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Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode
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Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode Sujuan Hu a,b,∗, Baoling Wang a,b, Haidong Ju a,b, Lei Jiang a,b, Yinhai Ma a,b, Yongze Fan a a b
Department of Chemistry, Kunming University, Kunming, 650214, China Yunnan Engineering Technology Research Center for Plastic Films, Kunming University, Kunming, 650214, China
a r t i c l e
i n f o
Article history: Received 5 July 2017 Revised 12 August 2017 Accepted 13 August 2017 Available online xxx Keywords: Methanol oxidation Photo-assisted Photoelectrochemistry 1D TiO2 nanoarays
a b s t r a c t In this paper, one-dimensional (1D) TiO2 nanorods arrays (NRs) are adopted as Pt nanoparticles support for electrocatalytic methanol oxidation. We show that high aspect ratio 1D-TiO2 NRs not only own the advantages of excellent photogenerated carrier transport with low recombination rate but also serve to anchor the Pt nanoparticles for well-dispersing onto the surface of nanorods. The Pt-1D TiO2 NRs electrodes show improved activities toward electrocatalytic methanol oxidation when they exposed into the simulated solar light irradiation. Synergistic effects between electro- & photo-catalytic methanol oxidation as well as the efficient interfacial charger separation in Pt-1D TiO2 NRs electrode contribute to the enhanced electrocatalytic methanol oxidation activities. Our results suggest that 1D-TiO2 NRs can be used as promising photo-assisted Pt noble metal support in direct methanol fuel cell. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Direct methanol fuel cells (DMFCs) are regarded as ideal energy converters for the settled and mobile devices due to their high energy capacity and non-poisonous [1,2]. However, the most negative obstacle for large-scale application of DMFCs is developing highefficiency and durable anodic oxidation materials. As reported, efficient and conventional Pt noble metal is usually applied as anodic oxidation electrocatalyst in electrocatalytic methanol oxidation [3,4]. However, pure Pt electrocatalyst easily suffers from the reaction intermediate CO poisoning during the electrocatalytic process and finally results in inactivated [5–7]. Besides, the cost of pure Pt is too high that not favors to the commercialization of DMFCs. In order to improve utilization of expensive Pt noble metal and maximize the benefits of performances, supports are often adopted for dispersing Pt to gain large specific surface area and more active sites. Photo-excited semiconductors as promising supports for enhancing the electrocatalytic methanol oxidation activities and durable performances of DMFCs have attracted more and more researchers’ attentions since Kamat first employ semiconductor as support for electrocatalytic methanol oxidation under UV-light irradiation [8–12]. Among various semiconductors, TiO2 are always regarded as the most promising materials for the solar energy ∗ Corresponding author at: Department of Chemistry, Kunming University, Kunming, 650214, China. E-mail address: [email protected] (S. Hu).
conversion due to its high stability, low-cost and good scalability for manufacture [13,14]. Multiple dimensions of TiO2 including zero dimensional (0D), one dimensional (1D), two dimensional (2D) and three dimensional (3D) have been fabricated for application in different optoelectronics fields according to their respective unique morphology-depended performance [15–19]. Among them, well-designed 1D TiO2 NRs become the focus of study in electro& photo-catalytic area as their superior photoelectron transfer property. Herein, well-designed 1D TiO2 nanorods arrays were prepared by a simple hydrothermal method and we introduced noble metal Pt nanoparticles onto the surface of 1D TiO2 nanorods arrays by electro-deposition method. With the assistance of light, the electrocatalytic methanol oxidation properties and durable performances are improved on Pt-1D TiO2 NRs electrodes in an alkaline solution. These results display that special high aspect ratio 1D TiO2 NRs structure can serve as well support to improve the utilization efficiency of Pt noble metal along with reduce poisoning. Besides, during electro-catalytic methanol oxidation process, 1D TiO2 NRs as an excellent photocatalyst support can provide abundant photo-excited electrons and holes with the assistance of light. The photogenerated holes with strong oxidization can firstly oxidize OH− /H2 O to hydroxyl radicals (•OH) and then •OH oxidize methanol molecules to CO2 . What’s more, the carbon-containing intermediates that produced during electrocatalytic process can also be oxidized by highly active •OH, resulting in an efficient poisoning suppression. The synergy effects between the electro- & photo-catalytic devote to the improvement of methanol oxidation
http://dx.doi.org/10.1016/j.jtice.2017.08.024 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: S. Hu et al., Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.024
