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DOI: 10.1002/celc.201600456

Articles

Studies on the Electrochemical Stability of Preferentially (100)-Oriented Pt Prepared through Three Different Methods Zhi Liu,[a] Chao Ma,[b] Jie Liu,[b] Xu Chen,[a] Zhishuang Song,[b] Wenbin Hu,[a, b, c] and Cheng Zhong*[b, c] The electrochemical stability of the Pt(100) surface is investigated by electrochemical methods and morphological and structural characterization. Preferentially (100)-oriented Pt is prepared by three different methods: use of water-in-oil microemulsions, electrodeposition and electrochemical faceting. The loss of the Pt(100) sites particularly the wide (100) surface domains during the potential cycling is confirmed by the hydrogen-desorption profile in H2SO4 solution, the decreasing specific activity for ammonia electro-oxidation and the decreasing proportion of Pt(100) sites as characterized by irreversible Ge adsorption. This is consistent with TEM observations that show

that the well-defined cubic Pt nanocubes change to a polycrystalline structure, whereas the sharp tips of electrodeposited Pt nanoparticles become rounded/blunt or disappear after 300 potential cycles. Noticeably, the bulk Pt disk electrode treated by electrochemical faceting exhibits a much higher electrochemical stability compared to preferentially (100)-oriented Pt nanoparticles. The high electrochemical stability is probably related to its preparation procedure, which involves high-frequency selective electrodissolution/electrodeposition of Pt atoms coupled to the periodic adsorption/desorption of oxygen-containing species and hydrogen adatoms.

1. Introduction Owing to its unique electrocatalytic properties, platinum has been extensively investigated as a key electrocatalyst for a wide variety of important electrocatalytic applications, such as the electro-oxidation of a variety of alcohols,[1] formic acid,[2] and ammonia,[2b, 3] as well as the oxygen reduction reaction (ORR).[4] These reactions are of vital importance for energy storage and conversion applications such as fuel cells and metal– air batteries.[5] Because the electrochemical reaction occurs on the surface of the electrocatalysts, the catalytic properties (e.g. activity, selectivity and stability) of Pt strongly depend on the arrangements of the surface atoms and the surface crystallographic plane.[1a, 4b, 6] This has been widely demonstrated by

both experimental studies on Pt single-crystal surfaces and theoretical investigations.[7] For example, it has been widely reported that ammonia electro-oxidation is a highly structuresensitive reaction that takes place almost exclusively on Pt(100) sites.[3a, c, 7a, 8] Based on investigations on three different basal planes of Pt [i.e. Pt(111), Pt(110) and Pt(100)], Vidal-Iglesias et al.[8a] found that Pt(100) single crystals showed a considerably high activity, whereas Pt(111) and Pt(110) single crystals exhibited low reactivity in ammonia electro-oxidation. Likewise, the electro-oxidation of dimethyl ether has also been found to be extremely sensitive to the surface structure of Pt.[9] The Pt(100) single electrode shows a particularly high activity for dimethyl ether electro-oxidation compared to Pt(111) and Pt(110) single electrodes.[9] Such surface-structure-sensitive electrocatalytic properties have not only been demonstrated on bulk Pt single-crystal surfaces but also on Pt nanoparticles with certain preferential orientation.[3a, 7a, 8b, 10] For instance, previous studies, including our work, found that the specific activity of (100)-oriented Pt nanoparticles for ammonia electro-oxidation was 3–5 times higher than that of the polycrystalline Pt nanoparticles.[3a, 8b, 11] Both Inaba et al.[12] and Wang et al.[10c] reported that cubic Pt nanoparticles with preferential (100) orientation exhibited higher electrocatalytic ORR activity in H2SO4 solution compared to polycrystalline Pt nanoparticles. Lu et al.[10b] demonstrated that Pt nanocubes composed of (100) facets showed similar features to bulk Pt single-crystal electrodes, and their activity was nearly three times higher than that of commercial Pt black catalyst for the electro-oxidation of dimethyl ether. In the case of the electro-oxidation of methanol and ethanol, a Pt nanocube

[a] Z. Liu, X. Chen, Prof. W. Hu State Key Laboratory of Metal Matrix Composites Shanghai Jiao Tong University Shanghai 200240 (China) [b] C. Ma, Dr. J. Liu, Z. Song, Prof. W. Hu, Prof. C. Zhong Tianjin Key Laboratory of Composite and Functional Material School of Materials Science and Engineering Tianjin University, Tianjin 300072 (China) E-mail: [email protected] [c] Prof. W. Hu, Prof. C. Zhong Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Education), School of Materials Science and Engineering Tianjin University, Tianjin 300072 (China) The ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/celc.201600456. T 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

