observed at higher rotating speed, at lower overpotential, and at lower Pt loading. These rotating-speed-independent currents can be considered as the kinetic currents free from the external mass transfer limitation. This can be interpreted according to eq 5. When kinetic currents become significantly lower than the limiting current of external mass transfer, the recorded currents represent kinetic behavior and are independent of disk rotating speed. These rotating-speed-independent currents are considered as the apparent kinetic currents. If these apparent kinetic currents are the true kinetic currents, they are expected to follow the Butler-Volmer equation, that is, shown as a straight line in the Tafel plot. Furthermore, the true kinetic currents in terms of specific current (per cm2 Pt) should be independent of catalyst loading. Figure 4 shows the specific ORR currents of the 10% Pt/C commercial catalyst in the Tafel
plot. It shows clearly that the specific currents still depend on the catalyst loading, and at each loading a limiting current appears. The limiting specific current increases with a decrease of catalyst loading. It suggests that a second mass transfer limitation is involved and its hindrance effect is lower with a decrease of catalyst loading, that is, with decreasing thickness of the catalyst layer. This is specified as the internal mass transfer limitation hereafter. If the internal mass transfer limitation is assumed to contribute to the overall electrode performance according to the serial resistance model as in eq 6, then the apparent kinetic currents in Figure 4 can be further corrected by eq 6, using limiting currents read from Figure 4. (See Supporting Information for details regarding eq 6.) Thuscorrected specific currents are again plotted in Tafel format, as shown in Figure 5. The corrected specific currents, though scattered, become loading-independent and show a linear relation over a wider overpotential range. Thus-obtained ORR activity is considered the true ORR kinetic activity over the commercial Pt/C catalyst.
1/jRDE ) 1/jE.D,limiting + 1/jk,apparent ) (jE.D,limiting + jk,apparent)/ (jE.D,limiting‚jk,apparent) (5) 1/jk,apparent ) 1/jI.D,limiting + 1/jk,true
(6)
We recently reported a method to prepare the Pt sols which can be used directly as electrocatalysts after deposition.39,44 The prepared sols were mixed in this study with different amounts of carbon black support to obtain Pt/C catalysts of different metal loading. The obtained Pt/C catalysts maintain the same Pt particle size as the Pt sols, and the average Pt size is not changed by the Pt loading on the support. TEM images of the house-prepared Pt/C and that of the commercial Pt/C are shown
126 J. Phys. Chem. C, Vol. 112, No. 1, 2008
Shih et al.
Figure 5. Kinetic current-corrected Tafel plots for the ORR at various Pt loadings using a 10% Pt/C commercial catalyst.
Figure 3. Levich plots for the ORR currents of a 10% Pt/C commercial catalyst as a function of rotation speed at Pt loadings of (a) 6 and (b) 0.5 µg Pt/φ5mm disk. Solid lines represent the estimated apparent kinetic currents; dotted lines indicate the trend of ORR current change with rotation speed. The theoretical Levich slope under our operating conditions is included for comparison.
Figure 4. Tafel plots for the apparent ORR kinetic currents at various Pt loadings using a 10% Pt/C commercial catalyst.