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activities. Our experimental results demonstrate that 1D TiO2 NRs is a promising photocatalyst support in the fields of solar and chemical energy conversion. 2. Experimental 2.1. Preparation of 1D TiO2 NRs electrode A hydrothermal method was adopted to prepare 1D TiO2 NRs electrode [20]. Firstly, 30 mL of deionized water and 30 mL of concentrated HCl were mixed under stirring. Then, 1.2 mL of tetrabutyl titanate was added into the above solution dropwise. After that, the mixture was transferred into Teflon-lined stainless steel autoclave with cleaned FTO conductive glass (1.5 ∗ 2 cm) placed at an angle (70–75°) against the wall of the Teflon-liner, and then the sealed autoclave was placed in an oven at 150 °C for 12 h Finally, TiO2 NRs photo-electrodes were achieved after annealed at 450 °C for 30 min. 2.2. Preparation of Pt-1D TiO2 NRs electrode Pt nanoparticles loaded 1D TiO2 NRs electrode was fabricated by the galvanostatic electro-deposition technique with a current density of 1 mA for 750 s. Herein, we adopted the as-prepared 1D TiO2 NRs electrode as cathode, a platinum plate as anode, an Hg/HgO (1.0 M KOH) as reference electrode and an aqueous solution consisting of 0.2 mM H2 PtCl6 and 0.47 M H2 SO4 as electrolyte. According Faraday’s law, the electric quantity for reducing H2 PtCl6 to Pt metal is 0.75 C and the amount of Pt loading onto the surface of 1D TiO2 NRs electrode is 0.379 mg. 2.3. Characterization The crystal structures of the samples were recorded using an X-ray diffractometer with Cu Ka irradiation (X’ Pert pro. PANalytical B.V). The morphologies of the samples characterized by scanning electron microscopy (JEOL, JSM-IT100). X-ray photoelectron spectroscopy (XPS) was performed on an X-ray photoelectron spectrometer (KRATOS AXIS165). UV–vis diffuse reflectance spectra (DRS) were obtained on a UV-vis spectrophotometer (Shimadzu, UV-2450). 2.4. Electro- & photo-catalytic measurements The electro- & photo-catalytic measurements were performed in a custom-built container with quartz window that confirmed no blocking of simulated solar light by an electrochemical workstation (AUTOLAB, PGSTAT302N). A xenon lamp (100 W) was used as the simulated solar light. The area of photoexcited Pt-1D TiO2 NRs electrodes during the performance test is 1 cm2 . A threeelectrode system consisting of the as-prepared samples as working electrodes, a platinum plate as a counter electrode, an Hg/HgO (1.0 M KOH) as the reference electrode and an aqueous solution consisting of 1.0 M KOH and 1.0 M CH3 OH as electrolyte was used for electro- & photo-catalytic measurements. The cyclic voltammetries (CV) of electrodes were tested in 1.0 M CH3 OH and 1.0 M KOH solution. Chronoamperometry (CA) and photocurrent responses of the electrodes were recorded in 1.0 M CH3 OH and 1.0 M KOH solution without bias potential. The electrochemical active surface area (ECSA) was investigated by CV measurement in 1.0 M KOH solution at a scan rate of 0.1 V/s. Pulsed potential cycles were implemented in 1.0 M CH3 OH and 1.0 M KOH solution with 30 s alternatively at −0.4 V and 0.1 V (vs. Hg/HgO). Electrochemical impedance spectroscopies (EIS) with different potentials were performed from 0.1 to 10,0 0 0 Hz under amplitude of 50 mV in 1.0 M CH3 OH and 1.0 M KOH solution.
Fig. 1. XRD patterns of FTO, 1D TiO2 NRs electrode and Pt-1D TiO2 NRs electrode.
3. Results and discussion Fig. 1 is the XRD patterns of phase structures of F-doped tin oxide (FTO) conductive substrate, 1D TiO2 NRs electrode, and Pt1D TiO2 NRs electrode. FTO substrates show a tetragonal phase of SnO2 (JCPDS no. 41–1445). The peaks located at ca. 36.26°, 41.37°, 63.02° and 69.94° can be indexed to (101), (111), (002) and (301) crystal planes of rutile phase TiO2 (JCPDS no. 21–1276). However, diffraction peaks of Pt are not observed because of its low amount. The morphologies of 1D TiO2 NRs electrode and the Pt-1D TiO2 NRs electrode are shown in Fig. 2. TiO2 NRs are uniformly and vertically aligned onto FTO substrate with an average diameter of ca. 150 nm (Fig. 2A). From Fig. 2B to D, we can see that some nanoparticles successfully decorated onto the surface of 1D TiO2 NRs. Fig. 2E is the cross-section of Pt-1D TiO2 NRs, it can be seen that the length of the nanorods is ca. 4 μm and some nanoparticles loaded onto it. To further confirm the species of these nanoparticles and reveal the chemical composition of Pt-1D TiO2 NRs electrode, XPS spectra of the sample were performed, as shown in Fig. 3. Two peaks at binding energies of 464.78 and 459.06 eV can be attributed to the doublet of Ti 2p1/2 and Ti 2p3/2 , respectively (Fig. 3A) [21,22]. A doublet that originates from the spin-orbit splitting of the 4f 5/2 and 4f 7/2 states of Pt at 74.65 and 71.31 eV further reveals the existence of Pt in Pt-1D TiO2 NRs [23,24]. Two oxygen signals observed at 532.03 and 530.27 eV can be ascribed to the surface-adsorbed oxygen and lattice oxygen [25,26]. The optical properties of the samples were studied by UV-vis diffuse reflectance spectra (Fig 4). For 1D TiO2 NRs electrode, the DRS spectra present steep absorption edges at ca. 415 nm, which can be assigned to the intrinsic band gap of TiO2 (3.2 eV). The absorption intensity is largely enhanced and no extension of absorption edges is observed when Pt nanoparticles loaded onto the surface of 1D TiO2 NRs electrode. For estimating the performance of Pt-1D TiO2 NRs electrode, the electrochemically active surface area (ECSA) was studied by CV measurements in 1.0 M KOH solution at a scan rate of 0.1 V s−1 , as shown in Fig. 5 [27,28]. The ECSA of Pt-1D TiO2 NRs was investigated according the followed Eq. (1). Herein, QH is the amount of electron transfer in H-adsorption and H-desorption peak areas, which can be calculated by Eq. (2).
ECSA =
QH =
QH 0.21 × mPt
Qads + Qdes = 2
(1) Sads V
+ 2
Sdes V
(2)
Where mPt is the quality of Pt (g) loaded onto the electrode, 0.21 (mC cm−2 ) represents the maximum surface charge transferred to Pt during adsorption of a monolayer of H, Sads and Sdes
Please cite this article as: S. Hu et al., Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.024
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Fig. 2. SEM images of 1D TiO2 NRs electrode (A) and Pt-1D TiO2 NRs electrode (B, C, D and E).
