Mesoporous Nanotubes as Bifunctional Electrocatalysts for Oxygen

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PtM (M = Co, Ni) Mesoporous Nanotubes as Bifunctional Electrocatalysts for Oxygen Reduction and Methanol Oxidation Shuli Yin, Ziqiang Wang, Xiaoqian Qian, Dandan Yang, You Xu, Xiaonian Li, Liang Wang, and Hongjing Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00872 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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ACS Sustainable Chemistry & Engineering

PtM (M = Co, Ni) Mesoporous Nanotubes as Bifunctional Electrocatalysts for Oxygen Reduction and Methanol Oxidation Shuli Yin,‡ Ziqiang Wang,‡ Xiaoqian Qian, Dandan Yang, You Xu, Xiaonian Li, Liang Wang,* and Hongjing Wang*

State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China

*Corresponding

‡Shuli

authors’ E-mails: [email protected]; [email protected]

Yin and Ziqiang Wang equally contributed to this work.

ACS Paragon Plus Environment

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ABSTRACT: Tailoring the morphology and composition of Pt-based nanostructures is crucial to design high-performance electrocatalysts for direct methanol fuel cells. In this work, we propose a facile strategy to fabricate one-dimensional PtM (M = Co, Ni) mesoporous nanotubes (PtM MNTs), in which F127 and Te nanowires facilitate the formation of mesoporous exteriors and cannular interiors, respectively. Owing to the structural advantages of bimetallic mesoporous nanotube architectonics, the PtM MNTs exhibit superior electrochemical performance for the oxygen reduction reaction and methanol oxidation reaction. The proposed dual-template strategy is highly promising for designing active Pt-based mesoporous nanotubes with desired composition toward various electrocatalytic fields.

KEYWORDS: Pt-based catalyst; bimetallic alloy; mesoporous structure; nanotube; bifunctional electrocatalyst


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INTRODUCTION

The excessive consumption of fossil fuel and resultant environmental damage have greatly boosted the development of sustainable energy storage and conversion devices. Among them, direct methanol fuel cells (DMFCs) have attracted intensive attention due to their low operation temperature, high energy conversion efficiency, low cost, and easy storage and transfer of liquid methanol.1−5 A crucial issue for large-scale applications of DMFCs is to develop highly active, stable, and cost-efficient electrocatalysts for the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR). Currently, Pt-based nanomaterial is still the most active electrocatalyst for the ORR and MOR.6−15 However, the high cost, easy aggregation, and poor stability of Pt heavily hinder its commercial application in DMFCs. As such, it is highly desired to explore cost-effective Pt-based catalysts with high activity and excellent durability for DMFCs. To this end, the incorporation of transition metals into Pt is a promising strategy for reducing Pt usage and enhancing the catalytic performance.16−22 Bimetallic structure


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can cause electronic effect and strain effect, which promote the downshift of the d-band center of Pt and narrow the Pt–Pt bond distance,23,24 respectively. Consequently, the adsorption of oxygenated species on the catalyst surface is weakened, resulting in enhanced catalytic activity.25−27 Moreover, the introduced transition metals (e.g., Fe, Co, Ni, and Cu) can effectively inhibit the adsorption of produced CO on Pt,28−31 improving the long-term durability of Pt-based catalysts. Therefore, control synthesis of Pt-based alloys is an effective approach to prepare high-performance and cost-effective catalysts toward the ORR and MOR. It has been demonstrated the shape-dependent effect of catalysts on electrocatalytic performance. Among various structures, one-dimensional (1D) nanotubes (NTs) have been intensively investigated, which can facilitate mass/charge transfer, extend service life, restrain mechanical degradation, and tolerate volume expansion, favoring their electrocatalytic applications.32−37 Electrodeposition method was reported to synthesize Pt, Pd, Au, and Ag noble metal nanotubes in dimethyl sulfoxide solvent.38,39 However, it is difficult to develop well-defined nanotube structure due to the difficulty in controlling the nucleation and growth. In addition, sacrificial template is a flexible and effective


