Degradation of the Pt/C Electrode Catalyst Monitored by Identical ...

Electrochemistry The Electrochemical Society of Japan Article

Received: September 22, 2014 Accepted: November 9, 2014 Published: December 27, 2014 http://dx.doi.org/10.5796/electrochemistry.83.12

Electrochemistry, 83(1), 12–17 (2015)

Degradation of the Pt/C Electrode Catalyst Monitored by Identical Location Scanning Electron Microscopy during Potential Pulse Durability Tests in HClO4 Solution Taro KINUMOTO,* Hiromi NISHIHIRA, Miki MATSUOKA, Naoki EGUCHI, Tomoki TSUMURA, and Masahiro TOYODA Department of Applied Chemistry, Faculty of Engineering, Oita University, 700 Dannoharu, Oita 870-1192, Japan * Corresponding author: [email protected] ABSTRACT The degradation of Pt nanoparticles in the Pt/C electrode catalyst of PEFC (polymer electrolyte fuel cell) during a potential pulse durability test was investigated using identical location field emission scanning electron microscopy (IL-FE-SEM). The degradation was classified as disappearance, migration, precipitation, coalescence, transformation, or other processes, although the size of most particles changed. Furthermore, several combinations of consecutive degradation were first observed. Thus, the IL-FE-SEM observations allowed us to discuss the degradation mechanism semi-quantitatively. Disappearance was recognized if the particle was not observed at the original position or a neighboring position and the frequency of this phenomenon reached a maximum of 10%. Migration was recognized if the particle moved from its original position to a neighboring position. The frequency of migration increased to 10% after the 2,000th pulse, then decreased up to the 5,000th pulse, and increased again after further pulses. Precipitation was observed at a frequency that reached 4% after the 5,000th pulse. Interestingly, migration and transformation were observed to occur following precipitation of Pt particles. Coalescence following migration was found to be a comparatively minor phenomenon. Transformation was the major phenomenon recognized in this study and was frequently observed following coalescence or precipitation of Pt particles. © The Electrochemical Society of Japan, All rights reserved.

Keywords : PEFC, Pt/C Catalyst, Identical Location Scanning Electron Microscope, Potential Pulse Test

1. Introduction Several technical issues must be addressed to allow widespread commercialization of PEFC (polymer electrolyte fuel cell) as a power source for stationary co-generation systems or FCV (fuel cell vehicle). In particular, the development of an electrode catalyst that has high durability and an acceptable low cost is required. Currently, the carbon black-supported Pt nanoparticle catalyst, denoted as Pt/C, is commonly employed as the cathode catalyst. Unfortunately, Pt/C deteriorates through the loss of ECA (electrochemical surface area) under potential cycling operations and start-stop cycles, and is one of the major reasons for the deterioration of cell performance.1,2 There are four processes that have been considered to be responsible for the loss of ECA: a) growth via modified Ostwald ripening, b) coalescence following migration of Pt particles on the carbon support, c) detachment from the carbon support, and d) dissolution and precipitation in the electrolyte.2 Factors a), c), and d) are accompanied by chemical reactions; typically, factors a) and d) occur because of Pt dissolution, and factor c) occurs because of corrosion of the carbon support.2 Factor b) could be attributed to thermodynamic instability of Pt nanoparticles, as it was observed during gas-phase sintering below 573 K.3 In addition, the formation of hydrogen peroxide leads to migration and detachment of Pt particles.4 In order to develop a more durable cathode catalyst, it is important to clarify how individual Pt nanoparticles deteriorate during the degradation of PEFC. However, it is quite difficult to monitor the degradation behavior of Pt nanoparticles in an actual MEA (membrane-electrode assembly) during operation. Typically, degradation is studied by ex situ TEM (transmission electron microscopy) and FE-SEM (field emission scanning electron mi12