Fig. 3. XPS spectra of Ti 2p (A), Pt 4f (B) and O1s (C) of Pt-1D TiO2 NRs electrode.
Fig. 4. UV–vis DRS of 1D TiO2 NRs electrode and Pt-1D TiO2 NRs electrode. Fig. 5. CV of Pt-1D TiO2 NRs electrode in 1.0 M KOH solution.
(A∗ V) are the peak areas of H-adsorption and H-desorption process, respectively, and V (V s−1 ) is the scan rate of CV. As a result, the ECSA of Pt-1D TiO2 NRs is 0.197 m2 g−1 . The electro- & photo-catalytic performances were measured by methanol oxidation reaction in alkaline methanol solution. Fig. 6A and B are the cyclic voltammetry curves of Pt-1D TiO2 NRs electrode and 1D TiO2 NRs electrode with and without simulated solar light irradiation. As can be seen from Fig. 6A, the curves exhibit the typical profile for methanol electrooxidation, including
two strong oxidation peaks at ca. −0.1 and −0.2 V, respectively. The peak formed in the forward scan at ca. −0.1 V is attributed to the oxidative removal of adsorbed/dehydrogenated methanol fractions and CO2 and carbon-containing intermediates are formed during the process. This peak current density is usually used to evaluate the electrocatalytic performance of electrode [29]. The peak in the reverse scan at ca. −0.2 V represents the re-oxidation of carboncontaining intermediates formed in the forward scan [30,31].
Please cite this article as: S. Hu et al., Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.024
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Fig. 6. The 150th CV of Pt-1D TiO2 NRs electrode (A) and 1D TiO2 NRs electrode (B) in alkaline methanol solution (1.0 M KOH+ 1.0 M CH3 OH) with (a) and without (b) simulated solar light irradiation. Transient photocurrent density-time curves (C) of Pt-1D TiO2 NRs electrode and 1D TiO2 NRs electrode.
Fig. 7. CA of Pt-1D TiO2 NRs (A) and 1D TiO2 NRs (B) with (a) and without (b) simulated solar light irradiation. Methanol oxidation peaks current density of 250 cycles CV testing on Pt-1D TiO2 NRs (C). CVs of the Pt-1D TiO2 NRs electrode before (a) and after (b) 10 0 0 pulsed potential cycles with (D) and without (E) simulated solar light irradiation. The XRD patterns of Pt-1D TiO2 NRs electrode before and after long team test (F).
However, these peaks are not observed in 1D TiO2 NRs electrode with or without simulated solar light irradiation. The results demonstrated that 1D TiO2 NRs owns no electrocatalytic activities toward methanol oxidation and introducing of Pt nanoparticles greatly enhanced the electrocatalytic methanol oxidation activities. Besides, the peak (at ca. −0.1 V) current density of the Pt-1D TiO2 NRs is improved to 5.32 mA mg−1 under simulated solar light irradiation, which is 2.86 times than without light irradiation. Then, transient photocurrent density-time curves were measured in alkaline methanol solution for testing the photo-electrocatalytic property of the as-prepared electrodes, as shown in Fig. 6C. Rapid and stable photocurrent responses can be observed for 1D TiO2 NRs electrode and Pt-1D TiO2 NRs electrode. Besides, the value of current density on Pt-1D TiO2 NRs is ca. 4.2 times higher than that of 1D TiO2 NRs when they were exposed onto light. Further, CA, multiple cyclic voltammetries and pulsed potential cycle treatment were performed for testing long-time stability of Pt-1D TiO2 NRs electrode, as shown in Fig. 7. During the process
of 40 0 0 s testing (Fig. 7A and B), negligible current density can be observed on Pt-1D TiO2 NRs electrode and 1D TiO2 NRs electrode in alkaline methanol solution (1.0 M KOH+ 1.0 M CH3 OH) in the dark. However, the current density is greatly improved when the Pt-1D TiO2 NRs electrode and 1D TiO2 NRs electrode are exposed into light. Besides, almost no current density recession observed during 40 0 0 s testing. The Pt-1D TiO2 NRs electrode displays ca. 4 times higher current density for methanol oxidation than that of 1D TiO2 NRs electrode. Next, 250 cycles of cyclic voltammetries were executed for further understanding the photo-assistant electrocatalyst methanol oxidation [9,24]. As shown in Fig. 7C, we displayed the peak current density of each cycle for Pt-1D TiO2 NRs electrode with and without simulated solar light irraditaion. It can be seen that introduction of light significantly improved the peak current density of methanol oxdation. What’s more, after 250 cycles of cyclic voltammetries testing, smaller decreasing of peak current density is occurred under light irradiation (ca. 3.8% loss after 250 cycles), however, the peak current density decreased ca.
Please cite this article as: S. Hu et al., Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.024
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Scheme 1. The mechanisms of the enhanced methanol oxidation activity for Pt-1D TiO2 NRs electrode.