ChemElectroChem 2017, 4, 66 – 74

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T 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Articles 2. Results and Discussion

catalyst also showed a lower onset potential and a higher current density compared to a polycrystalline Pt nanocatalyst, indicating its higher catalytic activity.[10a] To date, the majority of the related work has focused on the activity of preferentially oriented Pt. However, studies on the electrochemical stability have been relatively limited.[13] The stability of the Pt catalyst is also an important issue, which significantly affects its practical applications. For example, although preferentially (100)-oriented Pt has high activity for a wide variety of electrocatalytic reactions,[3a, 8b, 10–12] the issue of its stability has attracted relatively limited attention[12, 14] and some studies have indicated that Pt(100) surface sites are unstable upon potential cycling beyond a certain threshold.[12, 14a] Inaba et al.[12] found that the features of Pt(100) were gradually lost during repeated potential cycling up to 1.4 V (vs. RHE), resulting in the decrease of activity in the ORR and for hydrogen peroxide formation. This was attributed to the surface structural change from Pt(100) to polycrystalline. Similar results were also reported by Devivaraprasad et al.[13b] On the other hand, it is interesting to note that in the early 1980s, Arvia’s group developed a novel electrochemical method (referred to as “electrochemical faceting”) to obtain a Pt surface with preferred (100) orientation.[15] This was achieved by the treatment of bulk polycrystalline Pt by the repetitive square-wave potentials at high frequency in acidic solution.[15] Through careful selection of the low/high potential limits and the frequency of the square-wave voltage, a preferentially (100)-oriented surface can be obtained on the Pt, which has been well demonstrated by various structure-sensitive electrochemical reactions.[15, 16] Nevertheless, the electrochemical stability of preferentially (100)-oriented Pt surfaces obtained by this special electrochemical treatment has also remained unclear. Based on the above considerations, in the present work, the electrochemical stability of the Pt(100) surface was investigated by electrochemical methods and structural characterization. In order to reveal the potential factors affecting the stability, three different types of model Pt(100) systems were investigated: bulk Pt with a preferentially (100)-oriented surface obtained by the electrochemical faceting method and (100)-oriented Pt nanoparticles prepared by chemical reduction and electrodeposition methods. The change of the Pt(100) surface was characterized by the hydrogen desorption profile in H2SO4 solution and the ammonia electro-oxidation reaction after repeated potential cycling. The variation of the proportion of Pt(100) sites was quantitatively evaluated by the irreversible adsorption of Ge. In addition, morphological and structural changes of the Pt surface were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is interesting to note that the preferentially (100)-oriented Pt prepared by the electrochemical faceting method exhibits unusually high electrochemical stability compared to other types of (100)-oriented Pt and those reported in previous studies.


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It is widely known that the hydrogen adsorption–desorption process is extremely sensitive to the surface structure of Pt and thus can be used to characterize the in situ surface structure change of Pt during the electrochemical tests.[17] Figure 1 a–c shows the variation of 300 cyclic voltammograms (CVs) of cubic Pt particles prepared by the water-in-oil microemulsion method (Pt-A), Pt particles obtained by the electrodeposition method (Pt-B), and electrochemically-treated bulk Pt electrodes (Pt-C) upon potential cycling in 0.5 m H2SO4 at a scan rate of 0.05 V s@1 (normalized by the geometric surface area). To reflect the change of the CVs over the whole testing time, the first cycle is also shown. All the CVs show three typical potential regions: the hydrogen adsorption/desorption potential region (@0.2–0.1 V vs. SCE), the double-layer potential region (@0.1–0.4 V vs. SCE) and the formation/reduction of surface Pt oxide (0.4–1.0 V vs. SCE).[18] In the lower potential range during the positive scan, three characteristic peaks (h1, h2 and h3) can be observed at approximately @0.15, @0.06 and 0.01 V (vs. SCE), respectively, in the hydrogen desorption region. The hydrogen adsorption/desorption behavior on Pt has been widely studied and it is generally accepted that peaks h1, h2 and h3 are associated with the hydrogen desorption on (110) sites, (100) steps and terrace borders (short-range ordered domains), and (100) terraces or wide (100) domains (long-range ordered domains), respectively.[19] All three types of Pt exhibit a more intense h2 peak compared to the h1 peak, and have a distinct h3 peak in the initial stage of potential cycling, clearly indicating the presence of a preferential (100) orientation. It is interesting to note that Pt-C has a much stronger h3 peak compared to Pt-A and Pt-B, suggesting its higher proportion of wide Pt(100) surface domains. Furthermore, the h3 peak of the Pt-C is also much higher than that of the previously reported Pt electrode treated by electrochemical faceting, as reported

Figure 1. 300 CVs of a) the cubic Pt particles prepared by the water-in-oil microemulsion method (Pt-A), b) the Pt particles obtained by the electrodeposition method (Pt-B) and c) the bulk Pt electrode subjected to the electrochemical treatment (Pt-C) during the potential cycling in 0.5 m H2SO4 at a scan rate of 0.05 V s@1. Only the first, 10th, 20th, 50th, 100th, 200th, and 300th cycles are shown.