croscopy) before and after the operation of a PEFC; unfortunately, with these methods it is impossible to monitor the morphological changes of a specific catalyst or location during the operation (i.e., in the course of the degradation).5–8 Technology that can be used to continuously monitor the degradation of specific Pt nanoparticles is required to further understand the degradation process. TEM is the one of the most favorable techniques for investigating the degradation behavior of the Pt/C catalyst owing to its high resolution, which is advantageous for monitoring nanoparticles.1–3,5–8 Unfortunately, an electron accelerating voltage of over 200 kV is usually employed for TEM observations. This voltage can damage the electrolyte, as well as the electrode catalyst, which prohibits the use of this technique for continuous monitoring. On the other hand, FE-SEM has nanometer scale resolution and uses moderate electron accelerating voltages of 10–20 kV. Though the resolution of FESEM is lower than that of TEM, this technique has the potential to continuously monitor the degradation of specific Pt nanoparticles in the Pt/C catalyst at acceptable low levels of damage. Furthermore, we believe that FE-SEM is more suitable than TEM to monitor the coalescence of Pt particles following migration on the carbon support because FE-SEM provides a surface image of the sample, whereas TEM images include the depth profile of the sample. Owing to the ease of continuous monitoring at low damage levels and the ability to monitor coalescence following migration of Pt nanoparticles, we selected FE-SEM to investigate the degradation of specific Pt nanoparticles in the Pt/C catalyst. Hodnik et al. reported the degradation of a Pt-Ni alloy catalyst and the Pt/C catalyst using an identical location SEM technique.9,10 Although their reports are very interesting and noteworthy, the particle size of the catalysts was somewhat large (over 3 nm) and analysis of the degradation, especially for specific Pt particles,

Electrochemistry, 83(1), 12–17 (2015) was not realized. In this study, we used identical location FE-SEM (IL-FE-SEM) to observe specific Pt nanoparticles (average particle size = 2.5 nm) in the Pt/C catalyst during a durability test and discuss the degradation process of the catalyst semi-quantitatively. As mentioned above, the ECA of the Pt/C catalyst is known to decrease severely under variation of the electrode potential (cell voltage).1,2 Based on the evidence regarding the deterioration of the Pt/C catalyst, FCCJ (Fuel Cell Commercialization Conference of Japan) has proposed evaluation protocols for the electrode catalysts in PEFC.11 In particular, evaluation of the durability under loadchange cycles is important for FCV, where a potential pulse test between 0.6 V and 1.0 V is recommended for a half-cell test. For the first time, we attempted to carry out the potential pulse durability test on a commercially available Pt/C catalyst while using IL-FE-SEM observations to analyze the degradation process of individual Pt nanoparticles. 2. Experimental 2.1 Electrochemical cell A glass half-cell equipped with an RHE (reversible hydrogen electrode) and a Pt mesh counter electrode was used in this study. Thus, all the potentials reported in this study are against RHE. Prior to use, the electrochemical cell was cleaned with a 1:1 (v/v) mixture of H2SO4 (97%) and HNO3 (60%), and rinsed with hot water to completely eliminate the acidic compounds. The electrolyte was a 0.1 mol dm¹3 HClO4 aqueous solution that was prepared by dilution of an HClO4 solution (60%, 32059-1B, Ultrapur, Kanto Chemical) with highly pure water. 2.2 IL-FE-SEM observation and the potential pulse durability test The experimental flow chart is shown in Scheme 1. Pt/C (TEC10E50E, Pt loading weight = 45.7 wt%) was purchased from Tanaka Kikinzoku Kogyo, Japan. To prepare the test electrode, we applied the method proposed by Schmidt et al. to evaluate the catalytic activity for the oxygen reduction reaction.12,13 In brief, the catalyst was ultrasonically dispersed in an aqueous ethanol solution (90 vol%) using an ice bath and applied dropwise to a polished GC (glassy carbon) disk (Tokai Carbon, diameter = 5 mm). The loading

Scheme 1. Experimental flow chart for this study.