in Figure 6. The calculated average Pt particle size is 2.3-2.7 nm for the three house-prepared Pt/C catalysts and it is very close to 2.0 nm calculated for the commercial 10% Pt/C catalyst. However, TEM images indicate a very different Pt distribution on carbon for the house-prepared Pt/C in comparison to the 10%
Pt/C commercial catalyst. In house-prepared Pt/C catalysts, Pt particles (dark smaller particles) are agglomerated whereas Pt particles are well dispersed in commercial Pt/C catalysts. When the ORR analysis is performed over the house-prepared Pt/C catalysts, limiting currents were also found when plotting the apparent ORR kinetic currents extracted from the Levich plot. After the correction according to eq 6, the specific ORR kinetic currents of the house-prepared 10% Pt/C catalysts are found to follow the same trend that agrees with the data of the commercial 10% Pt/C catalyst, as shown in Figure 7. This indicates that Pt particle distribution on carbon black may not affect the specific ORR activity. Figure 8 shows the corrected kinetic currents of house-prepared Pt/C catalysts with different metal loading on carbon black. The corrected specific currents are independent of the Pt loadings on carbon black support. Figure 9 shows the corrected specific ORR currents of different Pt samples, namely, the commercial 10% Pt/C catalyst, houseprepared Pt/C catalysts, Pt black, Pt sols, and Pt disk. The catalytic experiments were carried out under similar experimental conditions for all the samples. All the samples at the same loading show consistent specific ORR kinetic currents after correction of both external and internal diffusion limitations. Pt black has an average particle size around 10 nm and the Pt disk is also compared. This suggests that the ORR over Pt catalysts is a structure-insensitive reaction. The corrected activity in Figure 9 is similar to previously reported ORR activity in the 0.8-0.85 V range, but it is higher than previous data at 0.6 V.45 This indicates the presence of an internal diffusion limitation in the previous work, especially at higher overpotential. The ORR over Pt catalysts is concluded to be independent of particle size and independent of particle distribution within the test conditions of this study. A Tafel slope of -120 mV/dec seems adequate in describing the corrected kinetic data in Figures 6-9. The selectivity to H2O2 during the ORR over all Pt samples tested in this study is calculated from RRDE analysis, and the results are plotted against potential, as shown in Figure 10. The ORR reaction over Pt surfaces appears to proceed mainly via a four-electron transfer to produce H2O at 298 K, and the selectivity differences between different samples are relatively insignificant. 4. Discussion In this study, we demonstrate that the recorded ORR activity over film electrodes using a RDE or RRDE system is influenced
Electrode Pt Loading and Oxygen Reduction Reaction
J. Phys. Chem. C, Vol. 112, No. 1, 2008 127
Figure 6. TEM images of (a) a 10% Pt/C commercial catalyst and the house-prepared (b) 10%, (c) 30%, and (d) 50% Pt/C catalysts using Pt sols.
Figure 7. Tafel plots for the ORR kinetic-corrected specific currents at various Pt loadings using different Pt distributions on carbon compared to the 10% Pt/C.
by two types of mass transfer limitations. One is the mass transfer from bulk liquid to the electrode surface through a
Figure 8. Tafel plots of ORR kinetic-corrected specific currents at various Pt loadings using the house-prepared Pt/C.
boundary layer; this external diffusion limitation can be described by the Levich equation and has been accounted for the most studies in literature. The second diffusion and its
128 J. Phys. Chem. C, Vol. 112, No. 1, 2008
Figure 9. Tafel plots for the ORR over various Pt catalysts.
Figure 10. Selectivity of H2O2 vs potential during the ORR over various Pt catalysts.
correction have not been reported before. It is indicated by the presence of a Pt-loading-dependent specific activity and the presence of a limiting kinetic current after the external diffusion effect is corrected. Theoretically, the intrinsic specific ORR activity of the catalyst cannot be dependent on the amount of catalyst used in the film electrodes. The identification and correction of internal diffusion are important for the evaluation of diffusion-free intrinsic catalytic activity in the ORR. The catalyst-loading-independent specific ORR activity is considered to represent the intrinsic activity of the catalyst. The intrinsic activity can be useful as a basis to (i) design the cathodes for PEFCs, and (ii) evaluate the catalyst utilization in cathodes. The data scattering observed in Figures 7-9 were analyzed further and found to be