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carbon nanotubes composite
Synthetic Metals 257 (2019) 116191
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electrochemical oxidation of methanol on highly dispersed Pt nanoparticles supported on NiAl layered double hydroxide/carbon nanotubes composite
T
⁎
D. Yazdania, , Atefeh Yassemib a b
Institute of Nano Science and Nano Technology, Razi University, Kermanshah, 67149, Iran Faculty of Chemistry, Sensor and Biosensor Research Center (SBRC) & Nanoscience and Nanotechnology Research Center (NNRC), Razi University, Kermanshah, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Direct methanol fuel cell Platinum catalysts Electrodeposition Ni–Al layered double hydroxide Nanostructured materials Methanol electro-oxidation
Platinum catalysts play a major role in the large scale commercialization of direct methanol fuel cells (DMFCs). Here, we present a procedure to create a nanostructural NiAl Layered Double Hydroxide/Carbon Nanotubes composite (NiAl-LDH/CNTs) containing a small amount (2.6 μg) of platinum nanoparticles (Pt NPs). It is found that Pt/NiAl-LDH/CNTs electrocatalyst exhibits higher electrocatalytic activity, better anti-poisoning ability and stability for methanol oxidation comparing to pure Pt and commercial Pt/ CNTs catalyst. Such an enhancement is probably attributed to the synergistic effect of Ni- NiAl LDH and the uniform loading of Pt NPs on the LDH layers. The morphology and structure of composites were characterized by scanning electron microscopy, atomic force microscopy, energy dispersive spectroscopy and electrochemical methods. It was found that the Pt NPs are formed on the surface of NiAl-LDH/CNTs with small particle size, high loading density, and uniform dispersion.
1. Introduction Layered double hydroxides (LDHs) are a class of 2D inorganic layered matrices, which consist of positively charged layers, with anions and water molecules intercalated in the interlayer region. They are expressed by the general formula, [MII1-xMIIIx(OH)2]x+(An− x/n) mH2O, where M2+ and M3+ represent divalent and trivalent metal cations respectively, and An− is the interlayer anions [1–3]. Among the wide variety of transition metal LDHs, Ni-containing LDHs are capable of undergoing an inner redox reaction within a limited potential range in alkaline medium, have been exploited for electrochemical application [4,5]. Howsoever, the weak conductivity of LDH itself is unfavorable to electron transfer, which deteriorates the electrocatalytic performance of the material. Therefore, it is desirable to develop active and easy-to-make catalysts on the basis of LDH hybrid materials [6–9]. Direct-methanol fuel cells (DMFCs) have been regarded as a broad prospect for development in portable devices because of high conversion efficiency, lower or non-pollution, simple fabrication process, convenient transport and storage. Because the use of methanol as fuel has several advantages in comparison to hydrogen: it is a cheap liquid fuel, easily handled, transported, stored and high theoretical energy density, this is the main motivation for studying the of direct methanol oxidation in a fuel cell [10–13].
⁎
The high price and limited supply of Pt constitute a high barrier to commercialization of DMFCs. At the present stage, most of the commercial DMFCs are employed Pt black and Pt/C as anode catalyst to oxidize methanol. Many supporting materials with high specific surface area, good electric conductivity, and electrochemical stability have been employed to improve the composites catalytic effect and stability, such as nano-carbon materials [14,15], metal oxides [16,17] and hybrid materials. In this work, the novel Pt/NiAl-LDH/CNTs electrocatalyst is firstly synthesized by a two-step facile process. The key aspects of this electrocatalyst are the formation of ultrathin Pt/NiAl-LDH/CNTs nanoparticles and association with the uniform loading of Pt. The prepared Pt/NiAl-LDH/CNTs electrocatalyst displayed excellent electrocatalytic activity with a high peak current density and also good stability for methanol oxidation. 2. Experimental 2.1. Materials and methods Al(NO3)3·9H₂O, Ni(NO3)2.6H2O and Na2CO3 were purchased from Merck. K2PtCl6, methanol and acetic acid were obtained from Aldrich. Functionalized CNTs (with COOH groups, > 95% purity) were purchased from Chengdu Organic Chemical Co.Ltd. (Chengdu, China),
Corresponding author. E-mail address: [email protected] (D. Yazdani).
https://doi.org/10.1016/j.synthmet.2019.116191 Received 29 July 2019; Received in revised form 20 September 2019; Accepted 30 September 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
Synthetic Metals 257 (2019) 116191
D. Yazdani and A. Yassemi
transferred to the Teflon lined stainless steel reactor and solvothermal reacted at 180 ℃ for 36 h. The suspension was aged at 70 C for 5 h. The solid precipitate was rinsed with deionized water and ethanol, respectively, and then dried at 60 C for 10 h, the black NiAl-LDH/CNTs powders were prepared.