amount of the catalyst was 1.47 µg by Pt weight. A thin Nafion film with a calculated thickness of 80 nm was then formed by casting the corresponding ionomer (Aldrich, 274704-100ML).14 After the test electrode was prepared, the morphology of the catalyst was observed by FE-SEM (JSM-6701F, JEOL) and the observation point was defined for IL-FE-SEM observation. Then, the test electrode was fixed to a PTFE (polytetrafluoroethylene) electrode holder (AFE4TQ050, Pine) and attached to an electrode shaft (AFE3M, Pine), and subsequently, the assembly was transferred into the electrochemical half-cell for CV (cyclic voltammetry) measurements and the potential pulse durability test. Only the surface of the test electrode was immersed in the electrolyte solution. CV measurements were carried out between 0.05 V and 1.2 V at 333 K under nitrogen atmosphere. The potential sweep rate was 0.1 V s¹1. The ECA of Pt was calculated from the electrical quantity assigned to the underpotential deposition of hydrogen on the Pt surface between 0.05 V and 0.4 V. After CV measurements, the potential pulse durability test was carried out according to the FCCJ protocol.11 The lowest and the highest potentials were 0.6 V and 1.0 V, respectively, and the pulse time was 3 s. The test was carried out at 333 K under continuously flowing nitrogen. The potential pulse durability test was performed up to 10,000 pulses. In order to check the degradation of the catalyst, the remaining ECA was investigated by CV measurements over the course of the durability test. Both the potential pulse durability test and the CV measurements were carried out using a potentiostat (VMP3, Bio-logic). The test electrode was taken out from the electrochemical cell after the 1,000th, 2,000th, 5,000th, 7,000th, and 10,000th pulse periods following the CV measurements. It was then transferred to the observation chamber of the FE-SEM for IL-FE-SEM observations. Both the defined region and the specified Pt nanoparticles were quickly observed at 300,000-fold magnification by FE-SEM. The vacuum pressure of the observation chamber was kept at ca. 10¹6 Pa before and during the observation, so that no volatiles were released during the observation. The electron accelerating voltage and the emission current were 15 kV and 10 µA, respectively. 3. Results and Discussion 3.1 ECA change during the durability test Figure 1 shows the change in the ECA of Pt during the durability test. The ECA decreased sharply to 75.6% by the 1,000th pulse, then decreased at a more moderate rate to 51.7% after the 7,000th pulse, and finally to 48.5% after the 10,000th pulse. The decrease of the ECA must be due to the four major mechanisms mentioned in the introduction.2 In particular, corrosion of the carbon support and GC

Figure 1. Change in the ECA of Pt during the potential pulse degradation test. The black symbols indicate when FE-SEM observations were made. 13


Figure 2. Representative IL-FE-SEM images of the Pt/C catalyst during the potential pulse durability test showing (a) before, (b) after the 1,000th pulse where both A and B shrank, (c) 2,000th pulse where A coalesced with B to form E and C shrank, (d) 5,000th pulse where C disappeared, (e) 7,000th pulse where E migrated and transformed, and (f ) 10,000th pulse where E migrated. Table 1. Summary of the degradation behavior of some particles in Fig. 2 observed using identical location FE-SEM. (a)

(b)

(c)

(d)

(e)

(f )

Pulse No.

Before

1,000

2,000

5,000

7,000

10,000

ECA

100%

75.6%

73.5%

55.8%

51.7%

48.5%

#A

d = 2 nm

Shrink

Disappear

ab esse

#B

d = 4 nm

Shrink

Coalescence with A to from E

ab esse

#C

d = 2 nm

Shrink

Transformation

Disappear

#D

d = 4 nm

#E

ab esse

Remaining at the edge of support Formation via. coalescence

substrate can be considered to be a minor factor because the change in the pseudocapacity originating from carbon was not significant compared with the decrease in ECA in the cyclic voltammogram.15 3.2 IL-FE-SEM observations Figure 2 shows representative IL-FE-SEM images during the durability test. From these images, we successfully observed the defined region and specific Pt nanoparticles (IL-FE-SEM). Figure 2(a) shows the FE-SEM image of the Pt/C catalyst before the durability test. The sizes of the whitish spots are estimated to be 2–4 nm, which corresponds to that previously reported.16 Therefore, these objects were identified as the Pt particles and the larger particles that were ca. 50 nm in diameter were the carbon supports. Significant degradation behavior was observed for the Pt nanoparticles in the specified region of the IL-FE-SEM images (indicated by a square). The typical changes observed for several Pt particles are summarized in Table 1. Before the durability test, some individual Pt particles (A, B, C, and D) were observed [Fig. 2(a)]. After the 1,000th pulse [Fig. 2(b)], the sizes of A, B, and C decreased slightly, whereas 14

ab esse

Transformation

Migration and transformation

Migration for 2 nm

that of D had not significantly changed. The observation of such shrinkage suggests that Pt is chemically dissolved from each particle.2 After the 2,000th pulse, particle A disappeared and likely moved to the left side of particle B, resulting in the formation of particle E by coalescence [Fig. 2(c)]. On the other hand, the morphology of particle C was slightly changed and this particle remained in its original position. As the size of the particle attached to B corresponds to that of particle A, the coalescence of particles A and B occurred by either migration or detachment of particle A between the 1,000th and 2,000th pulse. The degradation behaviors of particles A, B, and C observed between the 1,000th and 2,000th pulse are illustrated in Fig. 3. As shown in this figure, particle A migrates ca. 2–3 nm toward particle B, which results in the formation of particle E by coalescence. Particle C and D remain in their original positions, but particle C undergoes transformation to an ellipsoidal morphology. Furthermore, particle C disappeared after the 5,000th pulse [Fig. 2(d)] and particle E was transformed to form an ellipsoid particle with major and minor axes of ca. 7 nm and 3 nm, respectively. Although the reasons for the disappearance of C and the transformation of E have not been clarified, either “modified


Figure 3. Illustration of the proposed migration of particle A and its aggregation with particle B to form particle E between the 1,000th and 2,000th pulses. Particle C undergoes transformation and particle D remains unchanged.