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Articles by Arvia and co-workers.[15] This feature is highly reproducible and is explained by the equipment and the preparation parameters used in our work being different from those reported by Arvia.[15] The quantitative analysis of the Pt(100) sites for different Pt is described below. Furthermore, it was observed that the potential cycling leads to the dramatic decrease of the h3 peak for Pt-A and Pt-B, indicating the rapid loss of wide (100) surface domains. However, the decrease of the h3 peak for Pt-C is not obvious compared to Pt-A and Pt-B. Furthermore, for all the three Pt samples, the intensity of the h1 peak originating from Pt(110) sites increases during the potential cycling. To observe the change of the voltammetric behavior of different Pt samples, Figure 2 a–c shows the variation of the h2/h1 peak current ratio, the h3/h1 peak current ratio and the electrochemically active surface area (ECSA) as a function of potential cycle number, respectively. For all three Pt samples, both the h2/h1 and h3/h1 ratios decrease with the cycle number, indicating the loss of Pt(100) sites. The h3/h1 ratio drops more quickly compared to the h2/h1 ratio. For example, the h2/h1 ratio for Pt-A retains 79.3 % of its initial value, whereas the h3/h1 ratio only preserves 40.6 % of its initial value after 300 cycles. This suggests that the wide Pt(100) surface domains are less stable than the (100) steps and terrace borders. It is notable that there is a significant difference between the bulk Pt electrode treated by electrochemical faceting (Pt-C) and the (100)-oriented Pt nanoparticles (Pt-A and Pt-B). The h2/h1 ratio and h3/h1 ratio of Pt-C decrease much more slowly than that of Pt-A and Pt-B. For (100)-oriented Pt nanoparticles (Pt-A and Pt-B), the h2/h1 ratio and h3/h1 ratio decrease sharply in the first 50 cycles and then decrease relatively slowly. On the contrary, the decrease of the h2/h1 and h3/h1 ratios of Pt-C is much slower, and is at a similar rate compared to (100)-oriented Pt nanoparticles. After 300 cycles, the h2/h1 ratio and h3/h1 ratio of the Pt-C were 93.4 % and 76.3 % of their initial values, respectively. These values are much higher than not only the Pt-A and Pt-B but also the reported values in the previous literature.[12, 13b, 14, 20] For instance, Garbarino et al.[20] prepared preferentially (100)-oriented Pt nanowires and found that the h2/h1 ratio decreases re-

markably by 31.8 % and the h3 peak totally disappears after 300 cycles. This suggests a much higher stability of the Pt(100) sites in Pt-C. Furthermore, it is interesting to note that the values of the h2/h1 and h3/h1 ratios of Pt-C are significantly higher than those of Pt-A and Pt-B. This indicates that the bulk Pt electrode treated by electrochemical faceting (Pt-C) has a much higher proportion of short-range and long-range ordered (100) surface domains compared to the (100)-oriented Pt nanoparticles. Notably, the h3/h1 ratio of the Pt-C is also much higher than that reported by Arvia et al.,[15] who first developed the electrochemical faceting method to treat bulk Pt. As mentioned previously, we modified the electrochemical faceting method reported by Arvia et al.,[15] which is described in the Experimental Section. This might result in the formation of a higher proportion of wide Pt(100) surface domains and thus the larger h3/h1 ratio compared to previous studies. Quantitative analysis of the wide Pt(100) surface domains is given in the following sections. With respect to the ECSA change, different types of Pt exhibit different behavior (Figure 2 c). For the Pt-A, its ECSA first increases within the first 20 cycles and then decreases by 8.6 % compared to the highest value after 300 cycles. The initial increase in the ECSA is attributed to the cleaning process by the electrochemical activation during the initial stage of the potential cycling. Similar behavior has also been reported in previous studies of Pt particles prepared by a wet chemical method.[21] Although a chemical washing with acetone, followed by ultrapure water was used for cubic Pt nanoparticles (Pt-A) prepared by the water-in-oil microemulsion method, it cannot fully clean the Pt particles.[21c] Since the reversible hydrogen desorption in H2SO4 solution is extremely sensitive to the Pt surface status, the potential impurities adsorbed on Pt(100) sites cause the decrease of h2 and h3 peak currents for the Pt-A. Therefore, the h2/h1 and h3/h1 ratio of the Pt-A are smaller than those of the Pt-B in the initial cycles. During the electrochemical potential cycling in H2SO4 solution, the active surface of Pt particles would be further exposed such that the ECSA of the Pt-A increases in the initial stage of the potential cycling. Note that some of the previous reports did not provide the initial CVs in the cleaning stage and only show the stable CVs.[21b, 22] In the present work, to reflect the voltammetric changes during the whole experiment and also for the comparison between (100)-oriented Pt prepared by different methods, Figure 1 a–c shows the very initial potential cycling stage, including the first CV. In the case of (100)-oriented Pt nanoparticles prepared by the electrodeposition method, which was developed in our previous work,[11d] the ECSA shows a decreasing trend during the whole testing cycle. Because the electrodeposition method avoids the use of any surfactants, capping agents and organic solvents, the obtained Pt nanoparticles are extremely clean and there is no electrochemical cleaning/activation process in the initial potential cycling. Previous studies have found that repeated potential cycling led to the loss of ECSA, due to the possible dissolution–redeposition, Ostwald ripening and aggregation processes.[13b, 20] This is consistent with the present work on Pt-A and Pt-B. However, an unexpected behavior was observed for the Pt-C. The ECSA increases slightly during the whole testing stage and

Figure 2. Variation of a) the h2/h1 peak current ratio, b) the h3/h1 peak current ratio and c) the ECSA as a function of potential cycle numbers.