62% loss after 250 cycles in dark. As a result, photo-responsive 1D TiO2 NRs as support for Pt nanoparticles showed more higher and durable methanol oxidation activities. For further verify the nice long-term stability of Pt-1D TiO2 NRs electrode, plused potential cycles which have reported as a more rigorous measurement than CA measurement to probe the durability [32]. Herein, 30 s alternatively at −0.4 V and 0.1 V (vs. Hg/HgO) and 10 0 0 cycles were implemented and the results show in Fig. 7D and E. Compare to the decreasing degree of methanol oxidation peak current density (ca. 73% loss after 10 0 0 cycles) in dark, introduction of light weakened the reduced degree (ca. 42% loss after 10 0 0 cycles). Therefore, we can draw a conclusion that TiO2 NRs as an excellent photocatalyst support provide nice photo-response activity during electro-catalytic methanol oxidation process and well-designed 1D structure can serves as well support for Pt to improve the utilization efficiency along with prolong the service life of electrode. Besides, we tested the XRD pattern of Pt-1D TiO2 NRs electrode after long-term test and compare the result to untested, as shown in Fig. 7F No obvious deviation in peak location occurred after long-team test indicating that the Pt-1D TiO2 NRs electrode possesses crystalline structure stability. The mechanism of the enhanced methanol oxidation activity and durability for Pt-1D TiO2 NRs electrode is proposed in Scheme 1. Traditionally, Pt-based anodes are used commonly due to its high activity toward electrocatalytic methanol oxidation. However, the scarcity and high-cost of Pt resources along with it easily suffers from the reaction intermediate CO poisoning spur us on to greater efforts of improve utilization of Pt. 1D TiO2 NRs as support can not only prevent the agglomeration of Pt active sites and improve the adsorption and desorption of target molecules during methanol oxidation reaction, but also this special high aspect ratio structure retains the dominances of excellent photogenerated carrier transport with low recombination rate that benefits to the activities and stabilities of Pt catalyst. On the other hand, TiO2 can be photoexcited and produce electron-hole pairs under light irradiation (Eq. (3)). The photogenerated electrons lead to the increase in current and the photogenerated holes with strong oxidization result in the exhaustive oxidation of methanol. To be specific, the holes with strong oxidization can directly react with surface adsorbed OH− /H2 O to form strong oxidative hydroxyl radicals (•OH) (Eq. (4)). The adsorbed methanol molecules on the
surface of Pt-1D TiO2 NRs can be also oxidized upon the •OH radicals, leading to a photooxidation reactions, as shown in Eq. (5). Besides, the carbon-containing intermediates are also be oxidized by highly active •OH radicals, resulting in an efficiently poisoning suppression. That is, electro- & photo-catalytic are worked together for increasing the efficiency of methanol oxidation.
Pt − 1D TiO2 NRs + hν −→ eCB− + hVB+
(3)
Pt − 1D TiO2 NRs − OH + h+ −→ Pt − 1D TiO2 NRs + ·OH
(4)
CH3 OH+ · OH −→ ·CH2 (OH ) + H2 O
(5)
·CH2 (OH ) + 5 · OH −→ CO2 + 4H2 O
(6)
Then, EIS applied with different potentials of Pt-1D TiO2 NRs were carried out for further verifying the above electro- & photo-catalytic methanol oxidation mechanism, as shown in Fig. 8. Fig. 8A and B are the Nyquist plots of Pt-1D TiO2 NRs in dark. As the increase of potential from −0.2 V to −0.05 V, the diameters of the impedance arcs increased in the first quadrant and the arc suddenly reversed to second quadrant as the potential continue to rise to 0 V . Impedance plot in the second quadrant indicates the polarization resistance is negative and that is to say the steady state oxidation current decreases with the increase of polarization potential. The reverse phenomenon is due to the oxidation of adsorbed CO intermediate and the recovery of the catalytic sites which has been reported previously [33,34]. At more positive potentials from 0 to 0.2 V (Fig. 8B), the arcs changed to the first quadrant. This is due to the absent of the CO intermediate and Pt might be covered by Pt oxides result in inhibiting the oxidation of methanol [35,36]. The smaller the diameters of the impedance arcs are observed in the all potential range when Pt-1D TiO2 NRs electrode is upon light irradiation (Fig. 8C and D). This result indicates that photo-assistance electrocatalytic methanol oxidation benefits to interfacial charge transfer on Pt-1D TiO2 NRs electrode, results in the smaller charge-transfer resistance. Besides, we tested the EIS of 1D TiO2 NRs electrode with and without light irradiation for comparison, as shown in Fig. S1 in supplementary material. It can be seen that Pt-1D TiO2 NRs electrode owns smaller resistance whether it is under light irradiation or dark condition.
Please cite this article as: S. Hu et al., Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.024
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Fig. 8. EIS of Pt-1D TiO2 NRs without (A and B) and with (C and D) simulated solar light irradiation under different potentials.
4. Conclusions Well-designed special high aspect ratio structure 1D TiO2 NRs electrodes decorated with Pt nanoparticles were synthesized by hydrothermal method and electro-deposited method. With the assistance of light, electrocatalytic methanol oxidation properties and durable performances are improved on Pt-1D TiO2 NRs electrodes in an alkaline solution. The enhanced catalytic performances show that the Pt-1D TiO2 NRs electrodes could act as a promising photoassistant material in the fields of solar and chemical energy conversion and these results also provide new insight into development of electro- & photo-catalytic methanol oxidation electrode for applications in fuel cells. Acknowledgements This study was supported by the Applied Basic Research Programs of Yunnan Science and Technology Department (2017FD085), the Program of Introducing Talents of Kunming University (YJL16003), Scientific Research Programs of Yunnan Provincial Department of Education (2016ZZX181), National Training Programs of Innovation and Entrepreneurship for Undergraduates (201611393001), Construction of Innovative Practice Base for Chemistry and Chemical Engineering Students (HXHG1703). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.08.024. References [1] Arico AS, Srinivasan S, Antonucci V. DMFCs: From fundamental aspects to technology development. Fuel Cells 2011;1:133–61. [2] Liu HS, Song CJ, Zhang L, Wang HJ, Wilkinson DP. A review of anode catalysis in the direct methanol fuel cell. J Power Sources 2006;155:95–110. [3] Cao MN, Wu DS, Cao R. Recent advances in the stabilization of platinum electrocatalysts for fuel-cell reactions. ChemCatChem 2014;6:26–45.
