Methanol Electro-Oxidation on the Pt Surface: Revisiting the Cyclic

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Methanol Electro-oxidation on Pt Surface: Revisiting the Cyclic Voltammetry Interpretation Dong Young Chung, Kyung Jae Lee, and Yung-Eun Sung J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12303 • Publication Date (Web): 19 Apr 2016 Downloaded from http://pubs.acs.org on April 19, 2016

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The Journal of Physical Chemistry

Methanol

Electro-oxidation

on

Pt

Surface:

Revisiting the Cyclic Voltammetry Interpretation Dong Young Chung, #,†,‡ Kyung-Jae Lee #,†,‡, ¶ and Yung-Eun Sung*,†,‡ †

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-747, South

Korea ‡

School of Chemical and Biological Engineering, Seoul National University, Seoul 151-747,

South Korea

# These authors contributed equally to this work.

Corresponding Author E-mail: [email protected]

ACS Paragon Plus Environment


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ABSTRACT Methanol is a promising fuel for direct methanol fuel cells in portable devices. A deeper understanding of its electro-oxidation is needed for evaluating electrocatalytic performance and catalyst design. Here we provide an in-depth investigation of the cyclic voltammetry (CV) of methanol electro-oxidation. The oxidation peak in backward scan is shown to be unrelated to residual intermediate oxidation. The origin of second oxidation peak (If2) is expected to the methanol oxidation on Pt-Ox. Electrochemical impedance spectroscopy coupled with CV reveals the origin of CV hysteresis to be a shift in the rate-determining step, from methanol dehydration to OH adsorption by water dissociation, induced by a change in Pt surface coverage with oxygenated species. The peak ratio between forward oxidation peak current (If) and backward oxidation peak current (Ib), which is If/Ib, is not related to the degree of CO tolerance but to the degree of oxophilicity indeed.

KEYWORDS: Methanol oxidation reaction, Pt, CO poisoning, oxophilicity, Cyclic voltammetry


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INTRODUCTION Given the demands for energy supplies for portable devices, methanol oxidation has a great potential for next-generation energy conversion system.1 However, its low kinetics impedes the development of a direct methanol fuel cell for commercialization. To fully oxidize methanol to carbon dioxide (CO2), a 6-electron-mediated reaction is required, which is very complicate.2 Another problem is the reaction intermediate, carbon monoxide (CO), which binds strongly to the Pt catalyst and deactivates it. Effort has been made over decades to develop effective catalysts to enhance the methanol electro-oxidation reaction (MOR). Because COad oxidation (the subscript ad implies the species is adsorbed on the surface) can be explained by the Langmuir-Hinshelwood mechanism3, facile water dissociation and easy intermediate removal are important. More oxophilic materials, such as Ru and Ir, which are known to be good candidates for facilitating water dissociation, can be used to generate alloys, followed by reducing CO poisoning.4-13 Alloying with Au has been shown to enhance the MOR because of the weak CO binding energy.14 Controlling the nanoparticle shape and morphology of the catalyst can also enhance methanol oxidation dramatically.15 Various efforts to enhance the MOR activity have been successful; however, some issues should still be clarified to understand clearly. Goodenough’s group suggested two anodic peaks in the forward scan.16 The first peak (If1) at approximately 0.9 V was considered to be the main peak of methanol oxidation. The second peak (If2) came from the oxidation of residual intermediate carbon species, rather than that of freshly chemisorbed methanol. Both If2 and the anodic peak (Ib) in the backward scan are attributed to the oxidation of residual intermediates, not methanol. Following this work, many researchers suggested the peak ratio (If1/Ib) is associated with the CO-poisoning factor.17-23 According to this interpretation, a small Ib means that residual intermediates (mostly CO) are fully eliminated during the forward scan, which corresponds to a lower level of CO poisoning. A high If/Ib factor, therefore, has been regarded as an important criterion for a good MOR electrocatalyst. However, other researchers using in situ Fourier transform infrared spectroscopy (FTIR) reached the opposite conclusion: Ib is not related to the residual CO but rather to freshly chemisorbed methanol.24 In order to understand the nature of Ib in terms of the electrochemical reactions in the MOR, herein we focus on the methanol oxidation cyclic voltammograms (CVs). CVs in different scan directions were used to evaluate the effects of the residual intermediates. Electrochemical impedance spectroscopy (EIS) was conducted to determine the reaction



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mechanism. CV results show that the backward oxidation (Ib) is not affected by the forward reaction, and hence Ib cannot come from a forward scan intermediate, however the origin of If2 may be attributed to the MOR on the platinum oxides (Pt-Ox). The hysteresis between the forward and backward scans is due to the change in the rate-determining step. We also suggest that the meaning If/Ib is not related to level of CO tolerance, but to degree of oxophilicity instead.

