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Catalytic performance of hybrid Pt@ZnO NRs on carbon fibers for methanol electro-oxidation
Catalytic performance of hybrid Pt@ZnO NRs on carbon fibers for methanol electro-oxidation Dongyan Li, Chen Gu, Feng Han, Zhaoxiang Zhong, Weihong Xing PII: DOI: Reference:
S1004-9541(17)30878-9 doi:10.1016/j.cjche.2017.08.013 CJCHE 911
To appear in: Received date: Revised date: Accepted date:
16 July 2017 23 August 2017 24 August 2017
Please cite this article as: Dongyan Li, Chen Gu, Feng Han, Zhaoxiang Zhong, Weihong Xing, Catalytic performance of hybrid Pt@ZnO NRs on carbon fibers for methanol electro-oxidation, (2017), doi:10.1016/j.cjche.2017.08.013
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ACCEPTED MANUSCRIPT Energy, Resources and Environmental Technology
Catalytic performance of hybrid Pt@ZnO NRs on carbon fibers ☆
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for methanol electro-oxidation
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Dongyan Li1,2, Chen Gu1, Feng Han1, Zhaoxiang Zhong1*, Weihong Xing1*
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Supported by the National Key R&D Program (2016YFC0204000), the National Natural Science Foundation of China (U1510202), and the Jiangsu Province Scientific Supporting Project (BE2014717, and BE2015023).
*Corresponding author. Tel: +86-25-83172163; Fax: +86-25-83172292; E-mail
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address: [email protected] (Z.X. Zhong), [email protected] (W.H. Xing)
Abstract: A novel Pt@ZnO nanorod/carbon fiber (NR/CF) with hierarchical structure was prepared by atomic layer deposition combined with hydrothermal synthesis and
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magnetron sputtering(MS). The morphology of Pt changes from nanoparticle to nanorod bundle with controlled thickness of Pt between 10 and 50 nm. Significantly, with the increase of voltage from 0 to 0.6 V (vs. standard calomel electrode), the prompt photocurrent generated on ZnO NR/CF increases from 0.235 to 0.725 mA. Besides, the Pt@ZnO NR/CF exhibited higher electrochemical active surface area (ECSA) value, better methanol oxidation ability and CO tolerance than Pt@CF, which demonstrated the importance of the multifunctional ZnO support. As the thickness of Pt increasing from 10 to 50 nm, the ECSA values were improved proportionally, leading to the improvement of methanol oxidation ability. More importantly, UV radiation increased the density of peak current of Pt@ZnO NR/CF towards methanol oxidation by additional 42.4 %, which may be due to the synergy catalysis of UV light and electricity.
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Keywords: Carbon fibers; ZnO nanorods; Pt; Magnetron sputtering; Methanol
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electro-oxidation
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1. Introduction
The use of fossil fuels has caused alarming environmental problems, including global warming, greenhouse effect, and air pollution.[1] To meet increasing energy
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demand without further impacting on the environment, the search of environmentally friendly alternative energy sources such as solar, nuclear and biology are imperative.
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A fuel cell converts chemical energy into electricity through an electrochemical reaction with higher energy conversion efficiency than conventional fossil fuel combustion.[2] Furthermore, fuel cell produces water as the only by-product,
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eliminating the pollution caused by fuel burning.[3,4] Among different type of fuel
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cells, direct methanol fuel cell (DMFC) has attracted more and more attention because of its abundant fuel source, high energy efficiency, low cost and ease to transportation
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and storage.[5, 6]
Pt catalysts own the highest catalytic activity for methanol electrooxidation. However, Pt catalysts has low CO tolerance.[7,8] Numerous studies were conducted
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in search of a solution to the low CO tolerance of Pt catalysts, such as alloying with other metals, combined function with conductive substrates, and metal oxide coupling.[9-11] The use of metal oxides such as ZnO and TiO2 has shown promising results in controlling the poisoning of Pt.[12,13] Furthermore, ZnO and TiO2 also generate photocurrent under UV illumination using composite catalysts when coupled with Pt, further enhancing methanol electro-oxidation ability. Carbon fibers (CFs) have many advantages in carrier material due to its superior electro conductivity, good chemical resistance and high mechanical strength.[14,15] Kong et al. modified carbon fiber with grown CoSe2 nanoparticles as three -dimensional electrodes, exhibiting excellent catalytic activity for hydrogen evolution reaction. Huang et al.[16] employed knitted carbon fibers for supercapacitors with
ACCEPTED MANUSCRIPT improved capacity, energy and power density. However, the low specific surface area of carbon fibers resulted in low catalyst loading.[17] In this study, a novel Pt@ZnO nanorod/carbon fibers (NR/CFs) with hierarchical
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structure with enhanced methanol electric-oxidation was fabricated. Although both ZnO and TiO2 have wide band gap, ZnO processes two to three orders of magnitude
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higher electron migration rate (115-155 cm2·V-1·s-1)[18] and exhibits higher exciton binding energy of around 60 meV, which could generates higher photocurrent under UV illumination. Besides, the existence of ZnO with high specific surface area which
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can efficiently increase the loading of catalytic composition and enlarge contact areas with electrolytes.[19,20] Therefore, it is significant to synthesize this multifunctional
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materials. To prepare the novel structure, ZnO nanorods were hydrothermally grown on CF after the immobilization of ZnO seeds on CF via atomic layer deposition
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(ALD). Then, Pt were dispersed on the surface of ZnO nanorods via MS. The
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coverage, size and chemical state of Pt can also be controlled precisely by MS. In the end, the crystal phase composition, morphology and elements of Pt@ZnO NR/CF
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were characterized by XRD, SEM and EDS. The prompt current during on/off switching of the UV lamp was recorded to investigate the photoelectric response of ZnO NR/CF. Chronoamperometry and Cyclic voltammetry (CV) were conducted in a
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standard three-electrode cell at room temperature to evaluate the effect of thickness of sputtered Pt and ZnO secondary support on the activity of methanol oxidation and the stability of the catalyst.
2. Experimental Section 2.1. Raw Materials Carbon fibers (T300-3k, Toray) with a diameter of 7 μm and length of 5 μm. The source of zinc and oxygen for ALD were diethyzinc(Zn(C2H5)2) and deionized water. Zinc nitrate hydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (HTMA, C6H12N4) were provided by Shanghai Lingfeng Co. Pt catalysts was purchased from Beijing Gaodewei metal Co. All chemical reagents were analytical grade in the experiment. 2.2. Hydrothermal synthesis of ZnO nanorods
ACCEPTED MANUSCRIPT Fig. 1 shows the synthesis route of Pt@ZnO NR/CFs. Firstly, CFs were placed in the ALD reactor to deposit ZnO seeding layers, and then the chamber was preheated to 130 °C with both sides exposing to N2. The ALD process will start when the
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vacuum in the chamber reached 1 Torr. Two precursor vapors of DEZ and water were heated to 40 °C and 60 °C, which were alternately delivered into the reaction chamber.