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2017) 1–7
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Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode Sujuan Hu a,b,∗, Baoling Wang a,b, Haidong Ju a,b, Lei Jiang a,b, Yinhai Ma a,b, Yongze Fan a a b
Department of Chemistry, Kunming University, Kunming, 650214, China Yunnan Engineering Technology Research Center for Plastic Films, Kunming University, Kunming, 650214, China
a r t i c l e
i n f o
Article history: Received 5 July 2017 Revised 12 August 2017 Accepted 13 August 2017 Available online xxx Keywords: Methanol oxidation Photo-assisted Photoelectrochemistry 1D TiO2 nanoarays
a b s t r a c t In this paper, one-dimensional (1D) TiO2 nanorods arrays (NRs) are adopted as Pt nanoparticles support for electrocatalytic methanol oxidation. We show that high aspect ratio 1D-TiO2 NRs not only own the advantages of excellent photogenerated carrier transport with low recombination rate but also serve to anchor the Pt nanoparticles for well-dispersing onto the surface of nanorods. The Pt-1D TiO2 NRs electrodes show improved activities toward electrocatalytic methanol oxidation when they exposed into the simulated solar light irradiation. Synergistic effects between electro- & photo-catalytic methanol oxidation as well as the efficient interfacial charger separation in Pt-1D TiO2 NRs electrode contribute to the enhanced electrocatalytic methanol oxidation activities. Our results suggest that 1D-TiO2 NRs can be used as promising photo-assisted Pt noble metal support in direct methanol fuel cell. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Direct methanol fuel cells (DMFCs) are regarded as ideal energy converters for the settled and mobile devices due to their high energy capacity and non-poisonous [1,2]. However, the most negative obstacle for large-scale application of DMFCs is developing highefficiency and durable anodic oxidation materials. As reported, efficient and conventional Pt noble metal is usually applied as anodic oxidation electrocatalyst in electrocatalytic methanol oxidation [3,4]. However, pure Pt electrocatalyst easily suffers from the reaction intermediate CO poisoning during the electrocatalytic process and finally results in inactivated [5–7]. Besides, the cost of pure Pt is too high that not favors to the commercialization of DMFCs. In order to improve utilization of expensive Pt noble metal and maximize the benefits of performances, supports are often adopted for dispersing Pt to gain large specific surface area and more active sites. Photo-excited semiconductors as promising supports for enhancing the electrocatalytic methanol oxidation activities and durable performances of DMFCs have attracted more and more researchers’ attentions since Kamat first employ semiconductor as support for electrocatalytic methanol oxidation under UV-light irradiation [8–12]. Among various semiconductors, TiO2 are always regarded as the most promising materials for the solar energy ∗ Corresponding author at: Department of Chemistry, Kunming University, Kunming, 650214, China. E-mail address: [email protected] (S. Hu).
conversion due to its high stability, low-cost and good scalability for manufacture [13,14]. Multiple dimensions of TiO2 including zero dimensional (0D), one dimensional (1D), two dimensional (2D) and three dimensional (3D) have been fabricated for application in different optoelectronics fields according to their respective unique morphology-depended performance [15–19]. Among them, well-designed 1D TiO2 NRs become the focus of study in electro& photo-catalytic area as their superior photoelectron transfer property. Herein, well-designed 1D TiO2 nanorods arrays were prepared by a simple hydrothermal method and we introduced noble metal Pt nanoparticles onto the surface of 1D TiO2 nanorods arrays by electro-deposition method. With the assistance of light, the electrocatalytic methanol oxidation properties and durable performances are improved on Pt-1D TiO2 NRs electrodes in an alkaline solution. These results display that special high aspect ratio 1D TiO2 NRs structure can serve as well support to improve the utilization efficiency of Pt noble metal along with reduce poisoning. Besides, during electro-catalytic methanol oxidation process, 1D TiO2 NRs as an excellent photocatalyst support can provide abundant photo-excited electrons and holes with the assistance of light. The photogenerated holes with strong oxidization can firstly oxidize OH− /H2 O to hydroxyl radicals (•OH) and then •OH oxidize methanol molecules to CO2 . What’s more, the carbon-containing intermediates that produced during electrocatalytic process can also be oxidized by highly active •OH, resulting in an efficient poisoning suppression. The synergy effects between the electro- & photo-catalytic devote to the improvement of methanol oxidation
http://dx.doi.org/10.1016/j.jtice.2017.08.024 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: S. Hu et al., Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.024
