TiO2 nanotubes electrode for electro-oxidation of methanol

Accepted Manuscript Title: A facile method to prepare Pt/C/TiO2 nanotubes electrode for electro-oxidation of methanol Author: Xiaoyu Yue Wenxiu Yang Xiangjian Liu Yizhe Wang Changyu Liu Qingyou Zhang Jianbo Jia PII: DOI: Reference:

S0013-4686(15)01337-7 http://dx.doi.org/doi:10.1016/j.electacta.2015.05.172 EA 25105

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

2-2-2015 15-5-2015 29-5-2015

Please cite this article as: Xiaoyu Yue, Wenxiu Yang, Xiangjian Liu, Yizhe Wang, Changyu Liu, Qingyou Zhang, Jianbo Jia, A facile method to prepare Pt/C/TiO2 nanotubes electrode for electro-oxidation of methanol, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.05.172 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Elsevier Editorial System(tm) for Electrochimica Acta Manuscript Draft Manuscript Number: SGS15-050R3 Title: A facile method to prepare Pt/C/TiO2 nanotubes electrode for electro-oxidation of methanol Article Type: Research Paper Keywords: Pt/C/TiO2 nanotubes electrode; Thermal decomposition; Methanol oxidation; Surface area; Activity Corresponding Author: Prof. Jianbo Jia, Corresponding Author's Institution: Changchun Institute of Applied Chemistry First Author: Xiaoyu Yue Order of Authors: Xiaoyu Yue; Wenxiu Yang; Xiangjian Liu; Yizhe Wang; Changyu Liu; Qingyou Zhang; Jianbo Jia Abstract: In this study, we prepared a Pt/C/TiO2 nanotubes (TiO2NTs) electrode by the facile pyrolysis of chloroplatinic acid and glucose on the TiO2NTs support simultaneously. By several characterization methods such as scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, it was found that with the thermal decomposition of H2PtCl6 and glucose simultaneously, Pt nanoparticles appeared with small particle size and anchored on the surface of TiO2NTs with carbon. Cyclic voltammetry and chronoamperometry were used to evaluate the electrocatalytic activity and stability of the prepared electrodes toward the oxidation of methanol. The Pt/C/TiO2NTs electrode possesses a large electrochemically active surface area and exhibits enhanced electrocatalytic activity and better stability for methanol oxidation than the Pt/TiO2NTs electrode. The Pt/C/TiO2NTs electrode provides a new approach for improving the catalytic activity and stability of Pt nanomaterials in renewable energy applications.

*Response to Reviewers

May 15, 2015 Professor Shi-Gang Sun Associate Editor Electrochimica Acta Dear Prof. Sun, Thank you for your letter concerning our manuscript SGS15-050R2 “A facile method to prepare Pt/C/TiO2 nanotubes electrode for electro-oxidation of methanol”. We thank the reviewers for their kind recommendation and thoughtful comments. We have addressed their comments and revised our manuscript accordingly. The explanations are given below.

Reviewer #1: The resubmitted manuscript was well revised in accordance with the comments. Therefore, this paper is worthy of publication of Electrochimica Acta. Answer: Thanks! We appreciate your valuable comments very much. Reviewer #2: In the revised manuscript, the authors inserted all of the reviewer comments. Now, it is acceptable for publish in Electrochimica Acta. Some points are below: 1. In the Figures 5 the unite of X and Y are not correct. It should be change. "the Z ohm change to ohm.cm2 , as well as Y. Answer: Thank you! We have changed the ohm to ohm.cm2 (the area of our electrode is 1 cm2 during the whole process of the experiment), as given in the following figure.

2. The equivalent circuit of the EIS should be inserted in manuscript. Answer: Thank you! We have inserted the equivalent circuit of the EIS in the revised manuscript as follow. The equivalent circuit includes the electrolyte resistance (Rs), Warburg impedance (Zw), double-layer capacitance (Cd) and the charge-transfer resistance (Rct) [Q. Q. Wang, Z. X. Zhou, Y. L. Zhai, L. L. Zhang, W. Hong, Z. Q. Zhang, S. J. Dong, Label-free aptamer biosensor for thrombin detection based on functionalized graphene nanocomposites, Talanta 141 (2015) 247-252.]

Fig. 5 (a) EIS spectra of the TiO2NTs, Pt/TiO2NTs, and Pt/C/TiO2NTs electrodes in 2.5 mM K3[Fe(CN)6/K4[Fe(CN)6] and 0.1 M KCl solution at a potential of 0.3 V and the equivalent circuit. Inset (b) and (c) are enlarged EIS spectra of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes, respectively.

*Revised Manuscript (including Abstract) Click here to view linked References

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A facile method to prepare Pt/C/TiO2 nanotubes electrode for electro-oxidation of methanol Xiaoyu Yue a,b, Wenxiu Yang a,b, Xiangjian Liu a,b, Yizhe Wang a, Changyu Liu a, Qingyou Zhang c, Jianbo Jia a  a) State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China b) University of Chinese Academy of Sciences, Beijing 100049, P. R. China c) Institute of environmental and analytical sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China

Abstract: In this study, we prepared a Pt/C/TiO2 nanotubes (TiO2NTs) electrode by the facile pyrolysis of chloroplatinic acid and glucose on the TiO2NTs support simultaneously. By several characterization methods such as scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, it was found that with the thermal decomposition of H2PtCl6 and glucose simultaneously, Pt nanoparticles appeared with small particle size and anchored on the surface of TiO2NTs with carbon. Cyclic voltammetry and chronoamperometry were used to evaluate the electrocatalytic activity and stability of the prepared electrodes toward the oxidation of methanol. The Pt/C/TiO2NTs electrode possesses a large electrochemically active surface area and exhibits enhanced electrocatalytic activity and better stability for methanol oxidation than the Pt/TiO2NTs electrode. The Pt/C/TiO2NTs electrode provides a new approach for improving the catalytic activity and stability of Pt nanomaterials in renewable energy applications.



