Pt Nanoparticles Densely Coated on SnO2-Covered Multiwalled

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Pt Nanoparticles Densely Coated on SnO-Covered Multi-walled Carbon Nanotubes with Excellent Electrocatalytic Activity and Stability for Methanol Oxidation Meihua Huang, Jianshuo Zhang, Chuxin Wu, and Lunhui Guan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07866 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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ACS Applied Materials & Interfaces

Pt Nanoparticles Densely Coated on SnO2Covered Multi-walled Carbon Nanotubes with Excellent Electrocatalytic Activity and Stability for Methanol Oxidation Meihua Huang,a,bJianshuo Zhanga,b, Chuxin Wua,b, and Lunhui Guan*a,b a

CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of

Research on the Structure of Matter, Chinese Academy of Sciences. Fuzhou 350108, China. b

Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences. Fuzhou 350108, China. KEYWORDS:

Pt

electrocatalysts;

SnO2;

Carbon

nanotubes;

Methanol

oxidation;

Electrochemical stability

ABSTRACT: A new electrocatalyst exhibit enhanced activity and stability is designed from SnO2-coverd multi-walled carbon nanotubes coated with 85wt% ratio Pt nanoparticles (MWCNTs@SnO2@Pt). This catalyst showed a mass activity 6.2 times as active as that of the commercial Pt/C Pt for methanol oxidation, owning to the unique one dimensional structure. Moreover, the durability and anti-poisoning ability were also improved greatly. The enhanced

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intrinsic performance was ascribed to the densely connected networks of Pt NPs on the SnO2 NPs.

1. INTRODUCTION Low mass activity, poor stability and high cost of pure Pt catalysts predominantly hindered the large-scale utilization of proton exchange membrane fuel cells(PEMFCs).1 Until now, the most commonly used anode and cathode catalysts are Pt NPs with 2–5 nm in diameters supported on active carbon. However, the aggregation of Pt NPs due to the Ostwald ripening of Pt NPs, together with the electrochemical corrosion of carbon, lead to a significant loss of the electrochemical active surface areas (ECASAs) of Pt, and finally fast reduction in power output.2 Thus, how to increase the activity and stability of Pt-based catalysts has received widespread researchattention.3-7 One of the successful approaches to reach this goal for methanol oxidation reaction (MOR) is to design one-dimensional (1D) Pt-based nanostructured catalysts with special structural characteristics, i.e., abundant surface defects, and fast electron transport.8-11

Over the past decades, some 1D Pt-based nanostructures have been synthesized by wet chemical strategy,12 template directed approach (such as anodized aluminum oxide,13 polycarbonate membrane,14 inorganic nanowires15 and elongated soft materials16), modified phase-transfer method17 and so on18. The typically Pt nanostructures synthesized were Pt or Ptbased alloy, and the lengths were usually from 100 nanometers to several micrometers and wall thicknesses from several nanometers to several tens of nanometers, resulting in low mass activity, poor CO anti-poisoning ability and general catalytic durability.19-25 Recent studies indicated that SnO2 could lower the onset potential and increase CO anti-poisoning ability for methanol or ethanol oxidation through the so-called bifunctional mechanism.26-29 Because SnO2


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can produce OH species at the Pt-SnO2 interface sites for the effective removal of surface COads.30 Meanwhile, strong metal-metal oxide interaction resulted in charge transfer from SnO2 to Pt, increasing the local electron density of Pt NPs.31 Herein, we first synthesized SnO2-covered multi-walled carbon nanotubes (MWCNTs@SnO2), then covered their surface with the connected networks of Pt NPs (MWCNTs@SnO2@Pt), dramatically enhancing the performance of Pt catalysts.

Figure 1. Schematic illustration of the synthesis process for 1D cable nanostructure of MWCNTs@SnO2@Pt. Figure 1 illustrates our concept on MWCNTs@SnO2@Pt. Three important features of this new 1D Pt-based nanostructures are highlighted: (1) Higher MOR mass activity and durability was


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achieved due to the well-connected Pt NP framework, which helps to increase the electrical conductivity of the network and reduce the Pt NPs aggregation;; (2) the specific activity, antipoisoning ability and stability of MWCNTs@SnO2@Pt for MOR were greatly improved, owning to the versatile interface formed between the Pt NPs and SnO2; (3) The SnO2-coverd MWCNTs can avoid the electrochemical corrosion of carbon and enhance electrochemically stability of Pt NPs.