governed by two sources of errors in the procedure used in this study. One originates from reading the limiting currents of internal diffusion, for example, from Figure 4. As can be noted from Figure 4, this kind of error can be more significant as the apparent kinetic current gets closer to the limiting current value. The other reason causing the data scattering is the error originated from background current subtraction. The relative error can be expected to be higher when the measured currents from disk electrode are lower, that is, at lower overpotential. These two errors cause the data scattering in Figures 7-9 to appear as a dumbbell shape. Since ORR is typically under a mixed control regime, eq 5 is usually used in the literature to resolve the kinetic behaviors. We found that a Levich plot such as Figure 3 can be extremely
Shih et al. useful. This is because the measured currents in certain test conditions can be independent of disk rotating speed. In such cases, using eq 5 without a Levich plot can possibly result in false interpretation. This study shows that both external and internal diffusion are presented in ORR at a catalyst loading of 10 µg Pt/φ 5mm (equivalent to 0.05 mg Pt/cm2) or lower. The method used in the present study provides a way to analyze the intrinsic activity of the catalyst using a RDE or RRDE system. The method for ORR analysis reported in this study is found also applicable to the anodic hydrogen oxidation reaction over Pt catalysts. This suggests that analyzing the effect of catalyst loading is an important approach in electrocatalyst characterization and it should always be checked. Furthermore, the method described in this study can work as a screening method for the new catalyst development. Similar Pt-loading effects and the presence of limiting currents in the ORR analysis after external diffusion correction can also be found in the literature. For example, Mo et al.41 showed a Pt loading dependency on the ORR currents over Pt black, but neither the background current nor the internal diffusion effect was corrected in their study. Many earlier reports43,46,47 show the presence of limiting currents after the external mass transfer effect is corrected. A nonlinear relation in Tafel plots was observed at lower overpotentials, while limiting currents are observed at higher overpotentials. Some studies described the nonlinear relationship in the Tafel plot without providing an explanation.34,48 However, many studies reported a change in the Tafel slope from -60 to -120 at a different potential range of ORR and attributed it to a change in cathodic charge-transfer coefficient.13,17,32 Neyerlin et al.38 recently interpretated and suggested that the change in the Tafel slope is an artifact and may be due to mass transfer limitation. Results in this study provide experimental support for this argument. The nonlinear Tafel plot in ORR is likely owing to the internal diffusion limitation within the film electrode. After the correction of the internal diffusion, a Tafel slope of -120 mV/dec can describe the data from 0.85 to 0.55 VRHE. The internal diffusion limitation is closely related to the thickness of the catalyst layer. The average thickness of the tested film electrodes in this study can be calculated using the catalyst loading, geometric area of the disk, and the catalyst particle size by assuming even distribution over the GC substrate. The thickness of the film electrodes of a 10% Pt/C catalyst at 5 µg Pt/φ5mm Pt loading (equivalent to 0.025 mg Pt/ cm2) is calculated to be almost 2 µm, where the Pt/C catalyst particle size is taken as the same as 40 nm size of the carbon support. Accordingly, a film electrode at Pt loading as low as 1 µg Pt/φ5mm contains an average of 10 layers of catalyst particles. When the surface reaction rate is fast, the oxygen diffusion from the surface of the film electrode to inside can limit the overall reaction rate. In this case, the overall electrode performance can be improved by reducing the thickness of the catalyst layer. This is consistent with Anderson et al.,48 who reported that the MEA performance decreased with an increase of electrode thickness while keeping constant Pt loading. As a consequence, thinner electrodes give higher efficiency than thicker electrodes owing to a lower internal diffusion limitation. As the interparticle distance of Pt over carbon support may not affect the intrinsic ORR activity of the Pt surface,16,23,26 using a high Pt loading on a carbon support or a non-supported Pt can reduce the thickness of the electrode and the internal diffusion effect. The corrected specific ORR activity presented in Figures 7-9 is considered to represent the intrinsic activity over Pt catalysts.