EXPERIMENTAL Methanol electro-oxidation reaction An Autolab PGSTAT 101 in a standard three-compartment electrochemical cell was used for the electrochemical measurements. The geometric surface area of the rotating disk electrode (RDE) was 0.196 cm2. In all experiments, a saturated calomel electrode (SCE) was used as a reference electrode. All potentials referred to in this paper were converted to the pHindependent reversible hydrogen electrode (RHE) using the hydrogen oxidation/evolution reaction by Pt electrode. Used Pt/C and PtRu/C was commercial catalyst (J.M. Company, Pt/C: 40 wt.% and PtRu/C: 40wt.% Pt + 20 wt.% Ru). All electrochemical measurements were conducted in 0.1 M HClO4 + 0.5 M CH3OH electrolyte (Perchloric acid: 70%, ACS reagent, Sigma Aldrich, methanol: Chromasolv for HPLC ≥ 99.9%). Before the CV measurement started, the equilibrium time (5 s) at each initial potential was applied. The CVs were obtained at a scan rate of 50 mVs-1.

CO electro-oxidation reaction CO was introduced for 15 min at a constant voltage of 0.05 VRHE (for COad oxidation) or 0.1 VRHE (to calculate PZTC), and the currents were recorded. After CO saturation on the Pt surface, Ar (99.999%) was introduced for 20 min to eliminate excess CO. The CVs were than obtained from 0.05 to 1.2 V vs. RHE at 20 mVs-1.

EIS measurement EIS was also performed using conventional three-component electrode systems, which is same with MOR conditions. Before the EIS measurements, the methanol oxidation reaction was preceded by potential cycling from 0.05 to 1.2 V vs. RHE to avoid catalyst activation effect. Prior to EIS, the equilibrium state was achieved by inducing each potential. After the


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current reached equilibrium state, EIS was conducted with a voltage amplitude of 5 mV and a frequency range of 100 kHz to50 mHz.

RESULTS AND DISCUSSION Figure 1 presents the MOR CVs of Pt/C and PtRu/C under 0.1 M HClO4 + 0.5 M CH3OH solution. If1 and If2 are the anodic peak currents and are in good agreement with those in previous results. In order to resolve the dispute over the meaning of Ib, here we directly confirm the origins of If1 and Ib using electrochemical methods: if Ib originates from the forward scan residual intermediate, then the reverse-direction forward scan features will be different from those in the typical backward scan direction. (Note: Methanol oxidation CVs were measured two different directions. The typical direction means that CV is recorded from forward sweep (0.05-1.2 V vs. RHE) to backward sweep (1.2-0.05 V vs. RHE). The reverse direction means backward sweep first (1.2-0.05 V vs. RHE) as shown in figure 2. Figure 2 c and d show that the methanol oxidation results are identical in both typical and reverse direction for both Pt/C and PtRu/C, respectively. Hence, the backward sweep of the typical scan direction (the so-called backward reaction, Ib) is unrelated to forward scan residual intermediate, COad. Results from the COad electro-oxidation reaction at the same electrolyte (Figures 3a and 3b) also support this conclusion. The COad was completely removed by oxidation at 1.1 V (vs. RHE) in both Pt/C and PtRu/C. The CV and COad electro-oxidation data, taken together, imply that the chemical origin of Ib is not the intermediate CO produced in the forward reaction. This result supports the finding from Tong’s group, in which Ib is not originated from the residual carbon oxidation.24 Two questions remain, however if the backward oxidation reaction is not dependent on the forward oxidation reaction. First, the origin of hysteresis between anodic and cathodic sweep is unclear. Second, if intermediate species are fully oxidized below 1.1 V (vs. RHE), what is the origin of If2? Considering that the current scales are very different with and without methanol, the origin of peak must be related to methanol. Previous studies have proposed the following steps for methanol oxidation.25-27 In step 3 below, the CO adsorbed on Pt surface, which poisons the catalyst,6,28 can be oxidized to CO2 by OH, an oxygen source produced in reaction 2 via the Langmuir-Hinshelwood mechanism.3 Reaction 1: CH OH + Pt → Pt − CO + 4H + 4 e Reaction 2: H O + Pt → Pt − OH + H + e