2.3. In situ electrodeposition of Pt NPs onto the NiAl-LDH/CNTs composite The electrodeposition of Pt NPs onto the NiAl-LDH/CNTs composite was performed in a conventional electrochemical cell, consisting of three electrodes held at room temperature. Firstly, a 0.5-wt% NiAl-LDH solution was prepared by diluting the 5-wt% NiAl-LDH solution with 0.1 M phosphate buffer solution (pH 7), and then 10 mg CNTs were added in a 1-mL 0.5 wt% NiAl-LDH solution, and ultrasound agitation was performed for 1 h to form a homogeneous solution. Then, 3 μl of resultant suspension was spread on the surface of the pretreated glassy carbon electrode (GCE), and the coating was dried at room temperature. The Pt NPs are deposited onto the NiAl-LDH/CNTs composite by using a potentiostatic method in a 0.1 M phosphate buffer solution (pH 7) containing 1.0 Mm K2PtCl6 for 20.0 min. The electrodeposition of the Pt particles were performed at constant potential of −0.2 V vs. Ag/AgCl for fabrication of Pt/NiAl-LDH/CNTs/GCE. The Pt amount was estimated from the equation: m / μg = Qnet M / FZ = 0.5Qnet , where m was calculated by using the integral of current-time curve (Qnet / mC ) during the deposition process by assuming that the reduction efficiency of Pt (IV)–Pt (0) is 100% [18–21]. M is the atomic weight of Pt (195.078 g mol −1), F(96485 C mol−1) is the Faraday constant, and Z( = 4) is the number of electrons transferred. The Qnet (net charge) consumed for electrodeposition can be calculated by subtraction of charges obtained from chronoamperogram of the electrodeposition and blank solutions. The amount of deposited Pt was controlled about 2.6 μg for 0.017 cm2 (150 μg cm−2) which corresponds to a charge of 0.3 C /cm2. After platinum deposition, the electrode was removed from the deposition solution thoroughly rinsed with water. The 2 M CH3OH and 1 M H2SO4 solution was purged with N2 for 30 min to eliminate oxygen before methanol oxidation. The potential was scanned between 0 to 1 V versus Ag/AgCl at a scan rate of 50 mVs−1.
Fig. 1. Raman spectra of CNTs.
where the materials were prepared by chemical vapor deposition (CVD), and used without further purification. Fig. 1 shows the Raman Spectrum of CNTs. As can be observed in Raman spectrum, the characteristic peaks of CNTs, named the D band at 1338 cm−1 and the G band at 1585 cm−1, approximately, are identified. The ratio between the intensity of the D band and the G band, noted ID/G, is related to the degree of disorder of the nanotube. The ID/G it was achieved 0.2. Nafion solution (5%) was provided by DuPont Co. (USA). Fuel cell grade commercial Pt/ C (Platinum on Vulcan xc 72) was acquired from Sainergy (USA). The surface morphologies of NiAl-LDH/CNTs/GCE and Pt/NiAlLDH/CNTs/GCE were observed with a scanning electron microscopy (SEM) (Philips X1-40 FEG) equipped with an energy dispersive spectroscopy (EDS). AFM images were obtained on a FEI QUANTA 200 F.
2.2. Preparation of NiAl-LDH/CNTs composite The preparation process of the NiAl-LDH/CNTs composite is schematically shown in Fig. 2. 0.3 g of functionalized CNTs was added to 50 ml of an alkali solution of NaOH (0.3 M) and Na2CO3 (0.15 M) and placed in an ultrasonication bath for 60 min. Subsequently, the solution was added to 50 ml of a salt solution of Ni(NO3)2.6H2O (0.20 M) and Al(NO3) 3·9H2O (0.0.1 M) under vigorous stirring at room temperature. The mixtures were
Fig. 2. The schematic diagram of preparation of Pt/NiAl-LDH/CNTs composite. 2
Synthetic Metals 257 (2019) 116191
D. Yazdani and A. Yassemi
Fig. 3. AFM images of GCE, NiAl-LDH/CNTs /GCE and Pt/NiAl-LDH/CNTs/GCE.