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To ensure the precursors enter into the samples thoroughly, exposure mode was set in the experimental section. Under the condition of N2 flow rate of 20 sccm and temperature of 130 °C, CFs were suffered over 300 cycles. Secondly, hydrothermal
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synthesis reactor for growing ZnO NRs was maintained at 90 oC for 3 h, which contained 5 mmol·L-1 zinc nitrate. The prepared ZnO NRs/CFs were washed several
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times with water and dried at 70 oC for 2 h.
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Fig. 1 Synthesis route of Pt@ZnO NRs/CFs composites.
2.3. Pt nanoparticles loading Pt
nanoparticles
loading
were
conducted
via
magnetron
sputtering
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(VTC-600-2HD, MTI, Shenyang) with a circular Pt target at room temperature. During the process of sputtering, the sample table was acted as the support for ZnO NRs/CFs substrates with rotation speed of 20 r·min-1 to guarantee the homogeneous dispersion of Pt nanoparticles. The working pressure and power of the apparatus chamber were set as 3.0 Pa and 30 W, respectively. Besides, the layer thickness of Pt nanoparticles (10 nm, 30 nm, and 50 nm) was controlled by thickness detector, which are defined as Pt@ZnO NR/CF-10, Pt@ZnO NR/CF-30, Pt@ZnO NR/CF-50, respectively. 2.4. Characterizations The phase composition was detected by X-ray diffractometer (XRD; D8-Advance, Bruker, Germany) with Cu Kα radiation (wavelength of 0.154 nm), operated at 15 mA, 40 kV, and a step width of 0.02º with a scanning range of 10–80º.
ACCEPTED MANUSCRIPT The microstructure was observed by field emission scanning electron microscope (FESEM, HitachiS-4800, Japan). 2.5. Measurements of photoelectrochemical response
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A three-electrode cell with CHI760E electrochemical system (Chenhua
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Instruments, Co., Shanghai) in which 24 W UV lamp (LTD, =365 nm) was used in
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photoelectrochemical measurements served as light source. The working electrode was 10 nm Pt@ZnO NRs/CFs. Linear sweep voltammetry (LSV) was used to record the current-voltage curves. First, the photoresponse was conducted by switching light
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lamp in the process of total 5 cycles and each cycle was set in the absence of light illumination from 0-40 s and then next 40-90 s under the continuous illumination of
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light. Finally, the lamp was turned off at 90 s measuring at 0 V (vs. SCE). Besides, we also investigated the effect of the applied voltage (0, 0.1, 0.3 and 0.6 V vs. SCE) on
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response current according to the experimental procedures mentioned above.
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2.6. Electrochemical and Photoelectrochemical reactions of methanol-oxidation The electrochemical performance of the prepared samples was conducted by a
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traditional three-electrode cell in electrochemical workstation. Pt was acted as counter electrode and saturated calomel electrode (SCE) was functioned as the reference electrode. Pt@ZnO NRs/CFs with thickness of 10 nm, 30 nm, 50 nm were used as
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working electrode. Cyclic voltammetry method was conducted to characterize electrochemical active surface area (ECSA) and catalytic capability. In the methanol electro-oxidation experiment, the condition of electrode was 0.5 mol·L-1 H2SO4 and the mixed solution of 1 mol·L-1 CH3OH and 0.5 mol·L-1 H2SO4, respectively. In addition, chronoamperometry experiments was applied to characterize the stability of as-prepared working electrode and the scan rate was set as 30 mV·s-1 in all electrochemical experiments. A UV lamp (LTD, 24 W, =365 nm) was used as light source to test the effect of light source on the methanol–oxidation reaction performance of samples.
3. Results and Discussions 3.1. Microstructure and morphology
ACCEPTED MANUSCRIPT X-ray diffraction patterns of the samples are shown in Fig. 2a. A obvious peak at 25° represented the existence of integral graphite structure.[21] This indicated that CFs deposited on ZnO seeds by ALD and ZnO NR/CF showed three main peaks
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appearing at angles 2θ= 31.94°, 34.60° and 36.42°, which could be attributed to (100), (002) and (101) planes of hexagonal wurtzite (JCPDS36-1451).[22.23] The pattern of
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sample of Pt@ZnO NR/CFs with 10 nm Pt coating were no Pt peaks, which may be due to the small size and amount of Pt nanoparticles.[24] However, the crystalline phase peaks of Pt gradually increased with thickness of Pt. At 50 nm thick, two
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diffraction peaks at 39.5° and 46.8° were detected, which corresponded to (111) and (200) planes of Pt, respectively.[25] The XRD analysis results confirmed the loading
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of Pt nanoparticles on the surface of ZnO NRs. Fig. 2b showed the the EDS spectrum of Pt@ZnO NRs/CFs sample. It was used to further prove the existence of Pt on the
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surface of ZnO NRs. From Fig. 2b, it can be obviously seen that the peak of Pt
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element existed on the surface of Pt@ZnO NRs/CFs. Furthermore, it also had C, Zn
Intensity (a.u.)
50nm Pt@ZnO NRs/CFs
30nm Pt@ZnO NRs/CFs
30
10nm Pt@ZnO NRs/CFs
ZnO NRs/CFs
20
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b
200
Zn Pt
100 002 101 111
a
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and O element.
ALD ZnO/CFs CFs
40
50
2(o)
60
70
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Fig. 2 The XRD spectrums of Pt@ZnO NRs/CFs with different stages and Pt thickness of 10, 30 and 50 nm(a), and the EDS spectrum of Pt@ZnO NRs/CFs sample(b).