Electrode Pt Loading and Oxygen Reduction Reaction All Pt samples tested in this study show similar specific activity, similar dependency on overpotential, and similar selectivity to H2O2, as these data after corrections of external and internal diffusion effect fall within the experimental error range (1 order of magnitude) observed in Figure 5. Thus it is concluded that differently prepared Pt catalysts have similar intrinsic activity and that the ORR over Pt catalysts is a structure-insensitive reaction. The specific activity as a function of overpotential can be described by using a Tafel slope of -120 mV/dec. This Tafel slope, in terms of -2.3RT/F, implies that the rate-determining step of ORR is a single electron-transfer step with the corresponding transfer coefficient near 0.5.49 The Tafel slope of -120 mV/dec has been reported over Pt single crystals,13 polycrystalline Pt,32 and some Pt/C catalysts.50 The specific ORR kinetic currents over all the Pt catalysts shown in Figure 9 are fitted to a Tafel equation. The resulting formula is VRHE ) -0.12 log jORR (mA/cm2 Pt) + 0.633 if the Tafel slope is set as -120 mV/dec, or VRHE ) -0.102 log jORR (mA/cm2 Pt) + 0.64 if the Tafel slope is not limited. At 0.6 VRHE, the estimated jORR is around 1.9 mA/cm2 Pt. For the Pt catalysts with an average size of 3 nm, the active Pt area would be about 1000 cm2 Pt/mg Pt, and this corresponds to 1900 mA/mg Pt at 0.6 VRHE or a Pt utilization of 0.88 g Pt/kW. This is slightly higher than the 0.6 g Pt/kW goal set by DOE (Department of Energy) of the United States for 2005 Pt utilization in PEFCs for automobile applications. If an error from this data regression and the temperature effect on ORR are taken into consideration, it would require that all the Pt particles are not limited by mass transfer to meet the DOE goal. The presence of significant internal diffusion limitation under conditions of this study suggests that the ORR rate is fast compared to mass transfer rate. It implies that internal diffusion limitation is severe and almost unavoidable in PEMFCs. For making PEMFCs applications realistic, a maximum catalyst utilization is desired. The design of electrode morphology with improved mass transfer efficiency can be an engineering solution. On the other hand, when the Pt catalysts preparation is concerned, the main goal would be to achieve the high active Pt area. This is because the ORR is concluded as a structureinsensitive reaction in this study. While the development of more active ORR catalyst is concerned, the activity comparison must be carried out using a method that can reveal the diffusion-free intrinsic activity. 5. Conclusion This study shows that the limiting currents observed in RDE experiments depend on both the electrode rotating speed and the catalyst loading. After the correction of external diffusion using the Levich equation, the ORR currents still cannot be used as an indication of the kinetic performance of the catalyst because they are dependent on the catalyst loading and limiting currents are presented. This is attributed to the internal diffusion limitation. With the correction of internal diffusion, the specific ORR currents are independent of the catalyst loading and are considered to represent the intrinsic ORR activity of the catalyst. The corrected specific ORR currents of differently prepared Pt catalysts show similar specific activity, a similar Tafel slope, and similar selectivity to H2O2 under the conditions of this study. The ORR over Pt catalysts is concluded to be a structureinsensitive reaction. The reaction proceeds mainly as a fourelectron-transfer pathway to yield H2O. The intrinsic specific ORR currents from 0.9 to 0.5 VRHE over Pt samples of this study can be described by a Tafel slope of -120 mV/dec.