3. Results and discussion 3.1. Morphologies and composition of Pt NPs supported electrodes The surface morphology of GCE, NiAl-LDH/CNTs/GCE and Pt/NiAlLDH/CNTs/GCE was investigated by AFM and SEM, the results are shown in Fig. 3 and Fig. 4. The AFM images of the NiAl-LDH/CNTs/GCE exhibited a regular and homogeneous structure. The Pt/NiAl-LDH/ CNTs/GCE have slightly different structure, since the surface showed a strong increase in heterogeneities. This change of topography is due to the Pt NPs incorporation on surface of the composite film (Fig.3). Fig. 4 shows the SEM images of the NiAl-LDH/CNTs/GCE before and after eletroreduction of Pt NPs. It was found that Pt NPs were deposited to the surface of NiAl-LDH/CNTs, which could from a passageway for electrons to carry out the reaction of electrodeposition. The images of Pt/LDH/CNTs/GCE shows Pt NPs deposited onto the composite surface homogenously covered with them. From the data obtained by SEM micrographs, the Pt NPs size histogram was drawn and the average particle diameter was determined (inset Fig. 5). The size distribution was small and narrow in and the average particle diameter was about 50 nm. Fig. 6 shows EDS spectrum of elemental analysis data of the modified electrode. From the EDS results Pt was major element, carbon with 73.69% content in product was derived from GCE and CNTs and O, Ni and Al elements were derived from functionalized CNTs and NiAl–LDH. The specimen is usually coated with an ultra-thin coating of electrically conducting material Gold is the most common conductive material in current use for specimen coating. Also, the weight ratio of CNTs / NiAlLDH prepared according to the recipe described in the Experimental
Fig. 5. Pt NPs size histogram of NiO-NPs obtained from TEM.
Section was about 3/5. 3.2. Electrochemical characterization After deposition of Pt NPs, modified electrodes were characterized in 0.5 M H2SO4 aqueous solution using cyclic voltammetry. No hydrogen adsorption and desorption peaks are observed at the NiAl-LDH/ CNTs/GCE, on the contrary, the hydrogen adsorption and desorption peaks can be seen at the Pt/NiAl-LDH/CNTs/GCE (Fig. 7). The sharp anodic peak at around -0.20 V versus Ag/AgCl after incorporation of Pt
Fig. 4. SEM images of GCE, NiAl-LDH/CNTs/GCE and Pt/NiAl-LDH/CNTs/GCE. 3
Synthetic Metals 257 (2019) 116191
D. Yazdani and A. Yassemi
Fig. 6. EDS pattern of Pt/NiAl-LDH/CNTs/GCE.
3.3. Application of Pt/NiAl-LDH/CNTs/GCE for methanol oxidation The electrochemical performance of the Pt NPs with different supports toward methanol oxidation was investigated by CV technique. For comparison, three controlled experiments at the Pt/CNTs/GCE, Pt/ NiAl-LDH/GCE and Pt/NiAl-LDH/CNTs/GCE were performed. Fig. 8A shows the cyclic voltammograms recorded at the Pt/CNTs/GCE, Pt/ NiAl-LDH/GCE and Pt/NiAl-LDH/CNTs/GCE. The methanol oxidation current of Pt/NiAl-LDH/CNTs/GCE is much larger than Pt/CNTs/GCE and Pt/NiAl-LDH/GCE. This significant improvement in the catalytic performance can be attributed the high level of dispersion of Pt NPs on NiAl-LDH/CNTs nanocomposite. Besides, the ratio of the forward anodic peak current (If) to the reverse anodic peak current (Ib) was considered to describe the catalyst tolerance to accumulation of carbonaceous species in some degree. Fig. 8B displays the steady-state CV curves of methanol oxidation at Pt/NiAl-LDH/GCE and commercial 20% Pt/C. Clearly, the Clearly, the Pt/NiAl-LDH/CNTs/GCE exhibits much larger current density and more negative onset potential (-0.55 V) compared to commercial Pt/C. Whole, compared with Yang et al. Report [24] performance of these catalysts a little less. Fig. 9A shows the effect of methanol concentration on the anodic current of methanol oxidation at the Pt/NiAl-LDH/CNTs/GCE. It is clearly observed that the anodic current increases with increasing methanol concentration and levels off at concentration higher than 2. M. In accordance with these results, the optimum concentration of
Fig. 7. Cyclic voltammograms of NiAl-LDH/CNTs/GCE and Pt/NiAl-LDH/ CNTs/GCE in 0.5 M H2SO4 solution with a scan rate of 50 mVs−1.