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activities. Our experimental results demonstrate that 1D TiO2 NRs is a promising photocatalyst support in the fields of solar and chemical energy conversion. 2. Experimental 2.1. Preparation of 1D TiO2 NRs electrode A hydrothermal method was adopted to prepare 1D TiO2 NRs electrode [20]. Firstly, 30 mL of deionized water and 30 mL of concentrated HCl were mixed under stirring. Then, 1.2 mL of tetrabutyl titanate was added into the above solution dropwise. After that, the mixture was transferred into Teflon-lined stainless steel autoclave with cleaned FTO conductive glass (1.5 ∗ 2 cm) placed at an angle (70–75°) against the wall of the Teflon-liner, and then the sealed autoclave was placed in an oven at 150 °C for 12 h Finally, TiO2 NRs photo-electrodes were achieved after annealed at 450 °C for 30 min. 2.2. Preparation of Pt-1D TiO2 NRs electrode Pt nanoparticles loaded 1D TiO2 NRs electrode was fabricated by the galvanostatic electro-deposition technique with a current density of 1 mA for 750 s. Herein, we adopted the as-prepared 1D TiO2 NRs electrode as cathode, a platinum plate as anode, an Hg/HgO (1.0 M KOH) as reference electrode and an aqueous solution consisting of 0.2 mM H2 PtCl6 and 0.47 M H2 SO4 as electrolyte. According Faraday’s law, the electric quantity for reducing H2 PtCl6 to Pt metal is 0.75 C and the amount of Pt loading onto the surface of 1D TiO2 NRs electrode is 0.379 mg. 2.3. Characterization The crystal structures of the samples were recorded using an X-ray diffractometer with Cu Ka irradiation (X’ Pert pro. PANalytical B.V). The morphologies of the samples characterized by scanning electron microscopy (JEOL, JSM-IT100). X-ray photoelectron spectroscopy (XPS) was performed on an X-ray photoelectron spectrometer (KRATOS AXIS165). UV–vis diffuse reflectance spectra (DRS) were obtained on a UV-vis spectrophotometer (Shimadzu, UV-2450). 2.4. Electro- & photo-catalytic measurements The electro- & photo-catalytic measurements were performed in a custom-built container with quartz window that confirmed no blocking of simulated solar light by an electrochemical workstation (AUTOLAB, PGSTAT302N). A xenon lamp (100 W) was used as the simulated solar light. The area of photoexcited Pt-1D TiO2 NRs electrodes during the performance test is 1 cm2 . A threeelectrode system consisting of the as-prepared samples as working electrodes, a platinum plate as a counter electrode, an Hg/HgO (1.0 M KOH) as the reference electrode and an aqueous solution consisting of 1.0 M KOH and 1.0 M CH3 OH as electrolyte was used for electro- & photo-catalytic measurements. The cyclic voltammetries (CV) of electrodes were tested in 1.0 M CH3 OH and 1.0 M KOH solution. Chronoamperometry (CA) and photocurrent responses of the electrodes were recorded in 1.0 M CH3 OH and 1.0 M KOH solution without bias potential. The electrochemical active surface area (ECSA) was investigated by CV measurement in 1.0 M KOH solution at a scan rate of 0.1 V/s. Pulsed potential cycles were implemented in 1.0 M CH3 OH and 1.0 M KOH solution with 30 s alternatively at −0.4 V and 0.1 V (vs. Hg/HgO). Electrochemical impedance spectroscopies (EIS) with different potentials were performed from 0.1 to 10,0 0 0 Hz under amplitude of 50 mV in 1.0 M CH3 OH and 1.0 M KOH solution.
Fig. 1. XRD patterns of FTO, 1D TiO2 NRs electrode and Pt-1D TiO2 NRs electrode.
3. Results and discussion Fig. 1 is the XRD patterns of phase structures of F-doped tin oxide (FTO) conductive substrate, 1D TiO2 NRs electrode, and Pt1D TiO2 NRs electrode. FTO substrates show a tetragonal phase of SnO2 (JCPDS no. 41–1445). The peaks located at ca. 36.26°, 41.37°, 63.02° and 69.94° can be indexed to (101), (111), (002) and (301) crystal planes of rutile phase TiO2 (JCPDS no. 21–1276). However, diffraction peaks of Pt are not observed because of its low amount. The morphologies of 1D TiO2 NRs electrode and the Pt-1D TiO2 NRs electrode are shown in Fig. 2. TiO2 NRs are uniformly and vertically aligned onto FTO substrate with an average diameter of ca. 150 nm (Fig. 2A). From Fig. 2B to D, we can see that some nanoparticles successfully decorated onto the surface of 1D TiO2 NRs. Fig. 2E is the cross-section of Pt-1D TiO2 NRs, it can be seen that the length of the nanorods is ca. 4 μm and some nanoparticles loaded onto it. To further confirm the species of these nanoparticles and reveal the chemical composition of Pt-1D TiO2 NRs electrode, XPS spectra of the sample were performed, as shown in Fig. 3. Two peaks at binding energies of 464.78 and 459.06 eV can be attributed to the doublet of Ti 2p1/2 and Ti 2p3/2 , respectively (Fig. 3A) [21,22]. A doublet that originates from the spin-orbit splitting of the 4f 5/2 and 4f 7/2 states of Pt at 74.65 and 71.31 eV further reveals the existence of Pt in Pt-1D TiO2 NRs [23,24]. Two oxygen signals observed at 532.03 and 530.27 eV can be ascribed to the surface-adsorbed oxygen and lattice oxygen [25,26]. The optical properties of the samples were studied by UV-vis diffuse reflectance spectra (Fig 4). For 1D TiO2 NRs electrode, the DRS spectra present steep absorption edges at ca. 415 nm, which can be assigned to the intrinsic band gap of TiO2 (3.2 eV). The absorption intensity is largely enhanced and no extension of absorption edges is observed when Pt nanoparticles loaded onto the surface of 1D TiO2 NRs electrode. For estimating the performance of Pt-1D TiO2 NRs electrode, the electrochemically active surface area (ECSA) was studied by CV measurements in 1.0 M KOH solution at a scan rate of 0.1 V s−1 , as shown in Fig. 5 [27,28]. The ECSA of Pt-1D TiO2 NRs was investigated according the followed Eq. (1). Herein, QH is the amount of electron transfer in H-adsorption and H-desorption peak areas, which can be calculated by Eq. (2).
ECSA =
QH =
QH 0.21 × mPt
Qads + Qdes = 2
(1) Sads V
+ 2
Sdes V
(2)
Where mPt is the quality of Pt (g) loaded onto the electrode, 0.21 (mC cm−2 ) represents the maximum surface charge transferred to Pt during adsorption of a monolayer of H, Sads and Sdes
Please cite this article as: S. Hu et al., Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.024
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Fig. 2. SEM images of 1D TiO2 NRs electrode (A) and Pt-1D TiO2 NRs electrode (B, C, D and E).