Corresponding author. Tel. & Fax: +86-431-85262251. E-mail address: [email protected]. 1

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Keywords:

Pt/C/TiO2 nanotubes electrode; Thermal decomposition; Methanol

oxidation; Surface area; Activity 1. Introduction Direct methanol fuel cells as prospective power sources have been attracting much attention for decades due to their high energy density and ambient operating conditions [1, 2]. Pt particles supported on carbon materials are the most popular electrocatalysts for methanol oxidation reaction (MOR). However, carbon supported Pt-based catalysts are confined owing to the dissolution of Pt particles and electrochemical corrosion of the carbon support [3]. It is reported that the properties of electrochemical anti-corrosion and the strong metal support interaction between Pt and metal oxides (WO3 [4], SnO2 [5], SiO2 [6], and TiO2 [7, 8]) could significantly enhance the stability, catalytic activity, and CO poisoning tolerance. It is widely recognized that the strong metal support interaction, generally found between Pt and TiO2 nanotubes (TiO2NTs), can drastically change the electronic structure by charge transfer, thereby affecting the activity and durability of the catalysts [9, 10]. Moreover, the Pt/TiO2NTs electrode can be gained by depositing Pt nanoparticles on TiO2NTs electrode [11-13], which shows good electrocatalytic activity toward MOR. Although TiO2NTs have a high surface area, the conductivity is disappointed in comparison with that of carbon materials. In order to improve conductivity and surface area of catalyst supports, considerable efforts have been devoted to combining metal oxides with carbon materials (N-doped carbon nanocomposite, carbon nanotube, graphene, and so on) based on various structural designs [2, 14-18]. 2

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So far, there are many reports focused on the Pt/C/TiO2 catalysts, which exhibited enhancement in the electrochemical stability and activity. Different methods were reported for the synthesis of Pt/C/TiO2 catalysts such as photo-deposition [19], chemical vapor deposition [20] or microwave-assisted polyol process [21]. For example, Fan and co-workers prepared the Pt/C/TiO2 catalysts with the synthesis of TiO2/C particles firstly, then the Pt/C/TiO2 catalysts were prepared by reducing H2PtCl6 with NaBH4. And the Pt/C/TiO2 catalysts exhibit good performances for MOR [22]. However, most of the Pt/C/TiO2 catalysts were prepared by multi-steps or under harsh conditions. It is necessary to explore simple methods to prepare the Pt/C/TiO2 catalysts with improved performances. In this work, we developed a facile method to prepare the Pt/C/TiO2NTs electrode by thermal decomposition simultaneously and studied its properties for MOR. The physicochemical properties of the Pt/C/TiO2NTs were examined by various characterization tools including scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). It is demonstrated that

the

Pt/C/TiO2NTs

electrode

leads

to

considerable

enhancement

on

electrochemically active surface areas (ECSAs), electrocatalytic activity, and stability toward MOR.

2. Experimental 2.1 Chemicals Titanium foil (purity > 99.7%) and H2PtCl6•6H2O were purchased from Aldrich. 3

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Methanol, sulfuric acid, hydrofluoric acid, nitric acid, acetone, absolute ethanol, potassium hydroxide, and glucose were of analytical grade from Beijing Chemical Reagent Company (Beijing, China) without further purification. Ultrapure water from Water Purifier (Sichuan Water Purifier Co., Ltd., China) was used in all the experiments. 2.2. Apparatuses The morphology was characterized with an XL30 ESEM FEG SEM at an accelerating voltage of 20 kV. XPS measurement was conducted with an ESCALAB-MKII spectrometer (VG Co., U. K.) with an Al Kα X-ray radiation as the X-ray source for excitation and a chamber pressure of 3.5 × 10-7 Pa. The crystallographic information of the Pt/TiO2NTs electrode is carried out with XRD (Philips X-ray diffractometer, model PW1700 BASED, Cu Kα radiation Ɩ =1.5406 Å). The electrochemical impedance spectroscopy (EIS) measurement was performed on a Zahner Zennium electrochemical workstation (ZAHNER-Elektrik GmbH & Co. KG, Germany). The other electrochemical measurements were measured on a standard three-electrode cell using a CHI 832B electrochemical workstation (CH Instruments, Shanghai) at room temperature, wherein the Pt/C/TiO2NTs or Pt/TiO2NTs electrode served as the working electrode. A Pt foil and an Ag/AgCl (saturated with KCl) electrode were worked as counter and reference electrodes, respectively. 2.3 Procedures of preparing Pt/C/ TiO2NTs electrode Highly ordered TiO2NTs electrode was obtained by anodic oxidation at room temperature according to the reported method [11]. Prior to anodization, the titanium 4

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foil was chemically etched by immersing in a mixture of HF, HNO3, and H2O (v/v/v = 1:4:5) for 40 s. Then the titanium foil was rinsed with acetone, absolute ethanol, and water for 10 min, respectively. The substrate was dried in air. Then, the TiO2NTs were formed by anodizing the titanium foil in 40 mL of 0.50 wt% HF solution at 20 V for 20 min with magnetic agitation. Anodized titanium foils were annealed at 500 ºC for 3 h in air atmosphere, and heating rate was kept at 1 ºC min-1. The Pt/C/TiO2NTs electrode was prepared by drop-coating 10 L of a mixed aqueous solution of glucose (0.030 M) and H2PtCl6•6H2O (0.096 M) on the TiO2NTs. After the electrode was dried at room temperature, it was thermally treated in a tube furnace at 500 ºC for 1 h with a heating rate of 5 ºC min-1 under N2 atmosphere. For comparison, Pt/TiO2NTs electrode was prepared by the similar process without the addition of glucose.