Fig. 3 shows the SEM images of CFs, ZnO NR/CFs and Pt@ZnO NR/CFs with different thickness of Pt. The initial morphology of pure CFs is shown in Fig. 3a. After ALD process, the CF substrates are covered with a uniform thin layer of ZnO seed (Fig. 3b). As shown in Fig. 3(c-d), it can clearly see that ZnO NRs were on the
ACCEPTED MANUSCRIPT surface of the CFs. From Fig. 3(e-f), Pt@ZnO NR/CF-10 shows similar morphology as that of ZnO NRs/CFs without observing the Pt nanoparticles deposited. MS has been demonstrated to be an effective and easy-controlled method to deposit coating
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layers on various substrates. With increasing loading of sputtered Pt, the surface of ZnO NRs becomes rougher, as shown in Fig. 3(g-j). From Fig. 3(i-j), when the
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deposited thickness of Pt reaching to 50 nm, the original surface of ZnO NR were obviously wrapped by small Pt nanoparticles. According to BET test data published on my previous paper, ZnO NRs acted as bonding layer between carbon fiber
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substrate and Pt nanoparticles, which provided higher active sites and contact
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areas.[20]
Fig. 3 The micromorphology of the prepared samples (a is original CFs; b is CFs deposited with a
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ZnO seed layer; c and d are the sample of ZnO NRs/CFs at different magnification; e, f are 10 nm Pt@ZnO NRs/CFs; g, h are 30 nm Pt@ZnO NRs/CFs; i, j are 50 nm Pt@ZnO NRs/CFs).
3.2. Photoelectrochemical response of ZnO NR/CF Photoelectrochemical response of ZnO NRs/CFs were investigated by using LSV method to record current-voltage (I-V) curves from 0 to 0.8 V with the scan rate of 10 mV·s-1(Fig. 4). The response current gradually increased along with applied potentials from 0 and 0.8 V regardless of light irradiation. However, the current in the presence of light was consistently higher than that in the dark at the same potential. This increment of current may be ascribed to the generation of photocurrent.[26]
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Fig. 4 The I-V curves of ZnO NRs/CFs in dark and light illumination.
Fig. 5 The On-off I-t curves of ZnO NRs/CFs at 0 V potential.
ZnO is a photosensitive material. Experiment with periodic irradiation was
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performed to investigate the photosensitive characteristics of ZnO by recording the photocurrent-time (I-t) data. At 0 V vs. SCE, the response current remains at -0.020 mA in the dark during the first 40 s (Fig. 5). When UV light illumination was switched on, the response photocurrent immediately changed from -0.020 to 0.30 mA. Besides, the photocurrent maintained about 0.30 mA under UV light illumination for 50 s. When the light was switched off, the current returned to the -0.020 mA in the dark. The generation and disappearance of photocurrent is rapid in the periodic irradiation experiment, which suggests that ZnO have excellent response to photoelectricity. The consistent photocurrent response under periodic irradiation indicates the stability of ZnO NR/CF.
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Fig. 6 The on-off I-t curves of ZnO NRs/CFs at different potentials (0, 0.1, 0.3, 0.6 V).
Fig. 6 shows the effects of applied potentials on the photocurrent generation of
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ZnO NR/CF. As shown in Fig. 6, the as-prepared sample of ZnO NR/CF showed increasing sensitivity to light irradiation with applied potential. However, ZnO NR/CF showed remarkable response regardless of the applied potentials. When the applied
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potential increased from 0 V to 0.6 V, the photocurrent generated under light
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irradiation increased from 0.235 to 0.725 mA. This is because the higher potential promotes more effective separation and transfer of photo-generated electron-hole while
suppressing
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pairs
electron
recombination,
leading
to
increasing
photocurrent.[27]
3.3. Role of ZnO Secondary Carrier Layer in Electro-Oxidation of Methanol
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Cyclic voltammetry (CV) is a common method to calculate ECSA of Pt catalyst. The actual value of ECSA not only revealed the number of active sites of the catalyst, but also estimated the catalytic performance and the choice of suitable substrates. The ECSA were calculated by the area ratio of hydrogen adsorption/desorption peaks in the range of negative potential.[28] CV diagram of Pt@CF-10 and Pt@ZnO NR/CF-10 catalyst were measured in 0.5 mol·L-1 H2SO4 solution as shown in Fig. 7. Both catalysts had distinct hydrogen adsorption/desorption peaks between -0.2 and 𝑄
0.1 V. According to equation ECSA = 210 H×𝑃𝑡 ,[29] the ECSA (cm2·g-1)values of Pt@CF-10 and Pt@ZnO NR/CF-10 catalysts are 10.78 m2·g-1 and 23.19 m2·g-1, respectively. The higher ECSA value of Pt@ZnO NR/CF-10 suggested that the existence of ZnO secondary carrier layer, which improved the distribution and the size
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improvement of catalytic efficiency.
Fig. 7 The electrochemically active surface areas measured in 0.5 M H2SO4 of 10 nm Pt@CFs and
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Pt@ZnO NRs/CFs with 30 mV·s-1 sweep rate.
Fig. 8 showed the CV curves of the catalytic activity for electron-oxidation
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methanol of ZnO NR/CFs, Pt@CFs-10 and Pt@ZnO NR/CFs-10. As shown in the Fig.
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8, it can be seen that CV curves of the three catalysts showed two similar current peaks located at the potentials of about 0.7 V in forward scan and 0.5 V in backward
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scan. The current peaks in 0.7 V and 0.5 V corresponded to the peaks of methanol oxidation (If) and CO oxidation peak (Ib), respectively. The peak current density at about 0.7 V was a crucial parameter to represent the catalytic ability for methanol
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oxidation.[30] Otherwise, Fig. 8 also showed that the peak of current density (If) of three catalysts resided in accordance with the following order: Pt@ZnO NR/CFs-10＞ Pt@CFs-10 ＞ ZnO NR/CFs. Pt@ZnO NR/CFs-10 possessed the highest current density for methanol oxidation because of the synergistic effect between the Pt nanoparticles and the ZnO NRs. Moreover, the ratio of If and Ib was an indication of catalyst tolerance towards CO, where higher ratio of If/Ib indicates higher CO tolerance of catalysts.[31] The calculated If/Ib value of Pt@ZnO NR/CFs-10 and Pt@CFs-10 were 1.72 and 1.41, respectively. The increased ratio of If/ Ib revealed that the existence of ZnO enhanced the CO tolerance by oxidizing CO in lower potential.[32] In summary, the existence of ZnO secondary carrier layer effectively improved the ECSA, the methanol catalytic oxidation activity and the CO tolerance.
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Fig. 8 Cyclic voltammograms of ZnO NRs/CFs, 10 nm Pt@CFs, 10 nm Pt@ZnO NRs/CFs in 0.5 mol·L-1 H2SO4 and 1 mol·L-1 methanol with 30 mV·s-1 sweep rate.
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3.4. Methanol Electro-Oxidation by Pt@ZnO NRs/CFs electrode with different Pt loadings
Fig. 9 showed the CVs curves of catalysts with different thickness of Pt (10, 30,
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and 50 nm) in 0.5 mol·L-1 H2SO4 solution between -0.2 and +1.2 V. As shown in Fig.