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Reaction 3: Pt − CO + Pt − OH → CO + 2Pt + H + e

EIS can observe electrochemical reactions with various time constants in terms of the frequency. Therefore, it is useful for investigating multi-step electrochemical processes.29 EIS data for methanol oxidation can be divided into five regions according to the potential shown in Figure 4a. Below we discuss each region in detail.

1) Region 1: 0.4~0.5 V (Figure 4) In this region, CO is produced as an intermediate and adsorbed onto the Pt surface as Pt-COad. The COad inactivates the Pt surface and thereby inhibits continuous methanol oxidation.6,28 As a result, there is a small steady state current, and the Nyquist plot shows a large semicircle in this region. The latter indicates reaction 1 occurs predominantly in this region (Figure 4b). Based on these results, the majority of the Pt surface can be deduced to be covered with COad.

2) Region 2: 0.6~0.75 V (Figure 5) The Nyquist plot in this region shows typical pseudo-inductive behavior: a capacitive semicircle at high frequencies and an inductive semicircle at low frequencies (Figure 5a). The inductive behavior in AC impedance spectroscopy has been well-known in reactions through an intermediate species, where the intermediate-to-product process is the rate-determining step (RDS).30-34 In the case of the MOR, the reagent, intermediate, and product are methanol, CO, and CO2, respectively. The theoretical oxidation potential of methanol is only 0.04 V (vs. RHE at 298 K);30 therefore, the MOR (reaction 1) should occur rapidly in this region. The slower reaction between CO and OH (reaction 3) to free catalyst sites on Pt is the RDS by definition. The impedance at low frequency (w→0) becomes smaller as the potential increases, which is consistent with the increasing steady-state current observed in this region. The peak at low frequencies in the Bode plot, which corresponds to the reaction between CO and OH, shifts to higher frequencies as the potential increases (Figure 5b). This shift indicates an acceleration of reaction 3, possibly due to increased OH coverage on Pt surface.27

3) Region 3: 0.8~0.85 V (Figure 5) The impedance begins to appear in the second and third quadrants of the Nyquist plot, and an abrupt jump in the phase angle is observed in the Bode plot (Figure 5c and 5d). Similar


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behavior has also been observed for glucose oxidation at a metal (M)-modified glassy carbon electrode, and this behavior is believed to be attributable to a change in the RDS: when the kinetics for the oxidation of M-intermediate to M-product is enhanced, the RDS becomes the oxidation of M-glucose to M-intermediate.35 Therefore, the oxidation of the intermediate CO by OH is shown to be faster than the oxidation of methanol on the Pt surface. In region 2, the kinetic of the oxidation of intermediate CO with OH constantly increases, therefore, the reverse of the kinetic between the oxidation of intermediate CO and methanol is predictable. In addition, the abrupt jump in phase angle can be interpreted as an overlap of two peaks because of the reversed kinetics of the two oxidation reactions.