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Articles adsorption of Ge [mC cm@2], Q0ð100Þ is the charge for Pt(100) sites obtained from a Pt(100) single crystal (0.209 mC cm@2), a is the coefficient according to the literature (0.56),[19b] I is the current density for the oxidation of Ge on Pt(100) terraces [mA cm@2], E is the potential related to I [V], and v is the scan rate [V s@1]. Table 1 shows the calculated proportions of Pt(100) sites for different Pt samples before and after 300 potential cycles. It should be noted that when using irreversible Ge adsorption to quantify the Pt(100) sites, only larger (100) surface domains or terrace sites that are wide enough for the Ge adsorption can

increases by 14.1 % after 300 potential cycles. The potential cycling between @0.2 and 1.0 V (vs. SCE) could be essentially considered as a triangle-wave perturbing treatment at a somewhat low frequency equal to 0.02 Hz. As is reported in the literature,[15a, b, 16] the low-frequency periodic potential perturbing can increase the roughness of the Pt disk surface, and increase the ECSA measured in 0.5 m H2SO4 solution. This might result in the slight increase in ECSA in the present work. The amount of the Pt(100) sites was quantified by measuring the irreversible adsorption of Ge.[19b, 23] Figure 3 shows the

Table 1. Calculated proportions of Pt(100) sites from irreversible Ge adsorption curves for different Pt specimens before and after 300 potential cycles.

voltammograms of the irreversible adsorption of Ge on different Pt samples in 0.5 m H2SO4 solution before and after 300 potential cycles. All the currents are normalized to the Pt ECSA. Compared to the data in Figure 1, the irreversible Ge adsorption results in the absence of the hydrogen adsorption/desorption peaks, suggesting that the Ge coverage blocks the Pt surface. A broad oxidation peak was observed at approximately 0.24 V (vs. SCE), which is attributed to the oxidation of Ge adatoms on Pt(100) sites.[19b, 23] The proportion of Pt(100) sites can be calculated by the oxidation charge after subtracting the background current contribution according to Equation (1):[19b] R Qtð100Þ a1 I dEv ¼ Q0ð100Þ Q0ð100Þ

ð1Þ

where Ptð100Þ is the proportion of Pt(100) sites (%), Qtð100Þ is the charge of the Pt(100) terraces resulting from the irreversible ChemElectroChem 2017, 4, 66 – 74


Pt(100) sites after 300 cycles [%]

Pt-A Pt-B Pt-C

14.71 16.27 36.72

33.75 33.79 52.53

be detected.[3a, 24] For all the three Pt samples, the proportion of wide (100) domains decreases after 300 cycles, agreeing with the change of the h3/h1 ratio in Figure 2 b. After 300 cycles, Pt-C exhibits a much higher proportion of wide (100) domains, which is over 125 % higher than Pt-A and Pt-B and is even slightly higher than the initial values of the proportion of wide (100) domains of the Pt-A and Pt-B before potential cycling. Again, this is consistent with the results shown in Figure 2 b (note that the h3/h1 ratio after 300 cycles for Pt-C is higher compared to Pt-A and Pt-B). Because the electro-oxidation of ammonia is extremely sensitive to the surface structure of Pt, and it takes place almost exclusively on Pt(100) sites, in particular the wide (100) surface domains,[3a, 7a, d, e, 11d, 25] it can be used to evaluate the change of the surface structure of Pt during the potential cycling. After certain numbers of potential cycles in 0.5 m H2SO4 solution, the Pt samples were rinsed thoroughly and transferred to an ammonia-containing solution (1 m KOH + 0.1 m ammonia) for CV measurements. Figure 4 a–c shows CVs measured on Pt-A, Pt-B and Pt-C, respectively, after different potential cycles. In order to reveal the specific activity (intrinsic activity), the current was normalized to the ECSA of the Pt. All of these CVs show a characteristic oxidation peak at around @0.37 V (vs. SCE), corresponding to the electro-oxidation of ammonia.[2b, 3c, 26] Our previous work showed that there was no such oxidation peak in the CV of a blank KOH solution without ammonia.[27] The peak oxidation current density of all the three Pt samples decreases with increasing cycle number, indicating the decrease of the specific activity. It has been widely reported that the peak oxidation current density for ammonia electro-oxidation has a strong relationship with the amount of wide (100) surface domains on Pt.[3a, 11c, d, 28] The peak oxidation current density of Pt for the ammonia electro-oxidation increases as the wide (100) surface domains increases.[3a, 7a, 11c, d, 28] Therefore, the de-

Figure 3. CVs of the irreversible adsorption of Ge on a, b) Pt-A, c, d) Pt-B and e, f) Pt-C in 0.5 m H2SO4 solution. a, c and e) before and b, d and f) after 300 potential cycles.