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9, the Pt@ZnO NRs/CFs with 10, 30 and 50 nm thickness all exhibited hydrogen adsorption/desorption peaks in scan range of -0.2–0.1 V. Calculated results indicated
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that ECSA of the three prepared samples of 10, 30 and 50 nm Pt@ZnO NRs/CFs are 23.19 m2·g-1，29.73 m2·g-1 and 43.65 m2·g-1, respectively. The results further indicated that the ECSA values of catalysts increases with the increasing loading of Pt,[33] thus
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improving the ability of methanol electro-oxidation.
Fig. 9 The electrochemically active surface areas measured in 0.5 mol·L-1 H2SO4 of Pt@ZnO NRs/CFs with different thickness of Pt (10, 30 and 50 nm) at a sweep rate of 30 mV·s-1.
ACCEPTED MANUSCRIPT Fig. 10 showed the CVs curves of samples with various deposited Pt thickness (10, 30 and 50 nm) in mixed solution between 0 to 1 V vs. As shown in Fig. 10, the forward anodic peak current (If) increased with the loading of Pt. The result was
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consistent with ECSA experiment (Fig. 9). The distribution, grain size and morphology of Pt component had direct effect on the catalytic performance. From Fig.
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10, the sample of Pt@ZnO NR/CF-50 had the highest forward anodic peak current density (If), which may be due to the unique nanorod bundles morphology of Pt caused by pressure and high temperature in the process of MS. The changed
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morphology of Pt could caused much great improvement in activity of methanol oxidation activity. For example, the peak current density of Pt@ZnO NR/CFs-50 was
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approximately 3.75 times than that of Pt@ZnO NR/CFs-30. However, the If value of Pt@ZnO NR/CFs-30 is only 2.5 times than Pt@ZnO NR/CFs-10. This enhancement
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may be due to the aggregation and growth of Pt cores resulting in the increase of
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contact area and active sites for methanol oxidation. However, the improved loadings of Pt bring about the decrement of CO tolerance of catalysts due to the easiness of Pt
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catalyst poisoning. [34]
Fig. 10 The CV diagrams of Pt@ZnO NRs/CFs with various thickness of Pt (10, 30, 50 nm) in 0.5 mol·L-1 H2SO4 and 1 mol·L-1 methanol with 30 mV·s-1 sweep rate .
Chronoamperometric analytic approach is usually used to investigate the electric stability of catalysts. The current density was recorded for 1800 s at 0.6 V (vs. SCE) with 30 mV·s-1 scan rate in electrolyte containing 0.5 mol·L-1 H2SO4 and 1 mol·L-1 methanol. At 0.6 V constant potential, the current density of ZnO NR/CFs,
ACCEPTED MANUSCRIPT Pt@CFs-10 and Pt@ZnO NR/CFs with different thickness of Pt (10, 30 and 50 nm) decreased with time (Fig. 11). This result revealed the formation of reaction intermediates and the oxide of Pt on the basis of proceeding in methanol oxidation
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reaction.[35] Pt@ZnO NR/CFs-50 consistently showed the highest current density among all samples tested. In addition, Pt@ZnO NR/CFs-10 possessed higher current
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density than Pt@CFs-10, which indicated the existence of ZnO secondary support
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was benefit to improve the CO tolerance and activity of Pt.
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Fig. 11 The recording of chronoamperometric curves at 0.6 V versus SCE of ZnO NRs/CFs, 10nm Pt@CFs, 10nm Pt@ZnO NRs/CFs, 30nm Pt@ZnO NRs/CFs and 50nm Pt@ZnO NRs/CFs.
3.5. Photoassosted Methanol Electro-Oxidation
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Fig. 12 showed the CV curves of Pt@ZnO NR/CFs-30 for methanol electro-oxidation under light illumination. As shown in Fig. 12, the peak current density in forward scan was 6.55 mA·cm-2 under UV light, which was approximately 1.42 times than in the dark (4.6 mA·cm-2). This improvement of peak current density may be attributed to the occurrence of light-assisted methanol electro-oxidation. The increase of peak current density by 42 % was attributed to the synergistic photocurrent produced on ZnO semiconductor.[36] Under UV light, the electrons on ZnO semiconductor transferred from valance band to conduct band, which facilitate the formation of electron-hole pairs, thus generating additional photocurrent.[26]
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Fig. 12 CV diagrams of methanol electrochemical oxidation at the electrode of 30 nm Pt@ZnO NRs/CFs with or without UV light.
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4. Conclusions
Firstly, a novel multilevel photocatalyst of Pt@ZnO NR/CFs was fabricated by ALD method united with hydrothermal synthesis and magnetron sputtering. With
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increasing loadings of Pt, the existing morphology of Pt changed from nanoparticles
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to nanorod bundles. ZnO NR/CFs is an excellent photosensitive material as
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demonstrated by the excellent and stable photoelectric response. The presence of ZnO can efficiently enlarge the ECSA of catalyst, promote the efficiency of methanol oxidation and improve CO tolerance on methanol oxidation of catalysts. In addition,
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the changed morphology of Pt could cause much great improvement of methanol oxidation activity. The peak current density of methanol oxidation on Pt@ZnO NR/CFs-30 under UV illumination increased significantly compared with dark environment. The synergistic catalysis of light and electricity resulted in the improvement of current for methanol oxidation. Most importantly, the novel catalysts show good application promising due to its high catalytic efficiency and tenacious CO tolerance.
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ACCEPTED MANUSCRIPT photocatalysis”, Ind. Eng. Chem. Res., 55 (22), 6413-6421 (2016). [23] Banerjee,
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photoluminescence studies on ZnO nanowires”, Nanotechnology, 15 (3), 404-409
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[24] Rosario, A.V., Pereira, E.C., “The role of Pt addition on the photocatalytic
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activity of TiO2 nanoparticles: The limit between doping and metallization”, Appl. Catal., B., 144, 840-845 (2014).
[25] Peng, X., Zhao, S., Omasta, T.J., “Activity and durability of Pt-Ni nanocage
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electocatalysts in proton exchange membrane fuel cells”, Appl. Catal., B., 203, 927-935 (2017).
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[26] Lee, M.K., Tu, H.F., “Au-ZnO and Pt-ZnO films prepared by electrodeposition as photocatalysts”, J. Electrochem. Soc., 155 (12), 758-762 (2008).
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[28] Sharma, S., Ganguly, A., Papakonstantinou, P., “Rapid microwave synthesis of CO tolerant reduced graphene oxide-supported platinum electrocatalysts for oxidation of Methanol”, J. Phys. Chem. C., 114 (45), 19459-19466 (2010).