3. Results and discussion Fig. 1 shows the typical SEM images of TiO2NTs (a), Pt/TiO2NTs (b), and Pt/C/TiO2NTs (c and d). It is observed that high-density and uniform TiO2NTs are synthesized by the titanium foil with the electrochemical anodic oxidation technique, as exhibited in Fig. 1(a). From Fig. 1(b), it can be seen that Pt nanoparticles aggregation randomly loaded on the surface of TiO2NTs substrate compared with the TiO2NTs, which resulted from the decomposition of H2PtCl6. Fig. 1(c) and (d) show the SEM images of Pt/C/TiO2NTs with different magnifications. The agglomeration of Pt nanoparticles can be inhibited and the average particle size of Pt on the Pt/C/TiO2NTs is smaller than that on the Pt/TiO2NTs, which should be resulted from 5

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the decomposition of glucose simultaneously. The addition of glucose may lead the carbon doping and the anchoring of the carbon layers, which may cause the strong interaction among TiO2, C, and Pt [23, 24]. Fig. 1 Fig. 2 shows the XRD patterns of TiO2NTs, Pt/TiO2NTs, and Pt/C/TiO2NTs. The diffraction peaks at 25.4° should be assigned to the diffraction of the (101) crystal plane of anatase TiO2, whereas the peaks at about 34.9 and 76.3° can be indexed to crystal faces of rutile TiO2 [25]. The 46.3 and 67.7° peaks can be assigned to the diffraction of the (200) and (220) crystal planes of Pt (JCPDS file no. 87−0646), respectively, which means that Pt forms the face-centered cubic (fcc) crystal structure. Pt (111) is not clearly observed due to that the main characteristic peak of Pt (ca. 39.7°) has a strongly overlap with the peak of Ti at ca. 40.3°. The other peaks of 38.6, 40.3, 53.1, 63.1, and 70.8° correspond to the diffraction of the (002), (101), (102), (110), and (103) crystal planes of Ti metal (JCPDS file no. 44−1294), respectively. Pt was not observed in the XRD result of TiO2NTs. Compared with the Pt/TiO2NTs, there is a small shift of the Pt diffraction peaks on the Pt/C/TiO2NTs, indicating that the decomposition product of glucose had an effect on the Pt-Pt bond distance [26]. The average Pt nanoparticle size was calculated using Scherrer’s equation: d(Å) = Kλ/βcosθ, where K is a coefficient (0.9), λ is the wavelength of X-ray used (0.154 nm), β is the full-width half maximum of respective diffraction peak (rad), θ is the angle at the position of peak maximum (rad) [11]. The average Pt nanoparticle size on the Pt/C/TiO2NTs is ca. 12 nm based on Pt (200), which is smaller than that of ca. 19 6

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nm on the Pt/TiO2NTs. The result is consistent with the SEM results. Fig. 2 The XPS Pt 4f spectra of Pt/TiO2NTs and Pt/C/TiO2NTs are shown in Fig. 3. The binding energies of all peaks are referenced to a C 1s value of 284.6 eV. For the Pt/TiO2NTs, the spectrum of Pt (4f) shows a doublet containing a low binding energy (4f7/2) at 70.4 eV and a high binding energy (4f5/2) at 73.7 eV, as given in Fig. 3a. For the Pt/C/TiO2NTs (Fig. 3b), the low binding energy peak is located at 71.4 eV, which is positively shifted by 1.0 eV, and the high binding energy peak (at 73.8 eV) is positively shifted by 0.1 eV compared to those of the Pt/TiO2NTs. The shift of the Pt 4f position for the Pt/C/TiO2NTs indicates that the electronic properties of Pt were changed by the incorporation of C, which could weaken the bonding energy between metal and strongly adsorbed poisoning species [27] and lead to the variable catalytic properties according to the so-called electronic effect [28]. As shown in Fig. 3, the Pt 4f signals of Pt/TiO2NTs and Pt/C/TiO2NTs, were deconvoluted into two pairs of doublets, which were attributed to Pt (0) and Pt (II) in PtO or Pt(OH)2, respectively. Many studies have reported that oxidized noble metal is important to achieve high activity of dispersed Pt catalyst [29]. Fig. 3 Fig. 4 shows the cyclic voltammograms (CVs) of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes in deaerated 0.50 M H2SO4 with a scan rate of 50 mV s-1. It can be clearly seen that the Pt redox peaks are considerably enlarged at the Pt/C/TiO2NTs electrode. The ECSAs of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes were compared as 7