4) Region 4: 0.9~0.975 V (Figure 6) According to previous reports27,36, the OH coverage continuously increases, decreasing the active Pt surface available for methanol oxidation. This leads to the monotonically decreasing steady-state current, causing reaction 1 to become the RDS in region 3. Therefore, the arc size in the second and third quadrants increases as the potential increases (Figure 6a), and the peak in the Bode plot that corresponds to the oxidation of methanol on the Pt surface (reaction 1) shifts towards low frequencies (Figure 6c). OH adsorption by water dissociation, another important aspect of COad oxidation, is the RDS at lower potential. The reaction accelerates after OH adsorption at approximately 0.6 V (vs. RHE). At higher potential, the reaction slows because the adsorbed OH reduces the available Pt surface coverage. In the backward step, however, methanol dehydrogenation is the RDS; as the Pt surface is almost completely covered by OH in the negative scan direction, the MOR can occur only when a fresh Pt site is revealed by OH reduction. The onset of the methanol oxidation in the backward step and the onset potential of OH reduction in the CVs are correlated, as shown in Figure 2 and 3. (The OH reduction onset peaks of Pt/C and PtRu/C occur at approximately 1.0 and 0.8 V (vs. RHE), respectively, supporting the proposed reaction mechanism). OH reduction is much easier on Pt/C than on PtRu/C, resulting in more free sites for methanol dehydrogenation, and therefore higher peak current and positive potential during the backward scan. Difference in the Pt surface coverage along the voltage sweep directions is the origin of the hysteresis. There are other evidences that implying the electrochemical reaction in backward step is affected by Pt coverage. In different potential limit experiment (Figure 7), the backward peak decreases as increasing the anodic limit potential in forward scan, which is same trend with



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previous report37. Liu et al. suggested that this result was related to the residual carbon species.37 However, as If2 is not related to the residual carbon oxidation, we interpret this phenomenon with surface coverage effect. As increasing the anodic limit potential, the Pt is highly oxidized to Pt-Ox and Pt reduction potential is negatively shifted.38,39 At the same potential in backward scan, the Pt is existed by the much oxide form in the case of high anodic limit potential that means lowering the surface coverage to react with methanol, which is similar features of surface structure dependent MOR activity.40 The trend of oxygenated Pt reduction potential change, which is directly correlated to the Pt surface coverage, well support Ib change as varying the limit potential (Figure 7). The surface coverage effect on the backward scan is also supported by oxygen reduction reaction (ORR) as shown in Figure 7. As increasing the anodic limit potential, the backward scan ORR curves are negatively shifted (Note: ORR curves on forward scan are exactly same as varying the anodic potential limit). The low ORR current during backward scan is attributed to the high surface coverage with oxygenated species.41 In other words, surface coverage to react with oxygen, which is (1θad), is much higher in forward scan, based on ORR rate expression as equation 142. I= nFkcO2 (1 - θad)x exp(-βFE/RT) exp(-γrθad/RT)

(1)

Because Pt surface in the forward scan is less covered by OH and oxide at given potential compared to that in the backward scan41,43 (Note: In perchloric acid, θad is primarily the OHad.). Based on the information mentioned above, the meaning of If/Ib in methanol electrooxidation can be deduced as follows. The lower the Pt surface covered with oxygenated species causes the higher the Ib in backward scan, implying that free Pt surface (not covered by oxygenated species) is related to backward scan current. As oxophilicity is the tendency of chemical compounds to form surface oxygenated species,44 the correlation between surface coverage with oxygenated species and Ib implies that If/Ib seems to be related to the degree of oxophilicity. The correlation between oxophilicity and Ib in backward scan is also corroborated by PtRu/C. PtRu/C has higher tendency to form oxygenated species, so called high oxophilicity45 (in other words, PtRu/C is easily oxidized) and is followed by the high If/Ib. In summary, the hysteresis between the forward and backward scans in the MOR has the same origin as the hysteresis in ORR: difference in the Pt surface coverage with oxygenated species. If/Ib does not represent the degree of CO tolerance. Instead, it is affected by the Pt


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coverage with oxygenated species, which is degree of oxophilicity.