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[29] Girishkumar, G., Rettker, M., Underhile, R., “Single-wall carbon nanotube-based proton exchange membrane assembly for hydrogen fuel cells”, Langmuir, 21 (18), 8487-8494 (2005). [30] Li, Y., Gao, W., Ci, L., “Catalytic performance of Pt nanoparticles on reduced graphene oxide for methanol electro-oxidation”, Carbon, 48 (4), 1124-1130 (2010). [31] Lin, Y.H., Cui, X.L., Yen, C.H., “PtRu/carbon nanotube nanocomposite synthesized in supercritical fluid: a novel electrocatalyst for direct methanol fuel cells”, Langmuir, 21 (24), 11474-11479 (2005). [32] Drew, K., Girishkumar, G., Vinodgopal, K., “Boosting fuel cell performance with a semiconductor photocatalyst: TiO2/Pt-Ru hybrid catalyst for methanol oxidation”, J. Phys. Chem. B., 109 (24), 11851-11857 (2005). [33] Oliveira, N.A., Dias, R.R., Tusi, M.M., “Electro-oxidation of methanol and
ACCEPTED MANUSCRIPT ethanol using PtRu/C, PtSn/C and PtSnRu/C electrocatalysts prepared by an alcohol-reduction process”, J. Power Sources., 166 (1), 87-91 (2007). [34] Lin, C.T., Huang, H.J., Yang, J.J., “A simple fabrication process of Pt-TiO2 hybrid
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S1004-9541(17)30878-9 doi:10.1016/j.cjche.2017.08.013 CJCHE 911
To appear in: Received date: Revised date: Accepted date:
16 July 2017 23 August 2017 24 August 2017
Please cite this article as: Dongyan Li, Chen Gu, Feng Han, Zhaoxiang Zhong, Weihong Xing, Catalytic performance of hybrid Pt@ZnO NRs on carbon fibers for methanol electro-oxidation, (2017), doi:10.1016/j.cjche.2017.08.013
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ACCEPTED MANUSCRIPT Energy, Resources and Environmental Technology
Catalytic performance of hybrid Pt@ZnO NRs on carbon fibers ☆
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for methanol electro-oxidation
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Dongyan Li1,2, Chen Gu1, Feng Han1, Zhaoxiang Zhong1*, Weihong Xing1*
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Supported by the National Key R&D Program (2016YFC0204000), the National Natural Science Foundation of China (U1510202), and the Jiangsu Province Scientific Supporting Project (BE2014717, and BE2015023).
*Corresponding author. Tel: +86-25-83172163; Fax: +86-25-83172292; E-mail
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address: [email protected] (Z.X. Zhong), [email protected] (W.H. Xing)
Abstract: A novel Pt@ZnO nanorod/carbon fiber (NR/CF) with hierarchical structure was prepared by atomic layer deposition combined with hydrothermal synthesis and
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magnetron sputtering(MS). The morphology of Pt changes from nanoparticle to nanorod bundle with controlled thickness of Pt between 10 and 50 nm. Significantly, with the increase of voltage from 0 to 0.6 V (vs. standard calomel electrode), the prompt photocurrent generated on ZnO NR/CF increases from 0.235 to 0.725 mA. Besides, the Pt@ZnO NR/CF exhibited higher electrochemical active surface area (ECSA) value, better methanol oxidation ability and CO tolerance than Pt@CF, which demonstrated the importance of the multifunctional ZnO support. As the thickness of Pt increasing from 10 to 50 nm, the ECSA values were improved proportionally, leading to the improvement of methanol oxidation ability. More importantly, UV radiation increased the density of peak current of Pt@ZnO NR/CF towards methanol oxidation by additional 42.4 %, which may be due to the synergy catalysis of UV light and electricity.
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Keywords: Carbon fibers; ZnO nanorods; Pt; Magnetron sputtering; Methanol
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electro-oxidation
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1. Introduction
The use of fossil fuels has caused alarming environmental problems, including global warming, greenhouse effect, and air pollution.[1] To meet increasing energy
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demand without further impacting on the environment, the search of environmentally friendly alternative energy sources such as solar, nuclear and biology are imperative.
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A fuel cell converts chemical energy into electricity through an electrochemical reaction with higher energy conversion efficiency than conventional fossil fuel combustion.[2] Furthermore, fuel cell produces water as the only by-product,
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eliminating the pollution caused by fuel burning.[3,4] Among different type of fuel
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cells, direct methanol fuel cell (DMFC) has attracted more and more attention because of its abundant fuel source, high energy efficiency, low cost and ease to transportation
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and storage.[5, 6]
Pt catalysts own the highest catalytic activity for methanol electrooxidation. However, Pt catalysts has low CO tolerance.[7,8] Numerous studies were conducted
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in search of a solution to the low CO tolerance of Pt catalysts, such as alloying with other metals, combined function with conductive substrates, and metal oxide coupling.[9-11] The use of metal oxides such as ZnO and TiO2 has shown promising results in controlling the poisoning of Pt.[12,13] Furthermore, ZnO and TiO2 also generate photocurrent under UV illumination using composite catalysts when coupled with Pt, further enhancing methanol electro-oxidation ability. Carbon fibers (CFs) have many advantages in carrier material due to its superior electro conductivity, good chemical resistance and high mechanical strength.[14,15] Kong et al. modified carbon fiber with grown CoSe2 nanoparticles as three -dimensional electrodes, exhibiting excellent catalytic activity for hydrogen evolution reaction. Huang et al.[16] employed knitted carbon fibers for supercapacitors with
ACCEPTED MANUSCRIPT improved capacity, energy and power density. However, the low specific surface area of carbon fibers resulted in low catalyst loading.[17] In this study, a novel Pt@ZnO nanorod/carbon fibers (NR/CFs) with hierarchical
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structure with enhanced methanol electric-oxidation was fabricated. Although both ZnO and TiO2 have wide band gap, ZnO processes two to three orders of magnitude
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higher electron migration rate (115-155 cm2·V-1·s-1)[18] and exhibits higher exciton binding energy of around 60 meV, which could generates higher photocurrent under UV illumination. Besides, the existence of ZnO with high specific surface area which
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can efficiently increase the loading of catalytic composition and enlarge contact areas with electrolytes.[19,20] Therefore, it is significant to synthesize this multifunctional
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materials. To prepare the novel structure, ZnO nanorods were hydrothermally grown on CF after the immobilization of ZnO seeds on CF via atomic layer deposition
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(ALD). Then, Pt were dispersed on the surface of ZnO nanorods via MS. The
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coverage, size and chemical state of Pt can also be controlled precisely by MS. In the end, the crystal phase composition, morphology and elements of Pt@ZnO NR/CF
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were characterized by XRD, SEM and EDS. The prompt current during on/off switching of the UV lamp was recorded to investigate the photoelectric response of ZnO NR/CF. Chronoamperometry and Cyclic voltammetry (CV) were conducted in a
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standard three-electrode cell at room temperature to evaluate the effect of thickness of sputtered Pt and ZnO secondary support on the activity of methanol oxidation and the stability of the catalyst.