Figure 4. Representative IL-FE-SEM images of the Pt/C catalyst during the potential pulse durability test showing (a) before, (b) after the 1,000th pulse where both F and G disappear and H is generated, (c) 2,000th pulse where H migrates and transforms, (d) 5,000th pulse where I precipitates, (e) 7,000th pulse where H shrank, J precipitated, and I migrated and transformed, and (f ) 10,000th pulse where I and J migrated.

Ostwald ripening” via Pt dissolution from C to precipitate on particle E or migration following coalescence of particle C and E are possible. The behavior of particle E after the 5,000th pulse was very interesting, as shown in Figs. 2(d)–2(f ). This particle transformed or rotated sideways and moved ca. 3 nm in the upward direction between the 5,000th and 7,000th pulse. Finally, particle E moved further toward particle D at a distance of ca. 2 nm without a significant change in the particle size. Thus, for the first time, we observed that the Pt/C catalyst deteriorated not only by individual mechanisms, but also through the combination of several consecutive processes. Generally, each observable change should be related to: a) growth via modified Ostwald ripening, b) coalescence following migration of Pt particles on the carbon support, c) detachment from the carbon support, and d) dissolution and precipitation in the electrolyte.2 If FE-SEM or TEM observations were only carried out before and after the durability test, the degradation mechanism could be interpreted incorrectly. For example, since particles A and C disappeared during the durability test and a particle was observed near the original position of particle B, one might assume that “particle A and C dissolved or detached from the carbon support and particle B grew simultaneously during the durability test”. However, this hypothesis should be rejected owing to the continuous IL-FE-SEM observation in this study. In addition, it was possible to successfully visualize

coalescence following migration of Pt particles on the carbon support, which would, in principle, be hard to monitor using TEM. The IL-FE-SEM images at a different location (indicated by a square) are shown in Fig. 4. The typical changes observed for several Pt particles are summarized in Table 2. Here, a change identified as modified Ostwald ripening was recognized for the precipitation of particles I and J. Particles F and G were located close together on the support, as shown in Fig. 4(a). After the 1,000th pulse, only one particle (H) was observed. The disappearance of particles F and G may be a result of precipitation via modified Ostwald ripening or coalescence following migration of particles F and G. It is worth noting that the location of particle H is different from the original positions of particles F and G. Furthermore, between the 1,000th and 2,000th pulse, particle H migrated and showed a slight morphological change (transformation). Interestingly, after the 5,000th pulse, a new Pt particle (I) appeared on the bare surface of the carbon support, which clearly indicates that Pt particles precipitate during the durability test. Particle I migrated and showed a simultaneously transformation by the 7,000th pulse, and then further migrated between the 7,000th and 10,000th pulse. On the other hand, the size of particle H decreased severely and a new particle (J) simultaneously precipitated between the 5,000th and 7,000th pulse. The synchronous phenomena of particles H and J indicated the occurrence of a typical 15

Electrochemistry, 83(1), 12–17 (2015) Table 2. Summary of the degradation behavior of some particles in Fig. 4 observed using identical location FE-SEM. (a)

(b)

(c)

(d)

(e)

(f )

Pulse No.