5) Region 5: 1.1~1.2 V (Figure 6) In region 4, most of the Pt surface is oxidized. As a result, the original methanol oxidation steps, in which methanol is oxidized to CO on the Pt surface and CO is oxidized to CO2 by OH, become impossible. However, the steady-state current increases, and a new semicircle is observed in the Nyquist plot (Figure 6c), which indicates that a new electrochemical reaction begins to occur under these new circumstances. The mechanism remains unclear, but a new route for the methanol oxidation on the Pt-Ox surface takes place in this region. We expect the origin of second peak (If2) in CV is related to the MOR on Pt-Ox, not on Pt-OH.: As potential increases, the OH coverage on Pt surface increases, and the steady-state current decreases, which means that Pt-OH cannot act as an independent active site for methanol oxidation, but can act as a supporting site for eliminating Pt-CO on Pt surface, which is based on Langmuir-Hinshelwood mechanism. As Pt-OH goes to Pt-Ox along the potential increases further, however, the second peak (If2) is observed, which implying that the methanol oxidation can occur on Pt-Ox. Pt surface transition from Pt-OH to Pt-Ox above 1.0 V (vs. RHE) is demonstrated by in situ X-ray absorption fine structure analysis as the potential increases.46 Further studies to reveal the mechanism underlying methanol oxidation on Pt-Ox will be challenging, because information regarding the reactions on the Pt-Ox surface remains not clear yet, such as the reversibility and the exact mechanism38,39. Finally, an equivalent circuit model of methanol oxidation can be proposed, as shown in Figure 8. Three electrochemical reactions—the methanol dehydration reaction, the reaction between CO and OH, and reaction on Pt-Ox—are connected in parallel. First, the methanol dehydration reaction is represented as a RC parallel circuit, similar to general electrochemical reactions. Second, the reaction between CO and OH is expressed using a RL series circuit, similar to electrochemical reactions with intermediates. Finally, the electrochemical reaction on the Pt-Ox surface is added parallel to the two electrochemical reactions in a RC parallel circuit. The quantitative analysis results using EIS fitting based on the proposed equivalent circuit were described in supporting information.

CONCLUSION This study presents a detailed investigation of methanol electro-oxidation by CV and EIS. CV and COad electro-oxidation analyses confirm that Ib is not related to the residual



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intermediate CO, but to the coverage of free Pt surface for methanol reaction. The origin of the hysteresis between If1 and Ib is attributed to the Pt surface coverage effect, which causes the switch of RDS from OH adsorption by water dissociation to methanol dehydration. Based on the EIS results and potential limit experiment, we suggest that the If/Ib is not related to degree of CO tolerance, but to degree of oxophilicity instead. The origin of If2 is expected to the methanol oxidation on Pt-Ox, however further study such as methanol adsorption on Pt-Ox will be required.

AUTHOUR INFORMATION Corresponding authors [email protected] Author contributions #

D.Y. C. and K. J. L. contributed equally to this work.

Current address ¶

K. J. L. Battery R&D, LG Chem, Daejeon 305-738, South Korea

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by IBS-R006-G1.

ASSOCIATED CONTENT Supporting Information Available: Quantitative analysis using EIS fitting results can be found in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org