2. Experimental Section 2.1. Raw Materials Carbon fibers (T300-3k, Toray) with a diameter of 7 μm and length of 5 μm. The source of zinc and oxygen for ALD were diethyzinc(Zn(C2H5)2) and deionized water. Zinc nitrate hydrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (HTMA, C6H12N4) were provided by Shanghai Lingfeng Co. Pt catalysts was purchased from Beijing Gaodewei metal Co. All chemical reagents were analytical grade in the experiment. 2.2. Hydrothermal synthesis of ZnO nanorods
ACCEPTED MANUSCRIPT Fig. 1 shows the synthesis route of Pt@ZnO NR/CFs. Firstly, CFs were placed in the ALD reactor to deposit ZnO seeding layers, and then the chamber was preheated to 130 °C with both sides exposing to N2. The ALD process will start when the
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vacuum in the chamber reached 1 Torr. Two precursor vapors of DEZ and water were heated to 40 °C and 60 °C, which were alternately delivered into the reaction chamber.
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To ensure the precursors enter into the samples thoroughly, exposure mode was set in the experimental section. Under the condition of N2 flow rate of 20 sccm and temperature of 130 °C, CFs were suffered over 300 cycles. Secondly, hydrothermal
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synthesis reactor for growing ZnO NRs was maintained at 90 oC for 3 h, which contained 5 mmol·L-1 zinc nitrate. The prepared ZnO NRs/CFs were washed several
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times with water and dried at 70 oC for 2 h.
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Fig. 1 Synthesis route of Pt@ZnO NRs/CFs composites.
2.3. Pt nanoparticles loading Pt
nanoparticles
loading
were
conducted
via
magnetron
sputtering
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(VTC-600-2HD, MTI, Shenyang) with a circular Pt target at room temperature. During the process of sputtering, the sample table was acted as the support for ZnO NRs/CFs substrates with rotation speed of 20 r·min-1 to guarantee the homogeneous dispersion of Pt nanoparticles. The working pressure and power of the apparatus chamber were set as 3.0 Pa and 30 W, respectively. Besides, the layer thickness of Pt nanoparticles (10 nm, 30 nm, and 50 nm) was controlled by thickness detector, which are defined as Pt@ZnO NR/CF-10, Pt@ZnO NR/CF-30, Pt@ZnO NR/CF-50, respectively. 2.4. Characterizations The phase composition was detected by X-ray diffractometer (XRD; D8-Advance, Bruker, Germany) with Cu Kα radiation (wavelength of 0.154 nm), operated at 15 mA, 40 kV, and a step width of 0.02º with a scanning range of 10–80º.
ACCEPTED MANUSCRIPT The microstructure was observed by field emission scanning electron microscope (FESEM, HitachiS-4800, Japan). 2.5. Measurements of photoelectrochemical response
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A three-electrode cell with CHI760E electrochemical system (Chenhua
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Instruments, Co., Shanghai) in which 24 W UV lamp (LTD, =365 nm) was used in
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photoelectrochemical measurements served as light source. The working electrode was 10 nm Pt@ZnO NRs/CFs. Linear sweep voltammetry (LSV) was used to record the current-voltage curves. First, the photoresponse was conducted by switching light
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lamp in the process of total 5 cycles and each cycle was set in the absence of light illumination from 0-40 s and then next 40-90 s under the continuous illumination of
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light. Finally, the lamp was turned off at 90 s measuring at 0 V (vs. SCE). Besides, we also investigated the effect of the applied voltage (0, 0.1, 0.3 and 0.6 V vs. SCE) on
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response current according to the experimental procedures mentioned above.
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2.6. Electrochemical and Photoelectrochemical reactions of methanol-oxidation The electrochemical performance of the prepared samples was conducted by a
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traditional three-electrode cell in electrochemical workstation. Pt was acted as counter electrode and saturated calomel electrode (SCE) was functioned as the reference electrode. Pt@ZnO NRs/CFs with thickness of 10 nm, 30 nm, 50 nm were used as
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working electrode. Cyclic voltammetry method was conducted to characterize electrochemical active surface area (ECSA) and catalytic capability. In the methanol electro-oxidation experiment, the condition of electrode was 0.5 mol·L-1 H2SO4 and the mixed solution of 1 mol·L-1 CH3OH and 0.5 mol·L-1 H2SO4, respectively. In addition, chronoamperometry experiments was applied to characterize the stability of as-prepared working electrode and the scan rate was set as 30 mV·s-1 in all electrochemical experiments. A UV lamp (LTD, 24 W, =365 nm) was used as light source to test the effect of light source on the methanol–oxidation reaction performance of samples.
3. Results and Discussions 3.1. Microstructure and morphology
ACCEPTED MANUSCRIPT X-ray diffraction patterns of the samples are shown in Fig. 2a. A obvious peak at 25° represented the existence of integral graphite structure.[21] This indicated that CFs deposited on ZnO seeds by ALD and ZnO NR/CF showed three main peaks
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appearing at angles 2θ= 31.94°, 34.60° and 36.42°, which could be attributed to (100), (002) and (101) planes of hexagonal wurtzite (JCPDS36-1451).[22.23] The pattern of
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sample of Pt@ZnO NR/CFs with 10 nm Pt coating were no Pt peaks, which may be due to the small size and amount of Pt nanoparticles.[24] However, the crystalline phase peaks of Pt gradually increased with thickness of Pt. At 50 nm thick, two
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diffraction peaks at 39.5° and 46.8° were detected, which corresponded to (111) and (200) planes of Pt, respectively.[25] The XRD analysis results confirmed the loading
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of Pt nanoparticles on the surface of ZnO NRs. Fig. 2b showed the the EDS spectrum of Pt@ZnO NRs/CFs sample. It was used to further prove the existence of Pt on the
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surface of ZnO NRs. From Fig. 2b, it can be obviously seen that the peak of Pt
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element existed on the surface of Pt@ZnO NRs/CFs. Furthermore, it also had C, Zn
Intensity (a.u.)