2. EXPERIMENTALSECTION

2.1. Preparation of MWCNTs@SnO2@Pt

First, the surface of multi-walled carbon nanotubes (MWCNTs) was functionalized by refluxing them in concentrated nitric acid (68%) at 140°C for 10 h under stirring. The functionalized MWCNTs were washed with deionized H2O until the pH value of the solution was nearly neutral. The obtained products were dried at 70°C in the vacuum. 20 mg of the treated MWCNTs were dispersed into 40mL deionized water containing 25 uLthioglycolic acid and sonicated for about 30 min. 50 uL of concentrated HCl (38%) and 170 mg of SnCl2.2H2O were subsequently added and sonicated for about 30 min. Thereafter, 105 mg of urea was introduced and sonicated for 30 min. The mixture solution was continued stirring for 10 h at 95°C. Then, the mixture solution was filtered, rinsed with deionized H2O and dried at 70°C in the vacuum. The obtain product is denoted as MWCNTs@SnO2. The weight percentage of SnO2 in the MWCNTs@SnO2 is about 79 wt%, obtained from the TGA results (Figure S1).

To prepare MWCNTs@SnO2@Pt, formic acid reduction of H2PtCl6 approach was used to deposit Pt NPs on MWCNTs@SnO2 to form Pt-connected networks by the formic acid reduction


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of H2PtCl6. 35 mg of MWCNT@SnO2, 10 mL of H2O and 55 mL of H2PtCl6-ethylene glycol (about 0.019 mmol mL-1) solution were added to 100 ml of ethylene glycol (EG) and ultrasonically treated for 5h. Then, the mixture was heated in the oil bath at 95oC. 0.5 mLof formic acid and 35 mL of EG were sonicated for 10 min and slowly dropped to the mixture under N2 atmosphere. The mixture was stirred for 12 h and naturally cooled to room temperature. Finally, the MWCNTs@SnO2@Pt was recovered and washed with copious DI water. The product was vacuum dried in an oven at ambient temperature for 12 h. The mass ratio of the Pt in the catalyst was 86 wt%, uncovered by the inductively coupled plasma-mass spectrometry (ICPMS, Ultima2). The loading ratio of Pt NPs on MWCNTs@SnO2 could be varied easily by this method (Figure S2 and S3). It’s noteworthy that lower loading ratio of Pt NPs on MWCNTs@SnO2 made the Pt NP nanotubes and SnO2 NP nanotubes detached off from the MWCNTs and fragmented to nanosheets. So MWCNTs@SnO2 with lower Pt NPs loading ratio was not studied further. Whereas, the product with higher Pt NPs loading ratio of with 86wt% (MWCNTs@SnO2@Pt) was discovered to be stable and selected for further study in this paper. All chemical reagents were with analytical grade.

2.2. Materials Characterizations

The crystal structures of the samples were studied by powder X-ray diffraction (XRD) using a Rigaku diffractometer (Miniflex600). Thermogravimetry analyse (TGA) was taken by the NETZSCH STA449C. The morphology of the obtained products was examined by fieldemission scanning electronmicroscopy (SEM, SU8010). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) images and scanning transmission electron microscopy with energy-dispersive X-rayspectroscopy (STEM-EDS) were obtained by the Tecnai F20. The


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surface electronic information and near surface composition of the catalysts were revealed by Xray photoelectron spectroscopy (XPS, ESCALAB 250Xi).