Fig. 3. XPS spectra of Ti 2p (A), Pt 4f (B) and O1s (C) of Pt-1D TiO2 NRs electrode.
Fig. 4. UV–vis DRS of 1D TiO2 NRs electrode and Pt-1D TiO2 NRs electrode. Fig. 5. CV of Pt-1D TiO2 NRs electrode in 1.0 M KOH solution.
(A∗ V) are the peak areas of H-adsorption and H-desorption process, respectively, and V (V s−1 ) is the scan rate of CV. As a result, the ECSA of Pt-1D TiO2 NRs is 0.197 m2 g−1 . The electro- & photo-catalytic performances were measured by methanol oxidation reaction in alkaline methanol solution. Fig. 6A and B are the cyclic voltammetry curves of Pt-1D TiO2 NRs electrode and 1D TiO2 NRs electrode with and without simulated solar light irradiation. As can be seen from Fig. 6A, the curves exhibit the typical profile for methanol electrooxidation, including
two strong oxidation peaks at ca. −0.1 and −0.2 V, respectively. The peak formed in the forward scan at ca. −0.1 V is attributed to the oxidative removal of adsorbed/dehydrogenated methanol fractions and CO2 and carbon-containing intermediates are formed during the process. This peak current density is usually used to evaluate the electrocatalytic performance of electrode [29]. The peak in the reverse scan at ca. −0.2 V represents the re-oxidation of carboncontaining intermediates formed in the forward scan [30,31].
Please cite this article as: S. Hu et al., Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.024
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Fig. 6. The 150th CV of Pt-1D TiO2 NRs electrode (A) and 1D TiO2 NRs electrode (B) in alkaline methanol solution (1.0 M KOH+ 1.0 M CH3 OH) with (a) and without (b) simulated solar light irradiation. Transient photocurrent density-time curves (C) of Pt-1D TiO2 NRs electrode and 1D TiO2 NRs electrode.
Fig. 7. CA of Pt-1D TiO2 NRs (A) and 1D TiO2 NRs (B) with (a) and without (b) simulated solar light irradiation. Methanol oxidation peaks current density of 250 cycles CV testing on Pt-1D TiO2 NRs (C). CVs of the Pt-1D TiO2 NRs electrode before (a) and after (b) 10 0 0 pulsed potential cycles with (D) and without (E) simulated solar light irradiation. The XRD patterns of Pt-1D TiO2 NRs electrode before and after long team test (F).
However, these peaks are not observed in 1D TiO2 NRs electrode with or without simulated solar light irradiation. The results demonstrated that 1D TiO2 NRs owns no electrocatalytic activities toward methanol oxidation and introducing of Pt nanoparticles greatly enhanced the electrocatalytic methanol oxidation activities. Besides, the peak (at ca. −0.1 V) current density of the Pt-1D TiO2 NRs is improved to 5.32 mA mg−1 under simulated solar light irradiation, which is 2.86 times than without light irradiation. Then, transient photocurrent density-time curves were measured in alkaline methanol solution for testing the photo-electrocatalytic property of the as-prepared electrodes, as shown in Fig. 6C. Rapid and stable photocurrent responses can be observed for 1D TiO2 NRs electrode and Pt-1D TiO2 NRs electrode. Besides, the value of current density on Pt-1D TiO2 NRs is ca. 4.2 times higher than that of 1D TiO2 NRs when they were exposed onto light. Further, CA, multiple cyclic voltammetries and pulsed potential cycle treatment were performed for testing long-time stability of Pt-1D TiO2 NRs electrode, as shown in Fig. 7. During the process
of 40 0 0 s testing (Fig. 7A and B), negligible current density can be observed on Pt-1D TiO2 NRs electrode and 1D TiO2 NRs electrode in alkaline methanol solution (1.0 M KOH+ 1.0 M CH3 OH) in the dark. However, the current density is greatly improved when the Pt-1D TiO2 NRs electrode and 1D TiO2 NRs electrode are exposed into light. Besides, almost no current density recession observed during 40 0 0 s testing. The Pt-1D TiO2 NRs electrode displays ca. 4 times higher current density for methanol oxidation than that of 1D TiO2 NRs electrode. Next, 250 cycles of cyclic voltammetries were executed for further understanding the photo-assistant electrocatalyst methanol oxidation [9,24]. As shown in Fig. 7C, we displayed the peak current density of each cycle for Pt-1D TiO2 NRs electrode with and without simulated solar light irraditaion. It can be seen that introduction of light significantly improved the peak current density of methanol oxdation. What’s more, after 250 cycles of cyclic voltammetries testing, smaller decreasing of peak current density is occurred under light irradiation (ca. 3.8% loss after 250 cycles), however, the peak current density decreased ca.
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Scheme 1. The mechanisms of the enhanced methanol oxidation activity for Pt-1D TiO2 NRs electrode.