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derived by a calculation of the hydrogen desorption area from CVs in 0.50 M H 2SO4 solution. The ECSAs are 254.1 and 379.4 cm2 g-1 for the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes, respectively. The ECSA of the Pt/C/TiO2NT electrode was 1.49 times higher than that of the Pt/TiO2NT electrode, indicating that more active sites are available at the Pt/C/TiO2NTs electrode surface in electrocatalytic process. The increased ECSAs may be ascribed to three factors: (1) the addition of carbon improves the dispersion of Pt nanoparticles as can be seen from the SEM images; (2) the addition of carbon further increases the electronic conductivity of support; (3) the anchoring effect of the carbon layers formed during carbonization of glucose [24]. Fig. 4 The EIS experiments were conducted from 100 kHz to 0.1 Hz in order to investigate the internal resistance of the electrodes, which reflects the conductivity of the catalyst [30]. The related data and the equivalent circuit were given in Fig. 5. The equivalent circuit includes the electrolyte resistance (Rs), Warburg impedance (Zw), double-layer capacitance (Cd) and the charge-transfer resistance (Rct) [31]. Generally, the smaller semicircle arc in the high-frequency region indicates the smaller charge transfer resistance and faster interfacial charge transfer. As shown in Fig. 5, the radius of semicircle arc of the Pt/C/TiO2NTs electrode is much smaller than those of TiO2NTs and Pt/TiO2NTs electrodes. It can be said that the charge transfer resistance of the Pt/C/TiO2NTs electrode is smaller. The smaller resistance implies a faster reaction [32], indicating that the Pt/C/TiO2NTs electrode has better electronic conductivity. Therefore, the Pt/C/TiO2NT electrode should exhibit more efficient 8

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electrocatalytic activity for methanol oxidation. Fig. 5 Fig. 6 shows the CVs of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes in 1.0 M CH3OH and 0.50 M KOH solution with a scan rate of 50 mV s-1. As shown in Fig. 6, the curves show typical methanol oxidation peaks. The onset oxidation potential for MOR at the Pt/C/TiO2NTs electrode is -0.58 V, which is shifted negatively 0.13 V compared to that of the Pt/TiO2NTs electrode (-0.35 V), indicating that MOR is easier to perform at the Pt/C/TiO2NTs electrode. The forward peak current density of methanol oxidation is evaluated to be 9.1 mA cm−2 for the Pt/TiO2NTs electrode. However, for the Pt/C/TiO2NTs electrode, the peak current density is enhanced to be 81.4 mA cm−2, which is 9 times higher than that of the Pt/TiO2NTs electrode, displaying much improved electro-catalytic activity for methanol oxidation. It is reported that the peak ratio of If (the forward peak current density) to Ib (the backward peak current density), If/Ib could be used to indicate the CO tolerance of Pt catalyst [33]. In general, the higher ratio of If/Ib indicates better CO tolerance. The If/Ib ratios for the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes are 4.0 and 4.6, respectively, indicating that the Pt/C/TiO2NTs electrode has a better CO tolerance than the Pt/TiO2NTs electrode, which could be attributed to the high dispersion of Pt nanoparticles on the C/TiO2 support, and the strong metal-support interactions among Pt nanoparticles, C, and TiO2 support [22]. The Pt/C/TiO2NTs electrode exhibits a higher current intensity and If/Ib ratio when it is compared with those of the previous report, as shown in Table 1. 9

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Fig. 6 and Table 1 For further evaluation of the performances of the Pt/C/TiO2NTs and Pt/TiO2NTs electrodes for MOR, they were investigated by the chronoamperometry at a potential of -0.17 V for a period of 2000 s in deaerated 1.0 M CH3OH and 0.50 M KOH solution, as shown in Fig. 7. Compared with the Pt/TiO2NTs electrode, the Pt/C/TiO2NTs electrode exhibits the enhanced maximum initial oxidation current density. In addition, the descending rates of the oxidation current from 200 to 2000 s are 68.6% and 78.7% for the Pt/C/TiO2NTs and Pt/TiO2NTs electrodes, respectively. The results indicated both the catalytic activity and stability of the Pt/C/TiO2NTs electrode are improved, which could be attributed to the aid of oxygen-containing species such as -OH formed on the Pt surface [36], the strong metal support interaction between the catalyst and the catalyst support [23], and the anchoring effect of the carbon layers formed during the carbon riveting process [24]. Fig. 7

4. Conclusions In conclusion, we have developed a facile method to prepare the Pt/C/TiO2NTs electrode by heat treatment. After the pyrolysis of H2PtCl6 and glucose simultaneously, the as-prepared Pt/C/TiO2NTs electrode exhibits improved catalytic activity and stability toward methanol electro-oxidation comparing with that of Pt/TiO2NTs electrode. The enhancement in catalytic performance of the Pt/C/TiO2NTs electrode should be ascribed to the high dispersion, small size of Pt nanoparticles, and the strong metal-support interactions among Pt nanoparticles, C, and TiO2 support. This work provides a simple approach to prepared Pt and carbon 10

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co-doping TiO2 electrode for DMFCs and sensors.

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process for electrooxidation of formic acid, Appl. Catal. B Environ. 123 (2012) 214−220. [22] Y. Fan, Z. J. Yang, P. Huang, X. Zhang, Y. M. Liu, Pt/TiO2−C with heterointerfaces as enhanced catalyst for methanol electrooxidation, Electrochim. Acta 105 (2013) 157−161. [23] Q. Lv, M. Yin, X Zhao, C. Y. Li, C. P. Liu, W. Xing, Promotion effect of TiO2 on catalytic activity and stability of Pt catalyst for electrooxidation of methanol, J. Power Sources 218 (2012) 93−99. [24] Z. Z. Jiang, Z. B. Wang, Y.Y. Chu, D.M. Gu, G.P. Yin, Ultrahigh stable carbon riveted Pt/TiO2–C catalyst prepared by in situ carbonized glucose for proton exchange membrane fuel cell, Energy Environ. Sci. (4) 2011 728–735. [25] L. Han, L. Bai, S. J. Dong, Self-powered visual ultraviolet photodetector with prussian blue electrochromic display, Chem. Commun. 50 (2014) 802−804. [26] K. Zhang, Q. L. Yue, G. F. Chen, Y. L. Zhai, L. Wang, H. S. Wang, J. S. Zhao, J. F. Liu, J. B. Jia, H. B. Li. Effects of acid treatment of Pt-Ni alloy nanoparticles@ graphene on the kinetics of the oxygen reduction reaction in acidic and alkaline solutions. J. Phys. Chem. C 115 (2011) 379−389. [27] J. L. Haan, K. M. Stafford, R. I. Masel, Effects of the addition of antimony, tin, and lead to palladium catalyst formulations for the direct formic acid fuel cell, J. Phys. Chem. C 114 (2010) 11665−11672. [28] Z. Wang, G. Chen, D. Xia, L. Zhang, Studies on the electrocatalytic properties of PtRu/C-TiO2 toward the oxidation of methanol, J. Alloys Compd. 450 (2008) 14