J. Phys. Chem. C, Vol. 112, No. 1, 2008 129 Acknowledgment. This study was inspired by discussion with Dr. C. Bock and Dr. B. MacDougall when S.D.L. visited ICPET, NRC in 2005. The financial support of this study is partly from the NRC-NSC-ITRI co-sponsored Fuel Cell Research Project and partly from the Technology Development Program for Academia, Department of Industrial Technology, Ministry of Economic Affairs, Taiwan. Supporting Information Available: The origin of eq 6, its variable meanings, and correlation between thickness of the catalyst layer, internal diffusion, and data analysis of ORR over RDE are provided. References and Notes (1) Winter, U.; Herrmann, M. Fuel Cells 2003, 3, 141. (2) Tuber, K.; Zobel, M.; Schmidt, H.; Hebling, C. J. Power Sources 2003, 122, 1. (3) Gigliucci, G.; Petruzzi, L.; Cerelli, E.; Garzixi, A.; La Mendola, A. J. Power Sources 2004, 131, 62. (4) Cleghorn, S. J. C.; Ren, X.; Springer, T. W.; Wilson, M. S.; Zawodzinski, C.; Zawodzinski, T. A.; Gottesfeld, S. J. Hydrogen Energy 1997, 22, 1137. (5) Kinoshita, K. J. Electrochem. Soc. 1990, 137, 845. (6) Camara, G. A.; Giz, M. J.; Paganin, V. A.; Ticianelli, E. A. J. Electroanal. Chem. 2002, 537, 21. (7) Antolini, E.; Passos, R. R.; Ticianelli, E. A. Electrochim. Acta 2002, 48, 263. (8) Srinivasan, S.; Velev, O. A.; Parthasarathy, A.; Manko, D. J.; Appleby, A. J. J. Power Sources 1991, 36, 299. (9) Yeager, E. B. Electrochim. Acta 1984, 29, 1527. (10) Huang, J. C.; Sen, R. K.; Yeager, E. J. Electrochem. 1979, 120, 786. (11) Markovic´, N. M.; Adzic, R. R.; Cahan, B. D.; Yeager, E. B. J. Electroanal. Chem. 1994, 377, 249. (12) Grgur, B. N.; Markovic´, N. M.; Ross, P. N. Can. J. Chem. 1997, 75, 1465. (13) Markovi’c, N. M.; Gasteiger, H. A.; Ross, P. N. J. Phys. Chem. 1995, 99, 3411. (14) Perez, J.; Villulas, H. M.; Gonazalez, E. R. J. Electroanal. Chem. 1997, 435, 179. (15) Takasu, Y.; Ohashi, N.; Zhang, X. G.; Murakami, Y. Electrochim. Acta 1996, 41, 2595. (16) Gamez, A.; Richard, D.; Gallezot, P. Electrochim. Acta 1996, 41, 307. (17) Genies, L.; Faure, R.; Durand, R. Electrochim. Acta 1998, 4, 1317. (18) Bregoli, L. Electrochim. Acta 1978, 23, 489. (19) Sauler, M. L.; Ross, P. N. Ultramicroscopy 1986, 20, 21. (20) Antoine, O.; Bultel, Y.; Durand, R. J. Electroanal. Chem. 2001, 499, 85. (21) Min, M.; Cho, J.; Cho, K.; Kim, H. Electrochim. Acta 2000, 45, 4211. (22) Kongkanand, A.; Kuwabata, S. J. Phys. Chem. B 2005, 109, 23190. (23) Giordano, N.; Passalacqua, E.; Pino, L.; Arico, A. S.; Antonucci, V.; Vivaldi, M.; Kinoshita, K. Electrochim. Acta 1991, 36, 1979. (24) Kinoshita, K. Electrochemical Oxygen Technology; John Wiley & Sons, Inc.: New York, 1992; p 307. (25) Watanabe, M.; Sei, H.; Stonehart, P. J. Electroanal. Chem. 1989, 261, 375. (26) Higuchi, E.; Uchida, H.; Watanabe, M. J. Electroanal. Chem. 2005, 583, 69. (27) Markovic´, M. N.; Gasteiger, H.; Ross, P. N. J. Electrochem. Soc. 1997, 144, 1591. (28) Macia´, M. D.; Campina, J. M.; Herrero, E.; Feliu, J. M. J. Electroanal. Chem. 2004, 564, 141. (29) Watanabe, M.; Igarashi, H.; Yoshioka, K. Electrochim. Acta 1995, 40, 329. (30) Markovic´, N. M.; Schmidt, T. J.; Stamenkovic, V.; Ross, P. N. Fuel Cells 2001, 1, 105. (31) Schmidt, T. J.; Paulus, U. A.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 2001, 508, 41. (32) Paulus, U. A.; Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 2001, 495, 134. (33) Inoue, H.; Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Electrochim. Acta 2002, 47, 3777. (34) Mayrhofer, K. J. J.; Blizanac, B. B.; Arengz, M.; Stamenkovic, V. R.; Ross, P. N.; Markovic´, N. M. J. Phys. Chem. B 2005, 109, 14433. (35) Murthi, V. S.; Urian, R. C.; Mukerjee, S. J. Phys. Chem. B 2004, 108, 11011.
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