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strategy to synthesize nanotubes. Until now, Te, Cu, Ag, and Se nanowires (NWs) as self-sacrificial templates have been employed to fabricate tubular nanostructures by galvanic replacement reaction.40−44 Among them, the Te NWs are considered as the promising templates due to their high reactivity, long-term stability, low cost, and simple synthesis.34,40,43,45 Well-developed Pt and Pd nanotubes were synthesized using Te NWs as a self-sacrificial template via galvanic replacement reaction.34 Nevertheless, these 1D materials lack sufficient porous structures on the surface, which limit their wide application for DMFCs. The combination of nanotube geometry with porous structure can achieve both high surface-to-volume ratio and sufficient active sites, which is highly attractive in designing effective electrocatalysts for DMFCs. Herein, we demonstrate a universal method to prepare PtM (M = Co, Ni) mesoporous nanotubes (MNTs). The PtM MNTs are prepared by a dual-template method, in which the Te NWs behave as the self-sacrificial templates to form the nanotubes and the surfactant F127 micelles serve as the pore-making agent to direct the formation of mesoporous structures. Benefiting from the bimetallic structure, rich mesoporosity, and


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cannular morphology, the PtM MNTs show superior electrocatalytic performance for the ORR and MOR. EXPERIMENTAL SECTION Materials and Chemicals. Na2TeO3, NH3·H2O (25%−28%), N2H4·H2O (98%), polyvinylpyrrolidone (PVP, Mw = 58000), ascorbic acid (AA), CoCl2·6H2O, NiCl2·6H2O, acetone, KOH, and concentrated HCl were purchased from Aladdin. Pluronic F127 (PEO-PPO-PEO), Brij58, K2PtCl4, RuCl3, and Nafion solution (5 wt%) were bought from Sigma-Aldrich. Commercial Pt/C (20 wt% Pt) was obtained from Alfa Aesar. Synthesis of Te Nanowires (Te NWs). The synthesis of Te NWs was based on previous report with minor modification.46 Typically, PVP (1 g) and Na2TeO3 (0.092 g) were dissolved in H2O (35.22 mL) with violent stirring, followed by adding NH3·H2O (3.35 mL) and N2H4·H2O (1.43 mL). Then the solution was transferred to a Teflon-lined stainless autoclave, which was sealed and maintained at 180 °C for 3 h. Finally, the production was mixed with acetone, and collected by centrifugation/washing cycles three times with water. The obtained Te NWs were dissolved in H2O (15 mL) for further use.


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Synthesis of PtM MNTs. In a typical synthesis of PtCo MNTs, F127 (20 mg), Te NWs (0.2 mL), K2PtCl4 (1.5 mL, 20 mM), CoCl2 (0.5 mL, 20 mM), and AA (2 mL, 0.1 M) were mixed and reacted for 2 h under slow stirring. After that, KOH (0.4 g) was added to the solution and maintained for 5 h to etch the residual Te NWs. Finally, PtCo MNTs were obtained by centrifugation at 5000 rpm for 10 min and washed with water three times. For comparison, the PtNi MNTs were also synthesized in the same conditions except for using NiCl2 solution instead of CoCl2 solution. Characterization. A Hitachi S-4800 was used to carry out scanning electron microscope (SEM) image at 5 kV. A JEOL-2100F was used to conducted transmission electron microscopy (TEM) image, high-resolution TEM (HRTEM) image, energydispersive X-ray spectroscopy (EDS) and element mapping image at 200 kV. A Rigaku Miniflex 600 was used to conduct X-ray diffraction (XRD) pattern at 15 mA and 40 kV. Electrochemical Measurements. The electrochemical measurements were conducted on a rotating ring-disk electrode (RRDE) rotator (ALS Co., Japan) connected to a CHI 852D electrochemical analyzer. Ag/AgCl (saturated KCl) and Pt wire were used as a reference electrode and counter electrode, respectively. Each catalyst was dispersed in


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mixed solution with the concentration of 2 mg mL−1, and each ink was coated on the working electrode with the same loading of Pt (2 μg). After drying, 2 μL of Nafion (0.5 wt%) was added to the electrode surface and dried at room temperature. Cyclic voltammograms (CVs) were performed in N2-saturated H2SO4 (0.5 M) solution at a scan rate of 50 mV s−1. The CVs were employed to calculate the electrochemical active surface areas (ECSAs) from the equation 1:

ECSA=

Q m×210

(1)

where Q (C) is the charge in the Hupd adsorption/desorption area obtained after the double layer correction, m (mg) is the mass of Pt, and 210 (µC cm−2) is the charge required for monolayer adsorption of hydrogen on Pt surface. In order to evaluate ORR performance, linear sweep voltammograms (LSVs) were measured in O2-saturated HClO4 (0.1 M) solution at a scan rate of 5 mV s−1. The electron transfer number (n) can be obtained by calculating the slope of the KouteckLevich (K-L) plots according to K-L equation 2 and 3:

1 1 1 = + 𝑗 𝑗𝑘 𝑗𝑑

(2)


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jd = 0.2nFD2/3v-1/6ω1/2 CO2

(3)

where j, jk, and jd are the measured, kinetic, and diffusion currents, respectively, and the F, D, v, ω, and CO2 are the Faraday constant (96485 C mol−1), diffusion coefficient of O2 (1.93 × 10−5 cm2 s−1), kinetic viscosity (1.13 × 10−2 cm2 s−1), rotation speed of electrode (rpm), and bulk concentration of O2 dissolved in the electrolyte (1.22 × 10−3 mol L−1), respectively. Moreover, the value of n also can be estimated by RRDE measurements according to equation 4. The peroxide percentage (%H2O2) was calculated from equation 5.

where N is the RRDE collection efficiency of the Pt ring (0.4286), ID is disk current, and

IR is ring current. RESULTS AND DISCUSSION A facile dual-template strategy is employed to fabricate bimetallic PtM MNTs in an aqueous solution at room temperature (Scheme 1). In the synthetic process, the Te


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NWs serve as a self-sacrificial template to direct the formation of the PtM nanotube structure via galvanic replacement reaction. The F127 is used as a soft template to facilitate the formation of mesoporous structure on the nanotube wall. The produced Te NWs display high aspect ratio and uniform size (Figure S1), which is beneficial to produce high-quality PtM nanotubes. We can see from the SEM image that uniform PtCo MNTs with an average diameter of 100 nm are obtained in a high yield (Figure 1a). The nanotube wall of the PtCo MNTs is consisted of continuous mesopores with a pore size of approximately 20 nm (Figure 1b). It is noted that nanotube structure can be clearly observed from the cross section of SEM image (inset of Figure 1b). TEM image of PtCo MNTs further confirms the formation of well-defined mesoporous structure and nanotube structure (Figure 1c). The selected-area electron diffraction (SAED) pattern exhibits ring-like pattern that can be assigned to (111), (200), (220), and (311) facets of metallic face-centered cubic (fcc) structure, indicative of the polycrystalline structure of the PtCo MNTs (Figure 1d). As shown in Figure 1e and 1f, clear lattice fringes with a distance of 0.22 nm at the edge of the PtCo MNTs can be observed, which can be indexed to (111) plane of fcc PtCo nanocrystals. In order to further verify the crystalline


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structure of the PtCo MNTs, XRD pattern is carried out (Figure S2). Four peaks at 40.3°, 46.8°, 67.9°, and 81.8° are attributed to the (111), (200), (220), and (311) planes of fcc metal structure, respectively, which is consistent with the SAED results. Due to the existence of Co, these diffraction peaks have a slight shift compared with the standard Pt peaks (JCPDS No. 04-0802). The structure and component of PtCo MNTs are also confirmed by the high-angle annular dark-field scanning TEM (HAADF-STEM) and corresponding elemental mapping images (Figure 2). Abundant continuous mesopores (~20 nm in diameter) are distributed uniformly on the nanotube wall, and the nanotube structure can be indicated from the different contrast of surrounding and center (Figure 2a). The elemental mapping images confirm the uniform distribution of Pt and Co elements in the product (Figure 2b-d). Above results support the successful synthesis of bimetallic PtCo MNTs. The synthetic method is universal and can be used to prepare PtNi MNTs by only replacing CoCl2 with NiCl2. Similar to PtCo MNTs, the PtNi MNTs also possess tubular structure (~100 nm in diameter) with uniform mesoporous structure (~20 nm in pore size) on the surface (Figure S3a-c). The SAED of the PtNi MNTs indicates the polycrystalline