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16

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Captions Table 1 Comparison of the catalytic performance of the different Pt/C/TiO2NTs electrodes for MOR. Fig. 1 SEM images of TiO2NTs (a), Pt/TiO2NTs (b), and Pt/C/TiO2NTs (c, d). Fig. 2 XRD patterns of TiO2NTs, Pt/TiO2NTs, and Pt/C/TiO2NTs. Fig. 3 Pt 4f region in the XPS spectra of Pt/TiO2NTs (a) and Pt/C/TiO2NTs (b). Fig. 4 CVs of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes in a N2-saturated 0.50 M H2SO4 solution at a scan rate of 50 mV s−1. Fig. 5 (a) EIS spectra of the TiO2NTs, Pt/TiO2NTs, and Pt/C/TiO2NTs electrodes in 2.5 mM K3[Fe(CN)6/K4[Fe(CN)6] and 0.1 M KCl solution at a potential of 0.3 V and the equivalent circuit. Inset (b) and (c) are enlarged EIS spectra of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes, respectively. Fig. 6 CVs of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes in a N2-saturated CH3OH (1.0 M) and KOH (0.50 M) solution at a scan rate of 50 mV s−1. Fig. 7 Chronoamperometric curves of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes recorded at -0.17 V in a N2-saturated CH3OH (1.0 M) and KOH (0.50 M) solution.

17

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Table 1 Electrode

Forward peak current density (mA cm-2)

If/Ib

Ref.

Pt/TiO2NTs

20.0

2.50

[11]

Pt-TiO2NTs/RGO

1.36

1.93

[32]

Pd/TiO2-C

3.93

0.84

[34]

Pt/Ti

13.0

2.17

[35]

Pt/C/TiO2NTs

81.4

4.60

This work

18

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(d)

Fig. 1

19

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Fig. 2

20

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Fig. 3

21

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Fig. 4

22

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Fig. 5

23

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Fig. 6

24

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Fig. 7

25

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A facile method to prepare Pt/C/TiO2 nanotubes electrode for electro-oxidation of methanol Xiaoyu Yue a,b, Wenxiu Yang a,b, Xiangjian Liu a,b, Yizhe Wang a, Changyu Liu a, Qingyou Zhang c, Jianbo Jia a  a) State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China b) University of Chinese Academy of Sciences, Beijing 100049, P. R. China c) Institute of environmental and analytical sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China

Abstract: In this study, we prepared a Pt/C/TiO2 nanotubes (TiO2NTs) electrode by the facile pyrolysis of chloroplatinic acid and glucose on the TiO2NTs support simultaneously. By several characterization methods such as scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, it was found that with the thermal decomposition of H2PtCl6 and glucose simultaneously, Pt nanoparticles appeared with small particle size and anchored on the surface of TiO2NTs with carbon. Cyclic voltammetry and chronoamperometry were used to evaluate the electrocatalytic activity and stability of the prepared electrodes toward the oxidation of methanol. The Pt/C/TiO2NTs electrode possesses a large electrochemically active surface area and exhibits enhanced electrocatalytic activity and better stability for methanol oxidation than the Pt/TiO2NTs electrode. The Pt/C/TiO2NTs electrode provides a new approach for improving the catalytic activity and stability of Pt nanomaterials in renewable energy applications.



Corresponding author. Tel. & Fax: +86-431-85262251. E-mail address: [email protected]. 26

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Keywords:

Pt/C/TiO2 nanotubes electrode; Thermal decomposition; Methanol

oxidation; Surface area; Activity 2. Introduction Direct methanol fuel cells as prospective power sources have been attracting much attention for decades due to their high energy density and ambient operating conditions [1, 2]. Pt particles supported on carbon materials are the most popular electrocatalysts for methanol oxidation reaction (MOR). However, carbon supported Pt-based catalysts are confined owing to the dissolution of Pt particles and electrochemical corrosion of the carbon support [3]. It is reported that the properties of electrochemical anti-corrosion and the strong metal support interaction between Pt and metal oxides (WO3 [4], SnO2 [5], SiO2 [6], and TiO2 [7, 8]) could significantly enhance the stability, catalytic activity, and CO poisoning tolerance. It is widely recognized that the strong metal support interaction, generally found between Pt and TiO2 nanotubes (TiO2NTs), can drastically change the electronic structure by charge transfer, thereby affecting the activity and durability of the catalysts [9, 10]. Moreover, the Pt/TiO2NTs electrode can be gained by depositing Pt nanoparticles on TiO2NTs electrode [11-13], which shows good electrocatalytic activity toward MOR. Although TiO2NTs have a high surface area, the conductivity is disappointed in comparison with that of carbon materials. In order to improve conductivity and surface area of catalyst supports, considerable efforts have been devoted to combining metal oxides with carbon materials (N-doped carbon nanocomposite, carbon nanotube, graphene, and so on) based on various structural designs [2, 14-18]. 27