50nm Pt@ZnO NRs/CFs
30nm Pt@ZnO NRs/CFs
30
10nm Pt@ZnO NRs/CFs
ZnO NRs/CFs
20
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b
200
Zn Pt
100 002 101 111
a
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and O element.
ALD ZnO/CFs CFs
40
50
2(o)
60
70
80
Fig. 2 The XRD spectrums of Pt@ZnO NRs/CFs with different stages and Pt thickness of 10, 30 and 50 nm(a), and the EDS spectrum of Pt@ZnO NRs/CFs sample(b).
Fig. 3 shows the SEM images of CFs, ZnO NR/CFs and Pt@ZnO NR/CFs with different thickness of Pt. The initial morphology of pure CFs is shown in Fig. 3a. After ALD process, the CF substrates are covered with a uniform thin layer of ZnO seed (Fig. 3b). As shown in Fig. 3(c-d), it can clearly see that ZnO NRs were on the
ACCEPTED MANUSCRIPT surface of the CFs. From Fig. 3(e-f), Pt@ZnO NR/CF-10 shows similar morphology as that of ZnO NRs/CFs without observing the Pt nanoparticles deposited. MS has been demonstrated to be an effective and easy-controlled method to deposit coating
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layers on various substrates. With increasing loading of sputtered Pt, the surface of ZnO NRs becomes rougher, as shown in Fig. 3(g-j). From Fig. 3(i-j), when the
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deposited thickness of Pt reaching to 50 nm, the original surface of ZnO NR were obviously wrapped by small Pt nanoparticles. According to BET test data published on my previous paper, ZnO NRs acted as bonding layer between carbon fiber
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substrate and Pt nanoparticles, which provided higher active sites and contact
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areas.[20]
Fig. 3 The micromorphology of the prepared samples (a is original CFs; b is CFs deposited with a
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ZnO seed layer; c and d are the sample of ZnO NRs/CFs at different magnification; e, f are 10 nm Pt@ZnO NRs/CFs; g, h are 30 nm Pt@ZnO NRs/CFs; i, j are 50 nm Pt@ZnO NRs/CFs).
3.2. Photoelectrochemical response of ZnO NR/CF Photoelectrochemical response of ZnO NRs/CFs were investigated by using LSV method to record current-voltage (I-V) curves from 0 to 0.8 V with the scan rate of 10 mV·s-1(Fig. 4). The response current gradually increased along with applied potentials from 0 and 0.8 V regardless of light irradiation. However, the current in the presence of light was consistently higher than that in the dark at the same potential. This increment of current may be ascribed to the generation of photocurrent.[26]
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Fig. 4 The I-V curves of ZnO NRs/CFs in dark and light illumination.
Fig. 5 The On-off I-t curves of ZnO NRs/CFs at 0 V potential.
ZnO is a photosensitive material. Experiment with periodic irradiation was
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performed to investigate the photosensitive characteristics of ZnO by recording the photocurrent-time (I-t) data. At 0 V vs. SCE, the response current remains at -0.020 mA in the dark during the first 40 s (Fig. 5). When UV light illumination was switched on, the response photocurrent immediately changed from -0.020 to 0.30 mA. Besides, the photocurrent maintained about 0.30 mA under UV light illumination for 50 s. When the light was switched off, the current returned to the -0.020 mA in the dark. The generation and disappearance of photocurrent is rapid in the periodic irradiation experiment, which suggests that ZnO have excellent response to photoelectricity. The consistent photocurrent response under periodic irradiation indicates the stability of ZnO NR/CF.
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Fig. 6 The on-off I-t curves of ZnO NRs/CFs at different potentials (0, 0.1, 0.3, 0.6 V).
Fig. 6 shows the effects of applied potentials on the photocurrent generation of
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ZnO NR/CF. As shown in Fig. 6, the as-prepared sample of ZnO NR/CF showed increasing sensitivity to light irradiation with applied potential. However, ZnO NR/CF showed remarkable response regardless of the applied potentials. When the applied
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potential increased from 0 V to 0.6 V, the photocurrent generated under light
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irradiation increased from 0.235 to 0.725 mA. This is because the higher potential promotes more effective separation and transfer of photo-generated electron-hole while
suppressing
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pairs
electron
recombination,
leading
to
increasing
photocurrent.[27]
3.3. Role of ZnO Secondary Carrier Layer in Electro-Oxidation of Methanol
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Cyclic voltammetry (CV) is a common method to calculate ECSA of Pt catalyst. The actual value of ECSA not only revealed the number of active sites of the catalyst, but also estimated the catalytic performance and the choice of suitable substrates. The ECSA were calculated by the area ratio of hydrogen adsorption/desorption peaks in the range of negative potential.[28] CV diagram of Pt@CF-10 and Pt@ZnO NR/CF-10 catalyst were measured in 0.5 mol·L-1 H2SO4 solution as shown in Fig. 7. Both catalysts had distinct hydrogen adsorption/desorption peaks between -0.2 and 𝑄
0.1 V. According to equation ECSA = 210 H×𝑃𝑡 ,[29] the ECSA (cm2·g-1)values of Pt@CF-10 and Pt@ZnO NR/CF-10 catalysts are 10.78 m2·g-1 and 23.19 m2·g-1, respectively. The higher ECSA value of Pt@ZnO NR/CF-10 suggested that the existence of ZnO secondary carrier layer, which improved the distribution and the size
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improvement of catalytic efficiency.
Fig. 7 The electrochemically active surface areas measured in 0.5 M H2SO4 of 10 nm Pt@CFs and
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Pt@ZnO NRs/CFs with 30 mV·s-1 sweep rate.
Fig. 8 showed the CV curves of the catalytic activity for electron-oxidation
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methanol of ZnO NR/CFs, Pt@CFs-10 and Pt@ZnO NR/CFs-10. As shown in the Fig.
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8, it can be seen that CV curves of the three catalysts showed two similar current peaks located at the potentials of about 0.7 V in forward scan and 0.5 V in backward
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scan. The current peaks in 0.7 V and 0.5 V corresponded to the peaks of methanol oxidation (If) and CO oxidation peak (Ib), respectively. The peak current density at about 0.7 V was a crucial parameter to represent the catalytic ability for methanol
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oxidation.[30] Otherwise, Fig. 8 also showed that the peak of current density (If) of three catalysts resided in accordance with the following order: Pt@ZnO NR/CFs-10＞ Pt@CFs-10 ＞ ZnO NR/CFs. Pt@ZnO NR/CFs-10 possessed the highest current density for methanol oxidation because of the synergistic effect between the Pt nanoparticles and the ZnO NRs. Moreover, the ratio of If and Ib was an indication of catalyst tolerance towards CO, where higher ratio of If/Ib indicates higher CO tolerance of catalysts.[31] The calculated If/Ib value of Pt@ZnO NR/CFs-10 and Pt@CFs-10 were 1.72 and 1.41, respectively. The increased ratio of If/ Ib revealed that the existence of ZnO enhanced the CO tolerance by oxidizing CO in lower potential.[32] In summary, the existence of ZnO secondary carrier layer effectively improved the ECSA, the methanol catalytic oxidation activity and the CO tolerance.