Ptð100Þ ¼

Pt sample Pt(100) sites before cycling [%]

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Figure 5. Variation of the specific activity for ammonia electro-oxidation with respect to potential cycle numbers.

cific activity of the electrochemically treated bulk Pt (Pt-C) is 1.8–3.4 times higher than that of the Pt-A and Pt-B during the whole testing time. This is also fully consistent with its larger h3/h1 ratio and higher proportion of wide (100) surface domains. Figure 6 a shows the TEM image of cubic Pt nanoparticles prepared by the water-in-oil microemulsion method (Pt-A). Because the Pt nanoparticles prepared by this method are relatively clean without capping agents and surfactants, it is difficult to totally avoid the aggregation of the nanoparticles. Similar phenomenon has also been reported by previous studies for nanoparticles with relatively high level of cleanliness.[11d, 28] Even certain aggregation is present in Figure 6 a, it is clearly seen that most of the Pt nanoparticles are cubic in shape with a size of about 10 nm. Additionally, some unexpected irregular nanoparticles can also be seen in Figure 6 a, which might lead to the reduced current density of ammonia electro-oxidation of the Pt-A compared with the highest value reported in the reference.[3a] Figure 6 b shows the high-resolution TEM (HRTEM) image for the Pt nanoparticles and the corresponding fast Fourier transform (FFT) from the marked region. The HRTEM image reveals the highly crystalline structure of the cubic Pt nanoparticle, showing clear lattice fringes parallel to its edges.

Figure 4. CVs of ammonia electro-oxidation in 0.1 m ammonia + 1.0 m KOH measured on a) Pt-A, b) Pt-B and c) Pt-C, respectively, after the first, 10th, 20th, 50th, 100th, 200th, and 300th potential cycles in 0.5 m H2SO4.

creasing specific activity indicates the loss of the wide (100) surface domains during the potential cycling in 0.5 m H2SO4 solution. The peak oxidation current density of Pt nanoparticles with preferential (100) orientation (Pt-A and Pt-B) shows a somewhat similar decreasing trend, which decreased by about 66.5 % after 300 cycles. The loss of the peak oxidation current density of Pt-C is much less than that of (100)-oriented Pt nanoparticles and only a 39.8 % decrease was observed after 300 cycles. This indicates the higher electrochemical stability of Pt-C compared to Pt-A and Pt-B. Additionally, as reported in most of previous studies, the typical values of peak oxidation current density (normalized to the ECSA) of Pt particles with well-defined preferential (100) orientation for ammonia electro-oxidation range from 1.0 to 1.3 mA cm@2.[8b, 11b, c, 28] Before the potential cycling, the value of the peak oxidation current density of (100)-oriented Pt nanoparticles obtained in this study is similar to those reported previously.[8b, 11b, c, 28] However, the peak current density of the Pt-C before potential cycling (2.22 mA cm@2) is nearly 1.8 times higher than that of Pt-A and Pt-B. This value has so far been the highest reported in the literature for the ammonia electro-oxidation. This also suggests that the Pt-C prepared in the present work has a significantly high proportion of wide (100) surface domains, which is consistent with both the hydrogen desorption profile which shows a stronger h3 peak (Figures 1 c and 2 b) and the Ge irreversible adsorption (Table 1). This is an interesting result that deserves further investigation, however, it is beyond the scope in this work and will be studied in detail in another work. Figure 5 shows the change of the specific activity as a function of potential cycle number. The specific activity is determined by the peak oxidation current density normalized by the Pt ESCA after subtracting the background contribution.[3c] Pt nanoparticles with preferential (100) orientation (Pt-A and Pt-B) exhibit quite similar specific activity during the whole potential cycling. This can be explained by their similar change of the h3/h1 ratio (Figure 2 b) and the proportion of wide (100) surface domains (Table 1) during the potential cycling. The speChemElectroChem 2017, 4, 66 – 74


Figure 6. a) TEM and b) HRTEM images of cubic Pt nanoparticles prepared by the water-in-oil microemulsion method before potential cycling. Inset: the corresponding FFT pattern. c) TEM and d) HRTEM images of cubic Pt nanoparticles after 300 potential cycles.

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Articles The distance between adjacent lattice fringes is 1.95 a, which corresponds to the (200) planes of the face-centered cubic (fcc) Pt. This indicates the formation of single-crystal cubic Pt particles enclosed by well-defined (100) facets. As shown in the FFT image (Figure 6 b, inset), the angle between OA and OB is close to 458 and the distance ratio of OA/OB is approximately 1.414, suggesting the zone axis is [001]. The above structural characterizations are in good agreement with the electrochemical measurements (Figure 1 a), which demonstrate the preferential (100) orientation of the cubic Pt particles before the potential cycling. Figure 6 c and d show the TEM and the HRTEM images, respectively, of the cubic nanoparticles (Pt-A) after 300 potential cycles. A dramatic change in the shape and structure of the cubic Pt particles was observed. Almost no cubic Pt particles were found and the shape of original cubic Pt particles changed to an irregular shape after 300 cycles (Figure 6 c). As shown in Figure 6 d, these irregular Pt particles are characterized by a polycrystalline structure. It should be noted that although TEM can provide direct information about the surface structure of the Pt particles, it can only characterize a limited number of the nanoparticles and thus cannot represent average information of an entire batch of nanoparticles. Therefore, in situ electrochemical measurements, including hydrogen adsorption/desorption (Figure 1) and the ammonia electro-oxidation (Figure 4) experiments, were also performed. These electrochemical measurements together with the irreversible Ge adsorption (Figure 3 and Table 1) demonstrate the loss of the Pt(100) sites for cubic Pt particles (Pt-A). This is supported by the structural characterization by TEM. Figure 7 a–d shows the high-magnification and low-magnification TEM images of electrodeposited Pt nanoparticles before and after potential cycling. Before cycling, the as-prepared Pt nanoparticles (Pt-B) are characterized by a prickly morphology with protruding tips on the surface (Figure 7 a). Each Pt nanoparticle consists of smaller Pt nanoparticles (Figure 7 b). The presence of preferential (100) orientation of freshly electrodeposited Pt nanoparticles (Pt-B) is confirmed by different types of the characterization methods including the hydrogen