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structure (Figure S3d). At the edge of the nanotube wall, HRTEM image clearly displays the distinct lattice fringes with a d-spacing of 0.22 nm, which are assigned to (111) plane of fcc metallic crystals (Figure S3e and 3f). Moreover, the XRD pattern and elemental mapping images further demonstrate the formation of the bimetallic PtNi MNTs (Figure S4 and S5). In order to investigate the effect of the precursor ratio on the morphology of PtM MNTs, a series of control experiments are carried out. For the PtCo MNTs, welldeveloped PtCo MNTs with uniform morphology are formed when the ratio of Pt/Co is 3/1. With the decrease of the Pt concentration (the ratio of Pt/Co is 2/2), the nanotube diameter and mesoporous size decreases (Figure 3a). When the concentration of Pt further decreases (the ratio of Pt/Co is 1/3), only a very thin 1D structure are obtained, and no mesoporous structure can be observed (Figure 3b). For the PtNi MNTs, there is the same trend (Figure S6). This phenomenon can be explained by the following reasons. Due to the much lower reduction potential of the M2+/M0 than that of the PtCl42−/Pt0, the reaction between MCl2 and Te NWs proceeds slowly, resulting in partial reduction. Thus, with reducing the amount of the Pt amount, the nanotubes gradually


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become thinner. Moreover, Pt precursors are easy to coordinate with water to form metal-aqua complexes, which can interact with hydrophilic PEO groups in F127 to form crown-ethers-like conformation.47,48 After reduction, the Pluronic chains adsorbed on the Pt surface could form cavities and then facilitate the formation of mesoporous structures. However, no mesoporous structure is observe on the surface of PtRuTe nanotubes by replacing the F127 by Brij 58 (Figure S7). When the Pt concentration is too low, there is a lack of sufficient micelles to form porosity. According to the above results, the structure and thickness of mesoporous layers can be tailored by changing the ratio of the Pt precursors and metal precursors. These results show that the optimal ratio of Pt/Co for synthesizing PtCo MNTs is 3/1. According to the above results, we can predict the synthetic process of the PtM MNTs. Firstly, Te NWs as self-sacrificial templates can react with the metal precursors via the galvanic replacement reaction due to lower reduction potential of Te. During this process, the PtM nanoshell grows inward to form the nanotube structure by consuming the Te template. Simultaneously, Pt coordinated water molecules can combine with hydrophilic PEO groups to form spherical micelles, which can stabilize the precursor


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clusters and disperse the reduced metal nanoparticles well. The mesoporous structure is produced after washing F127 out.49 As a reducing agent, AA is introduced to ensure the complete reduction of metal precursor and the formation of desired mesoporous structure. Without adding AA, no mesoporous structure can be obtained for the PtCo MNTs (Figure S8). After etching the remaining Te, the well-developed PtM MNTs with the mesoporous exteriors and cannular interiors are formed. Inspired by the attractive mesoporous bimetallic nanotube architecture, the PtM MNTs were measured as promising catalysts for the ORR and MOR. The ORR performance of the PtM MNTs is evaluated by comparing with commercial Pt/C catalyst. The ECSAs of the three catalysts can be determined by the hydrogen desorption peaks of CVs in 0.5 M H2SO4 aqueous solution (Figure S9). The ECSAs of the PtCo MNTs and PtNi MNTs are 49.7 and 48.5 m2 g−1, respectively, which are higher than that of Pt/C (43.6 m2 g−1). The result indicates that the 1D mesoporous tubular structure can offer higher specific surface area and expose more active sites. The LSVs of different catalysts are measured to evaluate ORR activity in O2-saturated 0.1 M HClO4 (Figure 4a). The limiting current densities of the PtCo MNTs and PtNi MNTs are 5.4 and 5.3 mA