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So far, there are many reports focused on the Pt/C/TiO2 catalysts, which exhibited enhancement in the electrochemical stability and activity. Different methods were reported for the synthesis of Pt/C/TiO2 catalysts such as photo-deposition [19], chemical vapor deposition [20] or microwave-assisted polyol process [21]. For example, Fan and co-workers prepared the Pt/C/TiO2 catalysts with the synthesis of TiO2/C particles firstly, then the Pt/C/TiO2 catalysts were prepared by reducing H2PtCl6 with NaBH4. And the Pt/C/TiO2 catalysts exhibit good performances for MOR [22]. However, most of the Pt/C/TiO2 catalysts were prepared by multi-steps or under harsh conditions. It is necessary to explore simple methods to prepare the Pt/C/TiO2 catalysts with improved performances. In this work, we developed a facile method to prepare the Pt/C/TiO 2NTs electrode by thermal decomposition simultaneously and studied its properties for MOR. The physicochemical properties of the Pt/C/TiO2NTs were examined by various characterization tools including scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). It is demonstrated that

the

Pt/C/TiO2NTs

electrode

leads

to

considerable

enhancement

on

electrochemically active surface areas (ECSAs), electrocatalytic activity, and stability toward MOR.

2. Experimental 2.1 Chemicals Titanium foil (purity > 99.7%) and H2PtCl6•6H2O were purchased from Aldrich. 28

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Methanol, sulfuric acid, hydrofluoric acid, nitric acid, acetone, absolute ethanol, potassium hydroxide, and glucose were of analytical grade from Beijing Chemical Reagent Company (Beijing, China) without further purification. Ultrapure water from Water Purifier (Sichuan Water Purifier Co., Ltd., China) was used in all the experiments. 2.2. Apparatuses The morphology was characterized with an XL30 ESEM FEG SEM at an accelerating voltage of 20 kV. XPS measurement was conducted with an ESCALAB-MKII spectrometer (VG Co., U. K.) with an Al Kα X-ray radiation as the X-ray source for excitation and a chamber pressure of 3.5 × 10-7 Pa. The crystallographic information of the Pt/TiO2NTs electrode is carried out with XRD (Philips X-ray diffractometer, model PW1700 BASED, Cu Kα radiation Ɩ =1.5406 Å). The electrochemical impedance spectroscopy (EIS) measurement was performed on a Zahner Zennium electrochemical workstation (ZAHNER-Elektrik GmbH & Co. KG, Germany). The other electrochemical measurements were measured on a standard three-electrode cell using a CHI 832B electrochemical workstation (CH Instruments, Shanghai) at room temperature, wherein the Pt/C/TiO2NTs or Pt/TiO2NTs electrode served as the working electrode. A Pt foil and an Ag/AgCl (saturated with KCl) electrode were worked as counter and reference electrodes, respectively. 2.3 Procedures of preparing Pt/C/ TiO2NTs electrode Highly ordered TiO2NTs electrode was obtained by anodic oxidation at room temperature according to the reported method [11]. Prior to anodization, the titanium 29

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foil was chemically etched by immersing in a mixture of HF, HNO3, and H2O (v/v/v = 1:4:5) for 40 s. Then the titanium foil was rinsed with acetone, absolute ethanol, and water for 10 min, respectively. The substrate was dried in air. Then, the TiO2NTs were formed by anodizing the titanium foil in 40 mL of 0.50 wt% HF solution at 20 V for 20 min with magnetic agitation. Anodized titanium foils were annealed at 500 ºC for 3 h in air atmosphere, and heating rate was kept at 1 ºC min-1. The Pt/C/TiO2NTs electrode was prepared by drop-coating 10 L of a mixed aqueous solution of glucose (0.030 M) and H2PtCl6•6H2O (0.096 M) on the TiO2NTs. After the electrode was dried at room temperature, it was thermally treated in a tube furnace at 500 ºC for 1 h with a heating rate of 5 ºC min-1 under N2 atmosphere. For comparison, Pt/TiO2NTs electrode was prepared by the similar process without the addition of glucose.

5. Results and discussion Fig. 1 shows the typical SEM images of TiO2NTs (a), Pt/TiO2NTs (b), and Pt/C/TiO2NTs (c and d). It is observed that high-density and uniform TiO2NTs are synthesized by the titanium foil with the electrochemical anodic oxidation technique, as exhibited in Fig. 1(a). From Fig. 1(b), it can be seen that Pt nanoparticles aggregation randomly loaded on the surface of TiO2NTs substrate compared with the TiO2NTs, which resulted from the decomposition of H2PtCl6. Fig. 1(c) and (d) show the SEM images of Pt/C/TiO2NTs with different magnifications. The agglomeration of Pt nanoparticles can be inhibited and the average particle size of Pt on the Pt/C/TiO2NTs is smaller than that on the Pt/TiO2NTs, which should be resulted from 30