desorption profile, ammonia electro-oxidation and Ge irreversible adsorption. To prepare (100)-oriented Pt nanoparticles, the electrodeposition was carried out under a large current density (high overpotential). We previously found that Pt nanoparticles with preferential (100) orientation can be formed by electrodeposition under high cathodic overpotentials due to enhanced hydrogen evolution.[11d] Hydrogen adsorption alters the surface free energies of different Pt facets and the surface free energy of Pt(100) is lower than that of the Pt(111) surface in the presence of hydrogen,[11d, 29] which favors the formation of (100)-oriented Pt nanoparticles. On the other hand, the electrodeposition at high overpotential resulted in dendrite growth of the Pt nanoparticles (Figure 7 a and b). After 300 potential cycles, the size of the Pt nanoparticles was almost unaffected but the sharp tips became rounded/blunt or disappeared (Figure 7 c and d). Previous studies found that sharp edges and corners are highly active sites that are prone to dissolution and reconstruction during the catalysis or the potential cycling.[13b, 30] This is also the case in our work. Unlike chemically synthesized Pt nanoparticles with well-defined cubic shape (Figure 6 a and b), electrodeposited Pt nanoparticles (Pt-B) do not have well-defined shapes. Therefore, it is not appropriate to use the HRTEM to compare the structural change of electrodeposited Pt nanoparticles with irregular shape before and after potential cycling because it cannot represent statistical or average information of the surface structure and thus might give misleading conclusions. As a result, HRTEM was not used in this case. Figure 8 a–f shows SEM images of the bulk Pt disk electrode treated by electrochemical faceting (Pt-C) before and after 300 potential cycles. After the electrochemical faceting treatment, the original polished surface changed significantly and exhibited a highly stepped feature with parallel-terrace-like morphol-

Figure 7. TEM images of electrodeposited Pt nanoparticles a, b) before and c, d) after 300 potential cycles at a, c) low magnification and b, d) high magnification.

Figure 8. SEM images of the bulk Pt disk electrode treated by the electrochemical faceting a–c) before and d–f) after 300 potential cycles with different magnifications.



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Articles ogy (Figure 8 a–c). This contributes to the remarkable change of the CV profile (i.e. higher h2/h1 ratio and strong h3 peak) of the electrochemically treated Pt surface (Figure 1 c) compared to the untreated polycrystalline bulk Pt. After 300 cycles, both low-magnification and high-magnification SEM images did not show obvious morphological change on the Pt surface (Figure 8 d–f). However, the repeated CV measurements in H2SO4 (Figure 1) and ammonia-containing solutions (Figure 4), as well as the irreversible Ge adsorption (Figure 3 and Table 1) clearly demonstrate the surface structural change of Pt-C during the potential cycling. This suggests that the surface structural change of Pt-C after 300 cycles is too small to be detected by SEM. During the potential cycling, the loss of the Pt(100) sites especially wide (100) domains for all three (100)-oriented Pt is revealed by the decreasing h2/h1 and h3/h1 ratios (Figure 2), proportion of wide (100) domains characterized by the Ge irreversible adsorption (Figure 3 and Table 1), and the specific activity for the ammonia electro-oxidation (Figure 4). This is due to the repeated formation and reduction of Pt surface oxide, where the replacement between surface Pt atoms and adsorbed oxygen species gradually reconstructs the well-defined (100) surface domains.[12, 31] Based on the scanning tunneling microscopy characterizations, Furuya et al.[14b] found that small islands of Pt atoms were formed on the Pt(100) terrace after electrochemical potential cycling. Furthermore, a review of the literature shows that the proportion of wide Pt(100) surface domains losses quickly during the potential cycling.[10b, 12–14, 20] This feature has been observed for various types of (100) preferentially oriented Pt including Pt nanoparticles prepared by chemical methods,[10b, 12, 13] Pt nanowires prepared by electrochemical methods,[14a, 20] and Pt single crystals.[14b] For example, in the case of (100)-oriented Pt nanoparticles, CV measurements showed that the current peak related to the wide Pt(100) surface domains (h3 peak) decreased rapidly and almost disappeared after 100–200 potential cycles.[12, 13b] Garbarino et al.[20] and Ponrouch et al.[14a] also observed a loss of the h3 peak after 300 potential cycles. By using a Pt(100) single crystal, Furuya et al.[14b] found that the h3 peak was almost absent after 100 potential cycles. In the present work, for (100)-oriented Pt nanoparticles prepared by two different methods [i.e. chemical (Pt-A) and electrochemical (Pt-B) methods], the h3 peak becomes very small after 300 potential cycles and the proportion of wide (100) surface domains decreases to 15–16 %, which is similar to that of polycrystalline Pt.[11a] These results are similar to those reported previously.[10b, 12–14, 20] However, it is rather surprising that the Pt-C still shows a strong h3 peak, a high proportion of wide (100) domains and a high specific activity value for the ammonia electro-oxidation (1.31 mA cm@2) after 300 potential cycles. Note that this specific activity is similar to that of (100)-oriented Pt before the potential cycling.[8b, 11d] All of these results are consistent and are evidence of the high stability of wide (100) surface domains of the Pt-C, which is quite different compared to the Pt-A and Pt-B and also those reported by previous studies including both (100)-oriented nano-Pt systems and Pt(100) single crystals.[10b, 12–14, 20] The unusually high stability of wide (100) surface domains of Pt-C is ChemElectroChem 2017, 4, 66 – 74