NPs onto the NiAl-LDH/CNTs/GCE is attributed to the hydrogen desorption on the surface of NiAl-LDH/CNTs. These data are in agreement with the results reported in the literature [22,23].
Fig. 8. A) Cyclic voltammograms recorded in 2 M CH3OH and 1 M H2SO4 solution at Pt/NiAl-LDH/GCE, Pt/CNTs/GCE, Pt/NiAl-LDH/CNTs/GCE and B) commercial Pt/C in the potential range from 0 to1.0 V at a scan rate of 50 mVs−1. 4
Synthetic Metals 257 (2019) 116191
D. Yazdani and A. Yassemi
Fig. 9. Cyclic voltammograms for methanol oxidation on the Pt/NiAl-LDH/CNTs/GCE with different concentrations of methanol (A) and different scan rate (B) in 2 M CH3OH and 1 M H2SO4 solution.
during the scanning in aqueous solutions, especially in the presence of organic compounds. 4. Conclusions Nanostructured NiAl-LDH/CNTs composite was successfully synthesized by a simple and facile hydrothermal method, then, Pt nanoparticles were successfully electrodeposited in situ, for the first time, on NiAl-LDH/CNTs nanocomposite. The Pt/NiAl-LDH/CNTs hybrid displayed the excellent methanol oxidation reaction activity. Compared to Pt/CNTs/GCE and Pt/NiAl-LDH/GCE, Pt/NiAl-LDH/CNTs/GCE displayed better electrocatalytic activity towards methanol oxidation in acidic medium. This result shows that the 2D ultrathin NiAl-LDH/CNTs can be used as promising support electrode in direct methanol fuel cell. References
Fig. 10. Chronoamperometry for methanol oxidation at the Pt/CNTs/GCE and Pt/NiAl-LDH/CNTs /GCE in 2.0 M methanol and 1 M H2SO4.
[1] X. Deng, J. Huang, H. Wan, F. Chen, Y. Lin, X. Xu, et al., Recent progress in functionalized layered double hydroxides and their application in efficient electrocatalytic water oxidation, J. Energy Chem. 32 (2019) 93–104. [2] L. Lv, Z. Yang, K. Chen, C. Wang, Y. Xiong, 2D layered double hydroxides for oxygen evolution reaction: from fundamental design to application, Adv. Energy Mater. 9 (17) (2019) 1803358. [3] K. Dox, M. Everaert, R. Merckx, E. Smolders, Optimization of phosphate recovery from urine by layered double hydroxides, Sci. Total Environ. 682 (2019) 437–446. [4] F. Cao, M. Gan, L. Ma, X. Li, F. Yan, M. Ye, et al., Hierarchical sheet-like Ni–Co layered double hydroxide derived from a MOF template for high-performance supercapacitors, Synth. Met. 234 (2017) 154–160. [5] X. Long, Z. Wang, S. Xiao, Y. An, S. Yang, Transition metal based layered double hydroxides tailored for energy conversion and storage, Mater. Today 19 (4) (2016) 213–226. [6] L. Mohapatra, D. Patra, Multifunctional hybrid materials based on layered double hydroxide towards photocatalysis, Photocatal. Funct. Mater. Environ. Remediation (2019) 215–241. [7] G. Abellan, C. Martí-Gastaldo, A. Ribera, E. Coronado, Hybrid materials based on magnetic layered double hydroxides: a molecular perspective, Acc. Chem. Res. 48 (6) (2015) 1601–1611. [8] S. Yu, Y. Zhang, G. Lou, Y. Wu, X. Zhu, H. Chen, et al., Synthesis of NiMn-LDH nanosheet@ Ni 3 S 2 nanorod hybrid structures for supercapacitor electrode materials with ultrahigh specific capacitance, Sci. Rep. 8 (1) (2018) 5246. [9] G.-H. Gwak, A.-J. Choi, Y.-S. Bae, H.-J. Choi, J.