Before

1,000

2,000

5,000

7,000

10,000

ECA

100%

75.6%

73.5%

55.8%

51.7%

48.5%

#F

d = 2 nm

Disappear

ab esse

#G

d < 2 nm

Disappear

ab esse

#H

ab esse

Generation


Shrink

Shrink

No change

#I

ab esse

Precipitation


Migration

#J

ab esse

Precipitation

Migration

Figure 5. Illustration of the proposed modified Ostwald ripening of particles H and J between the 5,000th and 7,000th pulses. Particle I also migrates on the carbon support. degradation process, recognized as growth via modified Ostwald ripening, as illustrated in Fig. 5. The proposed degradation behavior of particles I and H and the precipitation of particle J between the 5,000th and 7,000th pulse is shown in Fig. 5. The size of particle H was decreased by dissolution and particle J simultaneously appeared on a different carbon support by precipitation. Therefore, it is reasonable to consider that modified Ostwald ripening has taken place for particles H and J. On the other hand, particle I migrates ca. 2 nm on the carbon support. Finally, particle J moved ca. 2 nm from its original position. Thus, after precipitation, particles tend to not have a fixed location, but occasionally move small distances. 3.3 Degradation behavior The use of continuous IL-FE-SEM shows interesting results for the investigation of morphological changes on the nanometer scale (area and particle); in particular, this technique is promising for the investigation of the degradation mechanism of the Pt/C catalyst. Unfortunately, the precise size of the Pt particles could not be determined because the resolution of FE-SEM is inadequate.17 Therefore, we do not focus on changes in the particle size, but on the morphological changes of the Pt/C catalyst. The frequencies of the degradation processes observed for Pt nanoparticles during the durability test are summarized in Fig. 6. These frequencies were obtained from the continuous IL-FE-SEM observations of more than fifty Pt particles. Even though the size of most Pt particles changed, particles that changed size with no other observable change were classified as “other” because of the low 16

resolution of the IL-FE-SEM observations. Here, we define the degradation processes, such as disappearance, migration, precipitation, coalescence, and transformation, and then discuss each process below. Disappearance is caused by dissolution of Pt from the particles, particle migration from the original position, and detachment from the carbon support; unfortunately, it is very difficult to determine the major reason for disappearance, as described by Shao-Horn et al.2 Therefore, we classified an observed change as disappearance if the particle was not observed at the original position or a neighboring positions, e.g., the behavior of particle A from Figs. 2(b) to 2(c) and of particle C from Figs. 2(c) to 2(d). Meanwhile, migration was defined when the particle was not observed at its original position, but could be observed near the original position, e.g., the behavior of particle E from Figs. 2(e) to 2(f ). The frequency of this phenomenon increased at the early stages of the durability test up to the 2,000th pulse, then decreased by the 5,000th pulse, and increased again at further pulses. As the frequency of migration approached a maximum of ca. 10% after the 2,000th pulse, modification of both the surface properties of the carbon support and the relationship between the Pt particles and the support is essential to depress degradation owing to migration. The increase in migration at the latter stage of the durability test may be related to precipitation, as described below. Precipitation was defined as the appearance of a particle on the bare surface of the carbon support, e.g., the behavior of particle I in Fig. 4(d) and particle J in Fig. 4(e). The coupled shrinkage of


Pulse number

10,000 Disappearance Migration Precipitation Coalescence Transformation Other

7,000 5,000 2,000 1,000 0

20

40

60

80

100

Frequency / %

Figure 6. Frequency of the degradation processes of the Pt particles observed during the potential pulse durability test.

particle H and precipitation of particle J between the 5,000th and 7,000th pulses strongly suggested that precipitation via modified Ostwald ripening took place. Furthermore, the particles that appeared by precipitation were observed to be unstable and showed migration as well as transformation, which is one probable reason for the increase in the frequency of migration after the 5,000th pulse. Coalescence was defined as the aggregation of two or more Pt particles to form one particle. This phenomenon is shown by the behavior of particles A and B from Figs. 2(b) to 2(c); this was a minor degradation behavior in this study and was mainly observed up to 2,000th pulse. Finally, transformation was defined as a morphological change of an individual particle, e.g., the behavior of particle E from Figs. 2(d) to 2(e). This phenomenon should be related to the rearrangement of Pt atoms in an individual particle. The observations of particles E, H, and I indicated that transformation tends to take place following coalescence or precipitation of Pt particles. In general, the frequency of disappearance was 2% after the 1,000th pulse and gradually increased to 10% after the 5,000th pulse. This result clearly indicates that at least 10% of Pt nanoparticles are lost by the 5,000th pulse of the durability test. On the other hand, migration was observed to increase at the beginning of the durability test up to the 2,000th pulse, which is significant for depression of the degradation of the Pt/C catalyst. Thus, stabilization of Pt particles on the carbon support by modifying the surface properties of the carbon support, as well as the strengthening the interaction between the Pt particles and the support, promises to be effective for decreasing the degradation by migration. Precipitation was also very low after the 1,000th and 2,000th pulses; however, this phenomenon became significant after the 5,000th and 7,000th pulses. The frequency of coalescence was lower than that of migration, and was the lowest among all the degradation behaviors. Therefore, it is not inevitable that coalescence occurs after the migration of Pt nanoparticles. The frequency of transformation increased between the 2,000th and 7,000th pulse and then decreased by the 10,000th pulse. This behavior corresponds to the trend observed for the decrease in ECA. Therefore, the observation of transformation was a representative phenomenon of the instability of Pt particles over the course of the degradation. 4. Conclusion In this study, we used the IL-FE-SEM technique to investigate the degradation of specific Pt nanoparticles in the currently employed Pt/C catalyst during a potential pulse durability test under half-cell conditions. Although the resolution is lower than that of TEM, IL-FE-SEM is a useful technique because of the acceptable levels of electron damage, as well as the easy, semi in situ