probably associated with its preparation procedure. In the present work, the Pt disk electrode was electrochemically treated by the high-frequency periodic square-wave potential in H2SO4 solution. Pt is electrodissolved during the positive halfcycle, whereas it is electrodeposited on the surface during the negative half-cycle. The lower and upper potential limits attain the potential range of the adsorption and desorption of hydrogen adatoms and oxygen-containing species (e.g. OH). The adsorption and desorption of these species contribute to the selective electrodissolution and electrodeposition of Pt atoms. The adsorption of hydrogen adatoms that occurred in the negative half-cycle is one of the factors that favor the development of Pt(100) sites. Previous studies, including our own work, found that hydrogen adsorption plays an important role in the formation of Pt(100) sites, because hydrogen adsorption lowers the surface free energy of the Pt(100) surface.[11d, 14a, 20, 29] Furthermore, the periodic adsorption and desorption of oxygen-containing species is another important factor, promoting the disruption and the reconstruction of surface Pt atoms.[15b, c] This together with the hydrogen adsorption assisted formation of Pt(100) sites in the negative half-cycle, expanding the Pt(100) sites and leading to the formation of wide (100) surface domains of the Pt disk electrode. Under the highfrequency potential perturbation between the potential range of oxide formation/reduction and hydrogen adsorption/desorption, the surface Pt atoms move frequently from initial metastable positions to equilibrium locations, resulting in the formation of wide (100) surface domains with a much higher stability compared to those obtained by other methods and the Pt(100) single crystal. The detailed investigations on the formation mechanism of wide (100) surface domains of Pt-C are beyond the scope of the present work and will be reported in another publication.

3. Conclusions The electrochemical stability of different preferentially (100)oriented Pt was studied by various electrochemical methods and morphological/structural characterizations. Three different methods were used to prepare preferentially (100)-oriented Pt samples, which were Pt nanoparticles prepared by the waterin-oil microemulsion method and the electrodeposition method, respectively, and the bulk Pt disk electrode prepared by electrochemical faceting. During the potential cycling up to the potential region of Pt surface oxide formation in H2SO4 solution, there is a decrease in the hydrogen desorption peaks related to the Pt(100) sites including (100) steps and terrace borders and wide (100) domains. The loss of the Pt(100) sites was also quantitatively confirmed by irreversible Ge adsorption. As a result, the specific activity of preferentially (100)-oriented Pt for the electro-oxidation of ammonia decreases during the potential cycling. After 300 potential cycles, TEM observations showed that the well-defined cubic Pt nanoparticles change to polycrystalline structure, whereas the sharp tips of electrodeposited Pt nanoparticles become rounded/blunt or disappeared. Notably, the bulk Pt disk electrode treated by electrochemical faceting exhibits a much higher electrochemi72


Articles cal stability compared to preferentially (100) oriented Pt nanoparticles and previously reported Pt single crystals, as demonstrated by the CV measurements in H2SO4 solution and ammonia-containing solution, as well as the irreversible Ge adsorption. For example, the wide (100) surface domains of the Pt disk electrode, treated by electrochemical faceting, is over 2.2 times higher than that of the Pt nanoparticles after 300 potential cycles. The specific activity for the ammonia electro-oxidation of the former is about 3.4 times higher than that of the latter after 300 cycles. The formation of a preferentially (100)oriented bulk Pt electrode is associated with the high-frequency selective electrodissolution and electrodeposition of Pt coupled to the periodic adsorption and desorption of oxygen-containing species and hydrogen adatoms. During this process, the surface Pt atoms move frequently from their initial metastable positions to equilibrium locations, which may result in the formation of wide (100) surface domains with a higher stability.