2.3. Electrochemical Measurements

Electrochemical measurements of MOR were performed on an electrochemical workstation (CHI 660D). The electrochemical measurements were performed at ambient temperature in a threeelectrode system. The counter electrode was a piece of Pt foil and the reference electrode was a mercury sulfate electrode (MMS) for MOR. A glassy-carbon electrode (diameter: 5 mm, area: 0.196 cm2) was chosen as the working electrode. The catalyst ink was prepared by dispersing 2.5 mg of catalyst and 30µL of 5 wt % Nafion solution into 2.0 mL of isopropyl alcohol, followed by untrasonication for 30 min. Glassy carbon electrode spread with 11.7µL of the aforementioned catalyst ink was used as the working electrode. For comparison, the commercial Pt/C (Johnson Matthey, 20 wt% Pt) was used as the benchmark catalyst and the same Pt was dropped on glassy carbon electrode surface for MOR. The electrochemically active surface area (ECASA) was calculated by integrating the hydrogen adsorption charge on the CV acquired at room temperature in nitrogen-saturated 0.5 M H2SO4.32

The calculation methods are shown below:

ECASA = QH/[210×Pt loading]

The charge for QH was obtained in 0.5 M H2SO4 by integrating the peaks between 0.02 and 0.3 V as shown in Figure 6a. The ECASA was also calculated by CO stripping and was compared with the ECASA determined by H-adsorption. The QCO is the charge for the COads oxidation. The 420 µC cm−2 is the assumed charge required for the oxidation of a COads monolayer.


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ECASA = QCO/[420×Pt loading]

The accelerated durability tests (ADTs) were performed at ambient temperature in N2-saturated 0.5 M H2SO4+ 0.5 M CH3OH solutions.

The cyclic potential was set between 0 and 0.5 V

versus MMS at a sweep rate of 100 mV/s for 10000 cycles. For MOR, all solutions were purged with N2 for 30 min to expel dissolved oxygen before the experiments.

3. Results and discussion

Figure 2. SEM images of (a) MWCNTs, (b) MWCNTs@SnO2and (c) MWCNTs@SnO2@Pt.


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Figure 2 illustrates the micro-nanostructure of initial MWCNTs, intermediate product MWCNTs@SnO2, and finial product MWCNTs@SnO2@Pt. Compared with the pristine MWCNTs, the diameters of the prepared MWCNTs@SnO2 increased, indicating that SnO2 NPs fully covered the surface of the MWCNTs. After further coating Pt NPs, the diameters of the MWCNTs@SnO2@Pt continue to increase, providing clear evidence for the successful growth of Pt NPs on the MWCNTs@SnO2. Meanwhile, the length of theMWCNTs@SnO2@Pt is much less than that of the pristine MWCNTs, implying that the MWCNTs were broken to shorter nanotubes after the reaction, which is confirmed by TEM (Figure S4, S5 and S6). The Pt NPs loading ratio on MWCNTs@SnO2 could be easily controlled by this method (Figure S2 and S3). The lower loading ratio of Pt NPs on MWCNTs@SnO2 made the Pt NP nanotubes to separate from each other, preventing them from forming network structures, thus reduce their conductivity and activity. In this study, ~85% loading ratio of Pt NPs on MWCNTs@SnO2 was shown


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Figure 3. (a) and (b) TEM, HR-TEM images of MWCNTs@SnO2; (c) and (d) TEM, and HRTEM images of MWCNTs@SnO2@Pt.


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Figure 4. a) STEM-EDS images of MWCNTs@SnO2@Pt; b-e) EDS elemental mapping images showing the distribution of C, Sn, and Pt throughout the MWCNTs@SnO2@Pt. Figure 3a and 3b are TEM and HR-TEM images of MWCNTs@SnO2. It is observed that SnO2 NPs with diameters of ~3 nm totally covered the surface of MWCNTs. The nanostructure is further proved by EDS image and cross-sectional compositional line-scanning profile of MWCNTs@SnO2 (Figure S7 and S8). Figure 3c and 3d are TEM and HR-TEM images of the MWCNTs@SnO2@Pt. Obviously, Pt NPs with well crystal structures connected with each other, forming networks, as further presented by the STEM-EDS and HRTEM images, and crosssectional compositional line-scanning profile of MWCNTs@SnO2@Pt, respectively (Figure 4, Figure S9 and S10). Pt NPs with diameters of ~5 nm totally covered the surface of SnO2 NPs.