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the decomposition of glucose simultaneously. The addition of glucose may lead the carbon doping and the anchoring of the carbon layers, which may cause the strong interaction among TiO2, C, and Pt [23, 24]. Fig. 1 Fig. 2 shows the XRD patterns of TiO2NTs, Pt/TiO2NTs, and Pt/C/TiO2NTs. The diffraction peaks at 25.4° should be assigned to the diffraction of the (101) crystal plane of anatase TiO2, whereas the peaks at about 34.9 and 76.3° can be indexed to crystal faces of rutile TiO2 [25]. The 46.3 and 67.7° peaks can be assigned to the diffraction of the (200) and (220) crystal planes of Pt (JCPDS file no. 87−0646), respectively, which means that Pt forms the face-centered cubic (fcc) crystal structure. Pt (111) is not clearly observed due to that the main characteristic peak of Pt (ca. 39.7°) has a strongly overlap with the peak of Ti at ca. 40.3°. The other peaks of 38.6, 40.3, 53.1, 63.1, and 70.8° correspond to the diffraction of the (002), (101), (102), (110), and (103) crystal planes of Ti metal (JCPDS file no. 44−1294), respectively. Pt was not observed in the XRD result of TiO2NTs. Compared with the Pt/TiO2NTs, there is a small shift of the Pt diffraction peaks on the Pt/C/TiO2NTs, indicating that the decomposition product of glucose had an effect on the Pt-Pt bond distance [26]. The average Pt nanoparticle size was calculated using Scherrer’s equation: d(Å) = Kλ/βcosθ, where K is a coefficient (0.9), λ is the wavelength of X-ray used (0.154 nm), β is the full-width half maximum of respective diffraction peak (rad), θ is the angle at the position of peak maximum (rad) [11]. The average Pt nanoparticle size on the Pt/C/TiO2NTs is ca. 12 nm based on Pt (200), which is smaller than that of ca. 19 31

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nm on the Pt/TiO2NTs. The result is consistent with the SEM results. Fig. 2 The XPS Pt 4f spectra of Pt/TiO2NTs and Pt/C/TiO2NTs are shown in Fig. 3. The binding energies of all peaks are referenced to a C 1s value of 284.6 eV. For the Pt/TiO2NTs, the spectrum of Pt (4f) shows a doublet containing a low binding energy (4f7/2) at 70.4 eV and a high binding energy (4f5/2) at 73.7 eV, as given in Fig. 3a. For the Pt/C/TiO2NTs (Fig. 3b), the low binding energy peak is located at 71.4 eV, which is positively shifted by 1.0 eV, and the high binding energy peak (at 73.8 eV) is positively shifted by 0.1 eV compared to those of the Pt/TiO2NTs. The shift of the Pt 4f position for the Pt/C/TiO2NTs indicates that the electronic properties of Pt were changed by the incorporation of C, which could weaken the bonding energy between metal and strongly adsorbed poisoning species [27] and lead to the variable catalytic properties according to the so-called electronic effect [28]. As shown in Fig. 3, the Pt 4f signals of Pt/TiO2NTs and Pt/C/TiO2NTs, were deconvoluted into two pairs of doublets, which were attributed to Pt (0) and Pt (II) in PtO or Pt(OH)2, respectively. Many studies have reported that oxidized noble metal is important to achieve high activity of dispersed Pt catalyst [29]. Fig. 3 Fig. 4 shows the cyclic voltammograms (CVs) of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes in deaerated 0.50 M H2SO4 with a scan rate of 50 mV s-1. It can be clearly seen that the Pt redox peaks are considerably enlarged at the Pt/C/TiO2NTs electrode. The ECSAs of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes were compared as 32

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derived by a calculation of the hydrogen desorption area from CVs in 0.50 M H 2SO4 solution. The ECSAs are 254.1 and 379.4 cm2 g-1 for the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes, respectively. The ECSA of the Pt/C/TiO2NT electrode was 1.49 times higher than that of the Pt/TiO2NT electrode, indicating that more active sites are available at the Pt/C/TiO2NTs electrode surface in electrocatalytic process. The increased ECSAs may be ascribed to three factors: (1) the addition of carbon improves the dispersion of Pt nanoparticles as can be seen from the SEM images; (2) the addition of carbon further increases the electronic conductivity of support; (3) the anchoring effect of the carbon layers formed during carbonization of glucose [24]. Fig. 4 The EIS experiments were conducted from 100 kHz to 0.1 Hz in order to investigate the internal resistance of the electrodes, which reflects the conductivity of the catalyst [30]. The related data and the equivalent circuit were given in Fig. 5. The equivalent circuit includes the electrolyte resistance (Rs), Warburg impedance (Zw), double-layer capacitance (Cd) and the charge-transfer resistance (Rct) [31]. Generally, the smaller semicircle arc in the high-frequency region indicates the smaller charge transfer resistance and faster interfacial charge transfer. As shown in Fig. 5, the radius of semicircle arc of the Pt/C/TiO2NTs electrode is much smaller than those of TiO2NTs and Pt/TiO2NTs electrodes. It can be said that the charge transfer resistance of the Pt/C/TiO2NTs electrode is smaller. The smaller resistance implies a faster reaction [32], indicating that the Pt/C/TiO2NTs electrode has better electronic conductivity. Therefore, the Pt/C/TiO2NT electrode should exhibit more efficient 33

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electrocatalytic activity for methanol oxidation. Fig. 5 Fig. 6 shows the CVs of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes in 1.0 M CH3OH and 0.50 M KOH solution with a scan rate of 50 mV s-1. As shown in Fig. 6, the curves show typical methanol oxidation peaks. The onset oxidation potential for MOR at the Pt/C/TiO2NTs electrode is -0.58 V, which is shifted negatively 0.13 V compared to that of the Pt/TiO2NTs electrode (-0.35 V), indicating that MOR is easier to perform at the Pt/C/TiO2NTs electrode. The forward peak current density of methanol oxidation is evaluated to be 9.1 mA cm−2 for the Pt/TiO2NTs electrode. However, for the Pt/C/TiO2NTs electrode, the peak current density is enhanced to be 81.4 mA cm−2, which is 9 times higher than that of the Pt/TiO2NTs electrode, displaying much improved electro-catalytic activity for methanol oxidation. It is reported that the peak ratio of If (the forward peak current density) to Ib (the backward peak current density), If/Ib could be used to indicate the CO tolerance of Pt catalyst [33]. In general, the higher ratio of If/Ib indicates better CO tolerance. The If/Ib ratios for the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes are 4.0 and 4.6, respectively, indicating that the Pt/C/TiO2NTs electrode has a better CO tolerance than the Pt/TiO2NTs electrode, which could be attributed to the high dispersion of Pt nanoparticles on the C/TiO2 support, and the strong metal-support interactions among Pt nanoparticles, C, and TiO2 support [22]. The Pt/C/TiO2NTs electrode exhibits a higher current intensity and If/Ib ratio when it is compared with those of the previous report, as shown in Table 1. 34