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cm−2, respectively, which are higher than that of Pt/C (5.1 mA cm−2). The Tafel slopes calculated by LSVs can represent the ORR kinetics, which are measured to 62, 61, and 64 mV dec−1 for PtCo MNTs, PtNi MNT, and Pt/C, respectively (Figure 4b). The lower Tafel slopes of the PtM MNTs reveal that the rate-determining step is the first electron step. The onset potential (Eonset) and half wave potential (E1/2) of the PtCo MNTs are 1.01 and 0.91 V, respectively. The Eonset and E1/2 of PtNi MNTs are 1.00 and 0.90 V, respectively. The Eonset and E1/2 of PtCo MNTs and PtNi MNTs are more positive than those of Pt/C (0.94 and 0.88 V) (Figure 4c) and most previously reported Pt-based catalysts (Table S1). The above results confirm the superior ORR activity of the PtM MNTs. The ECSA-normalized specific activities of the PtCo MNTs and PtNi MNTs are 0.99 and 0.89 mA cm−2, which are 4.3 and 3.9 times that of Pt/C (0.23 mA cm−2) (Figure 4d), respectively, demonstrating the excellent catalytic performance. The LSVs of the PtM MNTs (Figure S10 and S11) and Pt/C (Figure S12) at different rotating speeds and corresponding electron transfer number (n) at different potentials (0.5, 0.6, 0.7, and 0.8 V) are used to further analyze the ORR kinetics. According to the K-L equation, the K-L plots with good linear relation reveal the one-step reaction of ORR


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kinetics for these samples. The n values of the PtCo MNTs (3.96, 3.95, 3.97, and 3.99), PtNi MNTs (3.97, 3.99, 3.95, and 3.94), and Pt/C (3.94, 3.92, 3.94, and 3.98) are calculated from the K-L slopes, and the results reveal the four-electron ORR pathway for all samples. Furthermore, the RRDE tests were performed to investigate the selectivity of the samples (Figure 4e). The ring current (IR) and disk current (ID) are used to determine the n and percentages of H2O2. Based on the equation 4, the n values of the PtCo MNTs, PtNi MNTs, and Pt/C are calculated to be 3.98, 3.98 and 3.94, respectively (Figure 4f), corresponding with the LSV analysis. The percentages of H2O2 for the PtCo MNTs, PtNi MNTs, and Pt/C are calculated from the equation 5, which achieve 0.7%, 0.7%, and 3.4%, respectively. The higher n and lower percentages of H2O2 indicate that the PtM MNTs have higher four-electron selectivity for the ORR. Stability of the catalysts is a crucial parameter for the ORR, thus accelerated durability tests of the samples are performed. After 5000 cycles, the LSVs of the PtCo MNTs (Figure 5a) and PtNi MNTs (Figure 5b) show a slight decrease in Eonset and E1/2 compared with the initial one. Unfortunately, the Eonset and E1/2 of Pt/C have obvious drop after 5000 cycles (Figure 5c). Moreover, chronoamperometric test is carried out to


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investigate the stability of the three catalysts. After stability test at 0.5 V for 5 h, the current densities of the PtCo MNTs, PtNi MNTs, and Pt/C decrease by 12.6%, 13.8%, and 40.5% (Figure 5d), respectively, indicative of the strong long-term durability of the PtM MNTs for the ORR. The MOR catalytic performance for the PtCo MNTs and PtNi MNTs is evaluated by CVs in 0.5 M H2SO4 containing CH3OH (1.0 M) at a scan rate of 50 mV s−1 (Figure 6a and 6b). The specific activities (normalized to the ECSAs) and mass activities (normalized to the Pt loading) of the PtCo MNTs, PtNi MNTs, and Pt/C are calculated and plotted in Figure 6c. The specific activities of the PtCo MNTs and PtNi MNTs at the peak potentials are 1.92 and 1.86 mA cm−2, which are 3.8 and 3.6 times that of Pt/C (0.51 mA cm−2), respectively. The mass activities of the PtCo MNTs and PtNi MNTs at the peak potentials are 0.95 and 0.90 mA µg−1Pt, which are 4.3 and 4.1 times that of Pt/C (0.22 mA µg−1Pt), respectively. These activities are also superior to those of the previously reported Pt-based catalysts (Table S2). In order to evaluate the stability of the three catalysts for MOR, the chronoamperometry is performed at 0.6 V for 3600 s (Figure 6d). During the MOR process, it can be seen from i-t curves that the current