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Due to the high loading ratio, the Pt NPs connected with each other. The EDS and Inductive Coupled Plasma Emission Spectrometer (ICP) results both indicated that the weight percentage of Pt, SnO2 and C in the MWCNTs@SnO2@Pt were 85%, 10%, 5%, respectively.

Figure 5. a) XRD patterns of different catalysts. b) Pt4f XPS spectra of MWCNTs@SnO2@Pt and Pt/C. Figure 5a shows the XRD patterns of the commercial Pt/C (Johnson Matthey, 20wt%), MWCNTs@SnO2 and MWCNTs@SnO2@Pt. The diffraction angles at 2θ = 26.3◦, 33.6◦, 51.8◦ and 65.2 ◦ are the (110), (101), (211), and (301) planes of SnO2 in the MWCNTs@SnO2, respectively (based on the JCPDS card no.41-1445).33 In the MWCNTs@SnO2@Pt, the broad peak near 26°-35°originates from the graphitic carbons of the MWCNTs and SnO2. The diffraction peaks of the MWCNTs@SnO2@Pt at about 39.7°, 46.4°, 67.8° and 81.7°can be assigned to Pt(111), Pt(200), Pt(220) and Pt(311) crystalline planes of face-centered-cubic (fcc) Pt.

The

average

crystalline

sizes

of

commercial

Pt/C,

MWCNTs@SnO2

and

MWCNTs@SnO2@Pt are 3.5, 3.8 and 5.1 nm, respectively, roughly estimated from the peak widths using the Scherrer formula:


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D = Kλ/βcosθ where K is shape factor with a value close to 0.89, λis the wavelength of the X-ray source (1.54056 Å for Cu Ka radiation), β is the full width at half height of the diffraction peak (in radians), and θ is the Bragg angle corresponding to the peak maximum.34 The results agree with the TEM results. The larger diameter of Pt NPs in the MWCNTs@SnO2@Pt is due to the continuous reduction from a large amount of Pt ions. This is disadvantageous for high-efficient utilization of Pt atoms. To decrease the sizes of Pt NPs and increase the utilization of Pt, the new synthesis methods need to be developed in the further studies. The electronic structure of the catalyst is also investigated by the XPS (Figure 5b).A high metallic Pt content in the MWCNTs@SnO2@Pt could be revealed by the XPS measurement. Compared to Pt/C, these Pt 4f peaks of MWCNTs@SnO2@Pt are significantly shifted to lower binding energies about 0.6 eV. Previous studies showed that the Pt 4f peaks of MWCNTs coated only with Pt nanoparticles are much sharper and slightly downshifted compared to that of the commercial Pt/C.25 The results indicates a strong metal-metal oxide interaction and a partial electron transfer from SnO2 to Pt.35-37 The electron transfer may change the electronic structure of the surface Pt atoms in the MWCNTs@SnO2@Pt and weaken the binding energy of the poisonous intermediates, which strongly adsorbed on the surface Pt atoms of MWCNTs@SnO2@Pt. In addition to the electronic modification, the SnO2 NPs in the MWCNTs@SnO2@Pt might also activate water, producing OHads to oxidize CO-like intermediates adsorbed at adjacent Pt atoms.38


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Figure 6. a) CVs recorded with a sweep rate of 50 mV s-1 in 0.5 M H2SO4 solutionpurged with N2. b) Mass activity and c) Specific activity of the catalysts in 0.5 M CH3OH + 0.5 M H2SO4 with a sweep rate of 50 mV s-1 d) and e) Current–time curves at 0.22 V and 0.09 V in 0.5 M CH3OH + 0.5 M H2SO4, respectively. f) Electrochemical CO-stripping curves of different catalysts.