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Fig. 8 Cyclic voltammograms of ZnO NRs/CFs, 10 nm Pt@CFs, 10 nm Pt@ZnO NRs/CFs in 0.5 mol·L-1 H2SO4 and 1 mol·L-1 methanol with 30 mV·s-1 sweep rate.
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3.4. Methanol Electro-Oxidation by Pt@ZnO NRs/CFs electrode with different Pt loadings
Fig. 9 showed the CVs curves of catalysts with different thickness of Pt (10, 30,
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and 50 nm) in 0.5 mol·L-1 H2SO4 solution between -0.2 and +1.2 V. As shown in Fig.
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9, the Pt@ZnO NRs/CFs with 10, 30 and 50 nm thickness all exhibited hydrogen adsorption/desorption peaks in scan range of -0.2–0.1 V. Calculated results indicated
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that ECSA of the three prepared samples of 10, 30 and 50 nm Pt@ZnO NRs/CFs are 23.19 m2·g-1，29.73 m2·g-1 and 43.65 m2·g-1, respectively. The results further indicated that the ECSA values of catalysts increases with the increasing loading of Pt,[33] thus
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improving the ability of methanol electro-oxidation.
Fig. 9 The electrochemically active surface areas measured in 0.5 mol·L-1 H2SO4 of Pt@ZnO NRs/CFs with different thickness of Pt (10, 30 and 50 nm) at a sweep rate of 30 mV·s-1.
ACCEPTED MANUSCRIPT Fig. 10 showed the CVs curves of samples with various deposited Pt thickness (10, 30 and 50 nm) in mixed solution between 0 to 1 V vs. As shown in Fig. 10, the forward anodic peak current (If) increased with the loading of Pt. The result was
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consistent with ECSA experiment (Fig. 9). The distribution, grain size and morphology of Pt component had direct effect on the catalytic performance. From Fig.
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10, the sample of Pt@ZnO NR/CF-50 had the highest forward anodic peak current density (If), which may be due to the unique nanorod bundles morphology of Pt caused by pressure and high temperature in the process of MS. The changed
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morphology of Pt could caused much great improvement in activity of methanol oxidation activity. For example, the peak current density of Pt@ZnO NR/CFs-50 was
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approximately 3.75 times than that of Pt@ZnO NR/CFs-30. However, the If value of Pt@ZnO NR/CFs-30 is only 2.5 times than Pt@ZnO NR/CFs-10. This enhancement
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contact area and active sites for methanol oxidation. However, the improved loadings of Pt bring about the decrement of CO tolerance of catalysts due to the easiness of Pt
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catalyst poisoning. [34]
Fig. 10 The CV diagrams of Pt@ZnO NRs/CFs with various thickness of Pt (10, 30, 50 nm) in 0.5 mol·L-1 H2SO4 and 1 mol·L-1 methanol with 30 mV·s-1 sweep rate .
Chronoamperometric analytic approach is usually used to investigate the electric stability of catalysts. The current density was recorded for 1800 s at 0.6 V (vs. SCE) with 30 mV·s-1 scan rate in electrolyte containing 0.5 mol·L-1 H2SO4 and 1 mol·L-1 methanol. At 0.6 V constant potential, the current density of ZnO NR/CFs,
ACCEPTED MANUSCRIPT Pt@CFs-10 and Pt@ZnO NR/CFs with different thickness of Pt (10, 30 and 50 nm) decreased with time (Fig. 11). This result revealed the formation of reaction intermediates and the oxide of Pt on the basis of proceeding in methanol oxidation
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reaction.[35] Pt@ZnO NR/CFs-50 consistently showed the highest current density among all samples tested. In addition, Pt@ZnO NR/CFs-10 possessed higher current
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density than Pt@CFs-10, which indicated the existence of ZnO secondary support
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was benefit to improve the CO tolerance and activity of Pt.
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Fig. 11 The recording of chronoamperometric curves at 0.6 V versus SCE of ZnO NRs/CFs, 10nm Pt@CFs, 10nm Pt@ZnO NRs/CFs, 30nm Pt@ZnO NRs/CFs and 50nm Pt@ZnO NRs/CFs.
3.5. Photoassosted Methanol Electro-Oxidation
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Fig. 12 showed the CV curves of Pt@ZnO NR/CFs-30 for methanol electro-oxidation under light illumination. As shown in Fig. 12, the peak current density in forward scan was 6.55 mA·cm-2 under UV light, which was approximately 1.42 times than in the dark (4.6 mA·cm-2). This improvement of peak current density may be attributed to the occurrence of light-assisted methanol electro-oxidation. The increase of peak current density by 42 % was attributed to the synergistic photocurrent produced on ZnO semiconductor.[36] Under UV light, the electrons on ZnO semiconductor transferred from valance band to conduct band, which facilitate the formation of electron-hole pairs, thus generating additional photocurrent.[26]
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Fig. 12 CV diagrams of methanol electrochemical oxidation at the electrode of 30 nm Pt@ZnO NRs/CFs with or without UV light.
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4. Conclusions
Firstly, a novel multilevel photocatalyst of Pt@ZnO NR/CFs was fabricated by ALD method united with hydrothermal synthesis and magnetron sputtering. With
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increasing loadings of Pt, the existing morphology of Pt changed from nanoparticles
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to nanorod bundles. ZnO NR/CFs is an excellent photosensitive material as
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demonstrated by the excellent and stable photoelectric response. The presence of ZnO can efficiently enlarge the ECSA of catalyst, promote the efficiency of methanol oxidation and improve CO tolerance on methanol oxidation of catalysts. In addition,
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the changed morphology of Pt could cause much great improvement of methanol oxidation activity. The peak current density of methanol oxidation on Pt@ZnO NR/CFs-30 under UV illumination increased significantly compared with dark environment. The synergistic catalysis of light and electricity resulted in the improvement of current for methanol oxidation. Most importantly, the novel catalysts show good application promising due to its high catalytic efficiency and tenacious CO tolerance.
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