62% loss after 250 cycles in dark. As a result, photo-responsive 1D TiO2 NRs as support for Pt nanoparticles showed more higher and durable methanol oxidation activities. For further verify the nice long-term stability of Pt-1D TiO2 NRs electrode, plused potential cycles which have reported as a more rigorous measurement than CA measurement to probe the durability [32]. Herein, 30 s alternatively at −0.4 V and 0.1 V (vs. Hg/HgO) and 10 0 0 cycles were implemented and the results show in Fig. 7D and E. Compare to the decreasing degree of methanol oxidation peak current density (ca. 73% loss after 10 0 0 cycles) in dark, introduction of light weakened the reduced degree (ca. 42% loss after 10 0 0 cycles). Therefore, we can draw a conclusion that TiO2 NRs as an excellent photocatalyst support provide nice photo-response activity during electro-catalytic methanol oxidation process and well-designed 1D structure can serves as well support for Pt to improve the utilization efficiency along with prolong the service life of electrode. Besides, we tested the XRD pattern of Pt-1D TiO2 NRs electrode after long-term test and compare the result to untested, as shown in Fig. 7F No obvious deviation in peak location occurred after long-team test indicating that the Pt-1D TiO2 NRs electrode possesses crystalline structure stability. The mechanism of the enhanced methanol oxidation activity and durability for Pt-1D TiO2 NRs electrode is proposed in Scheme 1. Traditionally, Pt-based anodes are used commonly due to its high activity toward electrocatalytic methanol oxidation. However, the scarcity and high-cost of Pt resources along with it easily suffers from the reaction intermediate CO poisoning spur us on to greater efforts of improve utilization of Pt. 1D TiO2 NRs as support can not only prevent the agglomeration of Pt active sites and improve the adsorption and desorption of target molecules during methanol oxidation reaction, but also this special high aspect ratio structure retains the dominances of excellent photogenerated carrier transport with low recombination rate that benefits to the activities and stabilities of Pt catalyst. On the other hand, TiO2 can be photoexcited and produce electron-hole pairs under light irradiation (Eq. (3)). The photogenerated electrons lead to the increase in current and the photogenerated holes with strong oxidization result in the exhaustive oxidation of methanol. To be specific, the holes with strong oxidization can directly react with surface adsorbed OH− /H2 O to form strong oxidative hydroxyl radicals (•OH) (Eq. (4)). The adsorbed methanol molecules on the
surface of Pt-1D TiO2 NRs can be also oxidized upon the •OH radicals, leading to a photooxidation reactions, as shown in Eq. (5). Besides, the carbon-containing intermediates are also be oxidized by highly active •OH radicals, resulting in an efficiently poisoning suppression. That is, electro- & photo-catalytic are worked together for increasing the efficiency of methanol oxidation.
Pt − 1D TiO2 NRs + hν −→ eCB− + hVB+
(3)
Pt − 1D TiO2 NRs − OH + h+ −→ Pt − 1D TiO2 NRs + ·OH
(4)
CH3 OH+ · OH −→ ·CH2 (OH ) + H2 O
(5)
·CH2 (OH ) + 5 · OH −→ CO2 + 4H2 O
(6)
Then, EIS applied with different potentials of Pt-1D TiO2 NRs were carried out for further verifying the above electro- & photo-catalytic methanol oxidation mechanism, as shown in Fig. 8. Fig. 8A and B are the Nyquist plots of Pt-1D TiO2 NRs in dark. As the increase of potential from −0.2 V to −0.05 V, the diameters of the impedance arcs increased in the first quadrant and the arc suddenly reversed to second quadrant as the potential continue to rise to 0 V . Impedance plot in the second quadrant indicates the polarization resistance is negative and that is to say the steady state oxidation current decreases with the increase of polarization potential. The reverse phenomenon is due to the oxidation of adsorbed CO intermediate and the recovery of the catalytic sites which has been reported previously [33,34]. At more positive potentials from 0 to 0.2 V (Fig. 8B), the arcs changed to the first quadrant. This is due to the absent of the CO intermediate and Pt might be covered by Pt oxides result in inhibiting the oxidation of methanol [35,36]. The smaller the diameters of the impedance arcs are observed in the all potential range when Pt-1D TiO2 NRs electrode is upon light irradiation (Fig. 8C and D). This result indicates that photo-assistance electrocatalytic methanol oxidation benefits to interfacial charge transfer on Pt-1D TiO2 NRs electrode, results in the smaller charge-transfer resistance. Besides, we tested the EIS of 1D TiO2 NRs electrode with and without light irradiation for comparison, as shown in Fig. S1 in supplementary material. It can be seen that Pt-1D TiO2 NRs electrode owns smaller resistance whether it is under light irradiation or dark condition.
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Fig. 8. EIS of Pt-1D TiO2 NRs without (A and B) and with (C and D) simulated solar light irradiation under different potentials.
4. Conclusions Well-designed special high aspect ratio structure 1D TiO2 NRs electrodes decorated with Pt nanoparticles were synthesized by hydrothermal method and electro-deposited method. With the assistance of light, electrocatalytic methanol oxidation properties and durable performances are improved on Pt-1D TiO2 NRs electrodes in an alkaline solution. The enhanced catalytic performances show that the Pt-1D TiO2 NRs electrodes could act as a promising photoassistant material in the fields of solar and chemical energy conversion and these results also provide new insight into development of electro- & photo-catalytic methanol oxidation electrode for applications in fuel cells. Acknowledgements This study was supported by the Applied Basic Research Programs of Yunnan Science and Technology Department (2017FD085), the Program of Introducing Talents of Kunming University (YJL16003), Scientific Research Programs of Yunnan Provincial Department of Education (2016ZZX181), National Training Programs of Innovation and Entrepreneurship for Undergraduates (201611393001), Construction of Innovative Practice Base for Chemistry and Chemical Engineering Students (HXHG1703). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.08.024. References [1] Arico AS, Srinivasan S, Antonucci V. DMFCs: From fundamental aspects to technology development. Fuel Cells 2011;1:133–61. [2] Liu HS, Song CJ, Zhang L, Wang HJ, Wilkinson DP. A review of anode catalysis in the direct methanol fuel cell. J Power Sources 2006;155:95–110. [3] Cao MN, Wu DS, Cao R. Recent advances in the stabilization of platinum electrocatalysts for fuel-cell reactions. ChemCatChem 2014;6:26–45.
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Please cite this article as: S. Hu et al., Photo-assisted electrocatalytic methanol oxidation based on an efficient 1D-TiO2 nanorods arrays support electrode, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.024