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Fabrication, characterization, and electrocatalytic properties. Nano Lett. 2009, 9, 4352-4358. 22. Yoo, E.; Okata, T.; Akita, T.; Kohyama, M.; Nakamura, J.; Honma, I. Enhanced electrocatalytic activity of Pt subnanoclusters on graphene nanosheet surface. Nano Lett. 2009, 9, 2255-2259. 23. Lee, Y.-W.; Lee, J. -Y; Kwak, D. -H.; Hwang, E. -T.; Sohn, J. I.; Park, K. -W. Pd@Pt core-shell nanostructures for improved electrocatalytic activity in methanol oxidation reaction. Appl. Catal. B Environ. 2015, 179, 178-184. 24. Hofstead-Duffy, A. M.; Chen, D. J.; Sun, S. G.; Tong, Y. J. Origin of the current peak of negative scan in the cyclic voltammetry of methanol electro-oxidation on Pt-based electrocatalysts: A revisit to the current ratio criterion. J. Mater. Chem. 2012, 22, 5205-5208. 25. Gasteiger, H. A.; Marković, N.; Ross Jr, P. N.; Cairns, E. J. Methanol electrooxidation on well-characterized Pt-Ru alloys. J. Phys. Chem. 1993, 97, 12020-12029. 26. Jarvi, T. D.; Sriramulu, S.; Stuve, E. M. Potential dependence of the yield of carbon dioxide from electrocatalytic oxidation of methanol on platinum(100). J. Phys. Chem. B 1997, 101, 3649-3652. 27. Hsing, I. M.; Wang, X.; Leng, Y. J. Electrochemical impedance studies of methanol electro-oxidation on Pt/C thin film electrode. J. Electrochem. Soc. 2002, 149, A615-A621. 28. Xia, X. H.; Iwasita, T.; Ge, F.; Vielstich, W. Structural effects and reactivity in methanol oxidation on polycrystalline and single crystal platinum. Electrochim. Acta 1996, 41, 711-718. 29. Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. W. Electrochemical and photoelectrochemical investigation of water oxidation with hematite electrodes. Energy & Environ. Sci. 2012, 5, 7626-7636. 30. Wu, G.; Li, L.; Xu, B. Q. Effect of electrochemical polarization of PtRu/C catalysts on methanol electrooxidation. Electrochim. Acta 2004, 50, 1-10. 31. Piela, P.; Fields, R.; Zelenay, P. Electrochemical impedance spectroscopy for direct methanol fuel cell diagnostics. J. Electrochem. Soc. 2006, 153, A1902-A1913. 32. Armstrong, R. D.; Henderson, M. Impedance plane display of a reaction with an adsorbed intermediate. J. Electroanal. Chem. 1972, 39, 81-90. 33. Otomo, J.; Li, X.; Kobayashi, T.; Wen, C. J.; Nagamoto, H.; Takahashi, H. ACimpedance spectroscopy of anodic reactions with adsorbed intermediates: Electro-oxidations of 2-propanol and methanol on carbon-supported Pt catalyst. J. Electroanal. Chem. 2004, 573, 99-109. 34. Melnick, R. E.; Palmore, G. T. R. Impedance spectroscopy of the electro-oxidation of methanol on polished polycrystalline platinum. J. Phys. Chem. B 2001, 105, 1012-1025. 35. Danaee, I.; Jafarian, M.; Forouzandeh, F.; Gobal, F.; Mahjani, M. G. Impedance spectroscopy analysis of glucose electro-oxidation on Ni-modified glassy carbon electrode. Electrochim. Acta 2008, 53, 6602-6609. 36. Melnick, R. E.; Palmore, G. T. R. Time-dependent impedance of the electrooxidation of methanol on polished polycrystalline platinum. J. Phys. Chem. B 2001, 105, 9449-9457. 37. Liu, Z.; Ling, X. Y.; Su, X.; Lee, J. Y. Carbon-supported Pt and PtRu nanoparticles as catalysts for a direct methanol fuel cell. J. Phys. Chem. B 2004, 108, 8234-8240. 38. Angerstein-kozlowsk, H.; Conway, B. E.; Sharp, W. B. A. The real condition of electrochemically oxidized platinum surfaces. Part I. Resolution of component processes. J. Electroanl. Chem. 1973, 43, 9-36. 39. Conway, B. E. Electrochemical oxide film formation at noble metals as a surfacechemical process. Prog. Surf. Sci. 1995, 49, 331-452. 40. Herrero, E.; Franaszczuk, K.; Wieckowski, A. Electrochemistry of methanol at low


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Figure 1. Cyclic voltammograms of methanol oxidation (Black: Pt/C, Red: PtRu/C)


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Figure 2. Experimental scheme of MOR (a) typical direction, (b) reverse direction and MOR CV results of (c) Pt/C and (d) PtRu/C (Black line: typical direction, Red dashed line: reverse direction).



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Figure 3. Cyclic voltammograms of COad oxidation: (a) Pt/C and (b) PtRu/C


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Figure 4. (a) Potential vs current curves of methanol oxidation on Pt/C (b) Nyquist and (c) phase-frequency plots of region (ⅰ)



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Figure 5. (a) Nyquist and (b) phase-frequency plots of region (ⅱ) (c) Nyquist and (d) phase-

frequency plots of region (ⅲ)


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Figure 6. (a) Nyquist plots of region (ⅳ) and (b) Nyquist plots of region (ⅴ)

and (c) phase-frequency plots of region (ⅳ&ⅴ)



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Figure 7. Methanol oxidation, CV in Ar and oxygen reduction reaction of Pt/C with different anodic limit voltage (methanol oxidation and CV was measured 50mVs-1 and oxygen reduction was measured 10mVs-1)


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Figure 8. Schematic illustration of methanol oxidation at different potentials and the equivalent circuit according to electrochemical impedance spectroscopy



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TOC


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Methanol Electro-Oxidation on the Pt Surface: Revisiting the Cyclic

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