monitoring of morphological changes on the nanometer scale. Unfortunately, changes in the size of nanoparticles, which are very important for the study of degradation phenomena, cannot be discussed presently because of the lower resolution of this technique. Nevertheless, for the first time, we demonstrated the use of IL-FE-SEM to study the degradation behavior of Pt nanoparticles, and semi-quantitatively discussion the degradation behavior as disappearance, precipitation, migration, coalescence, and transformation. This technique can be applied to various test conditions and can be used to study morphological changes on the nanometer scale. Thus, IL-FE-SEM will be valuable not only for investigating the degradation process but also for the development of a durable catalyst. We are currently investigating the degradation process of the Pt/C catalyst under various experimental conditions, such as different potentials, atmospheres, and temperatures. References 1. R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J. E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K.-i. Kimijima, and N. Iwashita, Chem. Rev., 107, 3904 (2007). 2. Y. Shao-Horn, W. C. Sheng, S. Chen, P. J. Ferreira, E. F. Holby, and D. Morgan, Top. Catal., 46, 285 (2007). 3. R. Sellin, C. Grolleau, S. Arrii-Clacens, S. Pronier, J.-M. Clacens, C. Coutanceau, and J.-M. Léger, J. Phys. Chem. C, 113, 21735 (2009). 4. T. Kinumoto, K. Takai, Y. Iriyama, T. Abe, M. Inaba, and Z. Ogumi, J. Electrochem. Soc., 153, A58 (2006). 5. K. J. J. Mayrhofer, J. C. Meier, S. J. Ashton, G. K. H. Wiberg, F. Kraus, M. Hanzlik, and M. Arenz, Electrochem. Commun., 10, 1144 (2008). 6. F. J. Perez-Alonso, C. F. Elkjær, S. S. Shim, B. L. Abrams, I. E. L. Stephens, and I. Chorkendorff, J. Power Sources, 196, 6085 (2011). 7. Y. Yu, H. L. Xin, R. Hovden, D. Wang, E. D. Rus, J. A. Mundy, D. A. Muller, and H. D. Abruña, Nano Lett., 12, 4417 (2012). 8. W. Sheng, S. Chen, E. Vescovo, and Y. Shao-Horn, J. Electrochem. Soc., 159, B96 (2012). 9. N. Hodnik, M. Zorko, M. Bele, S. Hočevar, and M. Gaberšček, J. Phys. Chem. C, 116, 21326 (2012). 10. N. Hodnik, M. Zorko, B. Jozinović, M. Bele, G. Dražič, S. Hočevar, and M. Gaberšček, Electrochem. Commun., 30, 75 (2013). 11. http://fccj.jp/pdf/23_01_kt.pdf. [in Japanese] 12. T. J. Schmidt, H. A. Gasteiger, G. D. Stäb, P. M. Urban, D. M. Kolb, and R. J. Behm, J. Electrochem. Soc., 145, 2354 (1998). 13. U. A. Paulus, T. J. Schmidt, H. A. Gasteiger, and R. J. Behm, J. Electroanal. Chem., 495, 134 (2001). 14. D. Chu, D. Tryk, D. Gervasio, and E. B. Yeager, J. Electroanal. Chem., 272, 277 (1989). 15. H.-S. Choo, T. Kinumoto, S.-K. Jeong, Y. Iriyama, T. Abe, and Z. Ogumi, J. Electrochem. Soc., 154, B1017 (2007). 16. M. Shao, A. Peles, and K. Shoemaker, Nano Lett., 11, 3714 (2011). 17. Japanese Industrial Standard Committee, Method for particle size determination in metal catalysts by electron microscope, JIS 7804: 2005.

17