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The MWCNTs@SnO2@Pt with this unique structure presented highly mass activity and catalytic durability for MOR. The ECASAs of the catalysts were obtained by measuring hydrogen desorption area after the double-layer region. Herein, the ECASAs of MWCNTs@SnO2@Pt and Pt/C are 97.3 and 71.7 m2g−1, respectively (Figure 6a). The results imply that the densely connected Pt NPs did not decrease the surface area of Pt. Cyclic voltammetey (CV), were applied to evaluate the activity and stability of the electrocatalysts, and the relative results are normalized by the Pt loading mass and the ECASAs of the catalysts. The electrocatalytic performance of MWCNTs@SnO2@Pt toward MOR was evaluated with a sweep rate of 50 mV s−1 in 0.5 M H2SO4+ 0.5 M CH3OH (Figure 6b). Compared with that of the Pt/C, the MWCNTs@SnO2@Pt exhibits a very high mass peak current density of 1701.6A gPt-1, which is almost 6.2 times higher than that of the commercial Pt/C. The specific peak current density of the MWCNTs@SnO2@Pt is 1.75 mA cmPt-2, which is 4.6 times greater than that of the Pt/C (Figure 6c). Furthermore, an obviously lower onset potential was observed with the MWCNTs@SnO2@Pt, showing that the MOR on the MWCNTs@SnO2@Pt is much easier. Here, we use the peak current ratio of the forward scan (If) to backward scan (Ib) current ratio , If/Ib, represent the anti-poisoning ability of carbonaceous species.39 The If/Ib of the MWCNTs@SnO2@Pt is 0.74, which is a little higher than that of Pt/C (0.67). This result represents an example with superior performance to previously reported Ptbased catalyst for MOR (Table S1).40-45 Chronoamperometry was further applied to investigate the electrocatalytic activity and stability of the MWCNTs@SnO2@Pt at 0.22 V and 0.09 V vs. Hg/HgSO4 (MMS) in 0.5 M H2SO4 + 0.5 M CH3OH. As shown in Figure 6d-e, the chronoamperometry had high initial currents, that dropped quickly at the pristine stage due to the concentration gradient and poison of the


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intermediate species.44 The steady current density for the MWCNTs@SnO2@Pt was higher than that of Pt/C throughout the entire time range. A mass activity of 923.2 A gPt-1, which is 47.9 times higher than that of the Pt/C, was achieved at the end point with the MWCNTs@SnO2@Pt. This value changed to 544.2A gPt-1 at a different voltage of 0.09V, still 5.2 times higher than that of the Pt/C (Figure 6e). The long-term poisoning rate of the MWCNTs@SnO2@Pt and the Pt/C was 0.015 and 0.047% s-1 at 0.22 V, respectively. By measuring the linear decay of the current, the stability was determined to be 0.01and 0.038% s-1 at 0.09 V, respectively. Clearly, the current density on the MWCNTs@SnO2@Pt decayed much slower, indicating that the MWCNTs@SnO2@Pt has a prominent higher anti-poisoning ability to the CO-like intermediate species generated during MOR than the Pt/C. The enhancement of MOR for the MWCNTs@SnO2@Pt was due to connected Pt NPs and modified electron structure of Pt by SnO2 NPs. Table 1. The EACSAs of MWCNTs@SnO2@Pt and Pt/C are calculated by both H adsorption and CO stripping. Here, the ECASAH is calculated by H adsorption. The ECASACO is calculated by CO stripping. The peak potential and onset potential for CO oxidation are vs. MMS. MWCNTs@SnO2@Pt Peak potential (mV)

Onset potential (mV)

Pt/C Peak potential (mV)

Onset potential (mV)

30

-400

180

0

ECASAH (m2 g-1Pt)

ECASACO (m2 g-1Pt)

ECASAH (m2 g-1Pt)

ECASACO (m2 g-1Pt)

97.3

89.6

71.7

75.1


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The anti-poisoning performance of the MWCNTs@SnO2@Pt was further explored by CO stripping experiments. Shown in Figure 6f and Table 1, the peak potentials of the MWCNTs@SnO2@Pt and the Pt/C for CO oxidation are 30, and 180 mV. The CO oxidation onset potential of the MWCNTs@SnO2@Pt was 400 mV negatively, compared with that of the Pt/C. As a result, the MWCNTs@SnO2@Pt shows a higher CO oxidation activity versus the Pt/C. The ECASA derived from CO stripping (ECO) was 89.6 and 75.1 m2 g-1for MWCNTs@SnO2@Pt

and

Pt/C

are

89.6

and,

respectively.