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Fig. 6 and Table 1 For further evaluation of the performances of the Pt/C/TiO2NTs and Pt/TiO2NTs electrodes for MOR, they were investigated by the chronoamperometry at a potential of -0.17 V for a period of 2000 s in deaerated 1.0 M CH3OH and 0.50 M KOH solution, as shown in Fig. 7. Compared with the Pt/TiO2NTs electrode, the Pt/C/TiO2NTs electrode exhibits the enhanced maximum initial oxidation current density. In addition, the descending rates of the oxidation current from 200 to 2000 s are 68.6% and 78.7% for the Pt/C/TiO2NTs and Pt/TiO2NTs electrodes, respectively. The results indicated both the catalytic activity and stability of the Pt/C/TiO2NTs electrode are improved, which could be attributed to the aid of oxygen-containing species such as -OH formed on the Pt surface [36], the strong metal support interaction between the catalyst and the catalyst support [23], and the anchoring effect of the carbon layers formed during the carbon riveting process [24]. Fig. 7

6. Conclusions In conclusion, we have developed a facile method to prepare the Pt/C/TiO 2NTs electrode by heat treatment. After the pyrolysis of H2PtCl6 and glucose simultaneously, the as-prepared Pt/C/TiO2NTs electrode exhibits improved catalytic activity and stability toward methanol electro-oxidation comparing with that of Pt/TiO2NTs electrode. The enhancement in catalytic performance of the Pt/C/TiO2NTs electrode should be ascribed to the high dispersion, small size of Pt nanoparticles, and the strong metal-support interactions among Pt nanoparticles, C, and TiO2 support. This work provides a simple approach to prepared Pt and carbon 35

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co-doping TiO2 electrode for DMFCs and sensors.

Acknowledgments We acknowledge financial supports from the National Natural Science Foundation of China (No. 21305132) and the Ministry of Science and Technology of China (No. 2013YQ170585). References [1] H. Huang, S. Yang, R. Vajtai, X. Wang, P. M. Ajayan, Pt-decorated 3D architectures built from graphene and graphitic carbon nitride nanosheets as efficient methanol oxidation catalysts, Adv. Mater. 26 (2014) 5160−5165. [2] J. Zhu, X. Zhao, M. Xiao, L. Liang, C. Liu, J. Liao, W. Xing, The construction of nitrogen-doped graphitized carbon–TiO2 composite to improve the electrocatalyst for methanol oxidation, Carbon 72 (2014) 114−124. [3] H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, D. P. Wilkinson, A review of anode catalysis in the direct methanol fuel cell, J. Power Sources 155 (2006) 95−110. [4] S. Jayaraman, T. F. Jaramillo, S. H. Baeck, E. W. McFarland, Synthesis and characterization of Pt-WO3 films as methanol oxidation catalysts for low-temperature polymer electrolyte membrane fuel cells, J. Phys. Chem. B 109 (2005) 22958−22966. [5] H. Huang, Y. Liu, Q. Gao, W. Ruan, X. Lin, X. Li, Rational construction of strongly coupled metal–metal

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Captions Table 1 Comparison of the catalytic performance of the different Pt/C/TiO 2NTs electrodes for MOR. Fig. 1 SEM images of TiO2NTs (a), Pt/TiO2NTs (b), and Pt/C/TiO2NTs (c, d). Fig. 2 XRD patterns of TiO2NTs, Pt/TiO2NTs, and Pt/C/TiO2NTs. Fig. 3 Pt 4f region in the XPS spectra of Pt/TiO2NTs (a) and Pt/C/TiO2NTs (b). Fig. 4 CVs of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes in a N2-saturated 0.50 M H2SO4 solution at a scan rate of 50 mV s−1. Fig. 5 (a) EIS spectra of the TiO2NTs, Pt/TiO2NTs, and Pt/C/TiO2NTs electrodes in 2.5 mM K3[Fe(CN)6/K4[Fe(CN)6] and 0.1 M KCl solution at a potential of 0.3 V and the equivalent circuit. Inset (b) and (c) are enlarged EIS spectra of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes, respectively. Fig. 6 CVs of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes in a N2-saturated CH3OH (1.0 M) and KOH (0.50 M) solution at a scan rate of 50 mV s−1. Fig. 7 Chronoamperometric curves of the Pt/TiO2NTs and Pt/C/TiO2NTs electrodes recorded at -0.17 V in a N2-saturated CH3OH (1.0 M) and KOH (0.50 M) solution.

42

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Table 1 Electrode

Forward peak current density (mA cm-2)

If/Ib

Ref.

Pt/TiO2NTs

20.0

2.50

[11]

Pt-TiO2NTs/RGO

1.36

1.93

[32]

Pd/TiO2-C

3.93

0.84

[34]

Pt/Ti

13.0

2.17

[35]

Pt/C/TiO2NTs

81.4

4.60

This work

43

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(d)

Fig. 1

44

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Fig. 2

45

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Fig. 3

46

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Fig. 4

47

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Fig. 5

48

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Fig. 6

49

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Fig. 7

50