The bulk Pt electrode with preferential (100)-oriented surface was prepared by an electrochemical faceting method, with modifications, which was based on the method developed by Arvia and coworkers.[15c] Prior to use, a Pt disk electrode (diameter = 5 mm, geometric surface area = 0.196 cm2) was polished sequentially with 1, 0.3 and 0.05 mm alumina slurry and cleaned in ultrapure water with sonication for 1 min. The Pt disk electrode was electrochemically treated by periodic square-wave potentials in H2SO4 solutions (1 m), with a multifunction generator (WF1973, NF Corporation, Yokohama, Japan) coupled with a power supply (BP4610, NF Corporation). A two-electrode cell was used, with a Pt disk electrode and a graphite rod electrode. The optimum values of the frequency, upper/ lower potential limits and the total time of periodic square-wave potentials for obtaining a Pt disk electrode with preferential (100) orientation were 2000 Hz, 1.8/@3.8 V, and 2 h, respectively.

Structural and Electrochemical Characterization The surface morphology of the Pt disk electrode was characterized by SEM (S-4800, Hitachi). The shape and morphology of the Pt nanoparticles were characterized by TEM (JEM-2100F, JEOL). The surface structure of the nanoparticles was characterized by HRTEM.

Experimental Section Reagents and Materials

Electrochemical measurements were performed on a PARSTAT 2273 electrochemical workstation, with a classical three-electrode cell. A GCE supported with as-prepared Pt nanoparticles or an electrochemically treated Pt disk electrode was used as the working electrode. A SCE and a Pt plate (2 V 2 cm) were used as the reference electrode and the counter electrode, respectively. The electrochemically active surface area (ECSA) of the Pt was determined by the hydrogen desorption charge obtained from steady-state CV at a scan rate of 0.05 V s@1 in H2SO4 solution (0.5 m),[32] which was described in detail in our previous studies.[1a] The proportion of wide (100) surface domains on Pt was estimated by the irreversible adsorption of Ge, which was described in previous studies.[19b] The GCE supported with Pt nanoparticles or the Pt disk electrode was immersed in NaOH solution (1 m) containing GeO2 (0.01 m) at @0.2 V (vs. SCE) for 1 min, and it was then transferred to a H2SO4 solution (0.5 m) with a droplet of the GeO2 solution covering the electrode surface. Voltammograms were recorded from @0.2 to 0.4 V (vs. SCE) at a scan rate of 0.05 V s@1. The charge associated with the oxidation of Ge adatoms was used to calculate the proportion of Pt(100) sites. The electro-oxidation of ammonia was carried out in ammonia solution (0.1 m) containing KOH (1 m) by cyclic voltammogram at a scan rate of 0.01 V s@1. In addition, the irreversible adsorption of Ge as well as the electro-oxidation of ammonia was tested right after each CV cycle in H2SO4 solution (0.5 m) in order to avoid any impurities and to show their intrinsic properties. In order to compare the intrinsic activity of Pt, all the currents in the CV curves were normalized by the ECSA of Pt. All the solutions for electrochemical measurements were deaerated by purging with high-purity Ar gas (99.999 %).

H2PtCl6, n-heptane, polyethylene glycol dodecyl ether (Brij 30), and GeO2 were purchased from Sigma–Aldrich. H2SO4, (NH4)2SO4, HCl, NaOH, KOH, and NaBH4 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were of analytical grade and used as received. All the solutions were prepared from ultrapure water (18.2 MW cm) obtained from a Milli-Q water purification system (Millipore, Billerica, MA). A glassy carbon electrode (GCE, diameter = 5 mm, geometric surface area = 0.196 cm2) was used as the working electrode. Before use, the GCE was sequentially polished with 1, 0.3 and 0.05 mm alumina slurries and washed in ultrapure water with sonication for 1 min.

Electrode Preparation Cubic (100)-oriented Pt particles were synthesized by the wet chemical reduction method (water-in-oil microemulsion method) developed by Mart&nez-Rodr&guez and co-workers.[3a] H2PtCl6 was reduced by adding NaBH4 to a microemulsion of water/Brij 30/nheptane (3/16.5/80.5 v/v/v). The aqueous solution contains H2PtCl6 (100 mm), NaBH4 (1 m) and HCl (25 % w/v). After the reduction, the Pt nanoparticles were precipitated by adding acetone to the microemulsion, and collected by removing the liquid phase. Afterwards, they were sequentially washed with acetone, acetone/water mixtures and water. To prepare Pt nanoparticles with preferential (100) orientation by the electrochemical method, experiments were performed on a PARSTAT 2273 electrochemical workstation (Princeton Applied Research, Oak Ridge, Tennessee, USA). A classical three-electrode cell was used, with a GCE as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a Pt plate (2 V 2 cm) as the counter electrode. (100)-oriented Pt nanoparticles were prepared on the GCE by the electrodeposition at a current density of 12 mA cm@2 in a solution of H2PtCl6 (5 mm) and HCl (0.5 m), which was based on our previously reported method[11d] with a slight modification. After the electrodeposition, the working electrode was removed from the cell and rinsed thoroughly with ultrapure water. ChemElectroChem 2017, 4, 66 – 74


Acknowledgements This work was supported by the National Key Research and Development Program (2016YFB0700205), National Natural Science Foundation for Distinguished Young Scholars (51125016) and the Tianjin Natural Science Foundation (16JCYBJC17600). 73


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Manuscript received: July 31, 2016 Accepted Article published: September 23, 2016 Final Article published: October 19, 2016

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