The

stability

of

the

MWCNTs@SnO2@Pt and the Pt/C for MOR was further studied using accelerated durability tests (ADTs), since the electrocatalyst decay at the electrode remains a critical issue for PEMFC applications (Figure 7).. After 6000 and 10000 cycles, the peak current densities of the MWCNTs@SnO2@Pt were about 81.0% and 71 % of its initial value (Figure 7a). Whereas, the peak current density of the Pt/C was about 39.5% of its initial value after 6000 cycles, (Figure 7b). Morphology study showed that the 1D connected structures of MWCNTs@SnO2@Pt after ADTs was still maintained (Figure 7c), but serious aggregation was observed in the Pt/C (Figure 7d). Therefore, the 1D cable feature of connected Pt NPs coated on SnO2-covered MWCNTs is the major contributor to the high durability of the MWCNTs@SnO2@Pt for MOR.


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Figure 7 a) The changes on mass activitiesof MWCNTs@SnO2@Pt before and after CVs. b) The changes on mass activities of Pt/C before and after 6k CVs. c)Comparison of TEM images of the MWCNTs@SnO2@Pt after10k CVs. d) Comparison of TEM images of the Pt/C after 6k CVs.


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4. CONCLUSIONS In summary, Pt NPs coated on SnO2-covered MWCNTs were designed and fabricated. Compared with the Pt/C, the MWCNTs@SnO2@Pt exhibits a significantly enhanced mass activity, cycling stability and anti CO poisoning for the MOR. Here, catalysis enhancement factors are integrated: (1) the introduction of SnO2 as a co-catalyst provide abundant OHads species, which can remove CO-like adsorbed poison species on Pt surface; (2)The Pt NP networks and SnO2 produce strong synergistic effects for the MOR; (3) the electronic state of Pt is modified by SnO2, which is beneficial for the MOR. The hybridization method based on the SnO2 and 1D cable structure will provide a new route for fabricating electrocatalysts with high activity and stability for PEMFC applications. ASSOCIATED CONTENT Supporting Information. Additional details are available, including the representative TEM, HRTEM, STEM-EDS images, and relevant electrochemical results for all of the samples analyzed, including control samples and references. AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This research was supported by NSF for Distinguished Young Scholars of Fujian Province (Grant no. 2017J07004), the Science and Technology Planning Project of Fujian Province (Grant No. 2014H2008) and the strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA09010402). REFERENCES (1) Wu, J. B.; and Yang, H. Platinum-Based Oxygen Reduction Catalysts. Acc. Chem. Res. 2013, 46, 1848-1857. (2) Yu, X. W.; Ye, S. Y. Recent Advances in Activity and Durability Enhancement of Pt/C Catalytic Cathode in PEMFC: Part I. Physico-Chemical and Electronic Interaction Between Pt and Carbon Support, and Activity Enhancement of Pt/C Catalyst. J. Power Sources 2007, 172, 133-144. (3) Xia, B. Y.; Wu, H. B.; Li, N.; Yan, Y.; (David) Lou, X. W.; and Wang, X. One-Pot Synthesis of Pt–Co Alloy Nanowire Assemblies with Tunable Composition and Enhanced Electrocatalytic Properties. Angew. Chem. Int. Ed. 2015, 54, 3797-3801. (4) Jiang, B.; Li, C. L.; Malgras, V.; Imura, M.; Tominaka, S.; and Yamauchi, Y. Mesoporous Pt Nanospheres with Designed Pore Surface as Highly Active Electrocatalyst. Chem. Sci. 2016, 7, 1575-1581. (5) Ataee-Esfahani, H.; Imura, M.; Yamauchi, Y. All-Metal Mesoporous Nanocolloids: SolutionPhase Synthesis of Core-Shell Pd@Pt Nanoparticles with a Designed Concave Surface. Angew. Chem. Int. Ed. 2013, 52, 13611-13615.


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Table of Contents


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Pt Nanoparticles Densely Coated on SnO2-Covered Multiwalled

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