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densities of the PtCo MNTs and PtNi MNTs are higher than that of Pt/C, indicating that PtCo MNTs and PtNi MNTs possess better stability than Pt/C for the MOR. After the ORR and MOR stability measurements, the morphology and structure of the PtCo MNTs were further investigated by SEM and SEM. As shown in Figure S13, there is no obvious change in the morphology and structure, indicating the excellent stability of the materials. The excellent catalytic performance of PtM MNTs for the ORR and MOR is mainly due to the advantages of the distinct structure and bimetallic component. The electronic structure of this catalyst can be modulated by introducing Co or Ni element to Pt element, which contributes to a great improvement of the catalytic performance. The tubular structure provides low diffusion resistance and high rate of electron transfer, which is beneficial to the electrochemical performance. Moreover, the mesoporous structure can make full use of the interior and exterior surfaces of the material and the inner wall of the channel, which can offer high accessible surface area and sufficient active sites for promoting the occurrence of electrochemical reactions. CONCLUSION


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In summary, well-defined bimetallic PtM (M = Co, Ni) nanotubes with continuous mesopores on the nanotube wall have been successfully fabricated by a dual-template method. The tubular structure is constructed by sacrificing Te NWs template and the mesoporous structure is formed through the surfactant F127 micelles. Different from the previous nanotube structure, the as-synthesized PtM MNTs have rich mesoporous structures on the wall, which provide fast electron transfer and sufficient active sites for electrocatalysis. Benefiting from the bimetallic component, nanotube geometry, and mesoporous structure, the obtained PtM MNTs exhibit superior electrochemical performance for the ORR and MOR. This dual-template strategy is highly valuable for designing active Pt-based bimetallic mesoporous nanotubes toward various catalytic fields. ASSOCIATED CONTENT

Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. SEM images; XRD patterns; TEM images; Elemental mapping images; CVs; polarization curves; Table S1; Table S2.

AUTHOR INFORMATION

Corresponding Author *E-mails: [email protected]; [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was financially supported by the National Natural Science Foundation of China (21601154, 21776255, and 21701141).


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Figures and Figure Captions

Scheme 1. Schematic illustration of the fabrication of the PtM MNT.


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Figure 1. (a, b) SEM images, (c) TEM image, (d) SAED pattern and (e) HRTEM image of the PtCo MNTs. (f) Fast Fourier transformation images of the square area in (e). The inset in (b) shows SEM image of the cross section of the PtCo MNTs.


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Figure 2. (a) HAADF-STEM image and (b−d) elemental mapping images of the PtCo MNTs.


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Figure 3. SEM images of the different samples with different ratios of Pt/Co precursors: (a) 2/2 and (b) 1/3.


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Figure 4. (a) ORR polarization curves, (b) Tafel slopes, (c) the comparison of Eonset and

E1/2, (d) the comparison of the specific activity, (e) RRDE tests of ORR, and (f) peroxide percentages and electron transfer number of the three catalysts under a rotation rate of 1600 rpm.


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Figure 5. ORR polarization curves before and after durability test of the (a) PtCo MNTs, (b) PtNi MNTs, and (c) Pt/C. (d) Chronoamperometric curves at 0.5 V.


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Figure 6. (a) ECSA-normalized and (b) mass-normalized CVs of different catalysts in 0.5 M H2SO4 containing 1.0 M CH3OH at a scan rate of 50 mV s-1. (c) The comparisons of the specific activities and mass activities. (d) Chronoamperometric curves at 0.60 V in 0.5 M H2SO4 containing 1.0 M CH3OH. The currents densities were normalized by the ECSAs.


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For Table of Contents Use Only

Synopsis

PtM (M = Co, Ni) mesoporous nanotubes have been synthesized via a dual-template strategy, exhibiting excellent performance for the ORR and MOR.


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Mesoporous Nanotubes as Bifunctional Electrocatalysts for Oxygen

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