-M. Oh, Electrophoretically prepared hybrid materials for biopolymer hydrogel and layered ceramic nanoparticles, Biomater. Res. 20 (1) (2016) 1. [10] H. Javan, E. Asghari, H. Ashassi-Sorkhabi, Fabrication and electrochemical kinetics studies of reduced carbon quantum dots-supported palladium nanoparticles as bifunctional catalysts in methanol oxidation and hydrogen evolution reactions, Synth. Met. 254 (2019) 153–163. [11] X. Zheng, C. Shang, J. Yang, J. Wang, L. Wang, Preparation and characterization of chitosan-crown ether membranes for alkaline fuel cells, Synth. Met. 247 (2019) 109–115. [12] J. Wu, Q. Shi, S. Mu, Synthesis of aniline copolymer and as an active catalyst layer for electrochemical reduction of carbon dioxide in water free of supporting electrolytes, Synth. Met. 255 (2019) 116109.
methanol to obtain a higher current density may be considered 2 M. The effect of potential sweep rates was studied on the electrocatalytic behavior of methanol onto Pt/NiAl-LDH/CNTs/GCE as can be seen in Fig. 9B. The linear relationship between the anodic peak current and square rate of potential sweep rates can be observed. This implied that the electrooxidation of methanol at the surface of the modified electrode may be controlled by diffusion process. 3.4. Chronoamperometry studies comparison of stability In order to evaluate the electrocatalytic activity of the catalyst and the poisoning of the active surface under continuous operation conditions, long-term chronoamperometry experiments were performed (Fig. 10). In two current-time curve for Pt/CNTs/GCE and Pt/NiAlLDH/CNTs/GCE, there was an initial current slower decay, but the current values obtained for the Pt/NiAl-LDH/GCE were higher than those obtained for Pt/NiAl-LDH/CNTs/GCE. The data indicated that the Pt/NiAl-LDH/CNTs/GCE have high activity for methanol oxidation than Pt/CNTs/GCE. The stability of the Pt/NiAl-LDH/CNTs/GCE was examined by using cyclic voltammetry technique (figure not shown). The anodic peak current decreases gradually with potential cycling. In general, the loss of catalytic activity with successive scans of potential may result from the consumption of methanol during the potential scanning in cyclic voltammetry. It also perhaps due to poisoning and the structure changes of the Pt particles as a result of the potentials perturbation 5
Synthetic Metals 257 (2019) 116191
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[19] J. Calderón Gómez, V. Celorrio, L. Calvillo, D. Sebastián, R. Moliner, M. Lázaro Elorri, Electrochemical behavior of Pt–Ru catalysts supported on graphitized ordered mesoporous carbons toward CO and methanol oxidation, Surfaces 2 (1) (2019) 1–15. [20] T. Unmüssig, J. Melke, A. Fischer, Synthesis of Pt@ TiO 2 nanocomposite electrocatalysts for enhanced methanol oxidation by hydrophobic nanoreactor templating, J. Chem. Soc. Faraday Trans. (2019). [21] T. Radhakrishnan, N. Sandhyarani, Pt-Ag nanostructured 3D architectures: a tunable catalyst for methanol oxidation reaction, Electrochim. Acta 298 (2019) 835–843. [22] M. He, G. Fei, Z. Zheng, Z. Cheng, Z. Wang, H. Xia, Pt nanoparticle-loaded graphene aerogel microspheres with excellent methanol electro-oxidation performance, Langmuir 35 (10) (2019) 3694–3700. [23] J. Klein, F. Argast, A.K. Engstfeld, S. Brimaud, R.J. Behm, Electro-oxidation of methanol on Ru-core Pt-shell type model electrodes, Electrochim. Acta 311 (2019) 244–254. [24] Z. Yang, N. Nakashima, A simple preparation of very high methanol tolerant cathode electrocatalyst for direct methanol fuel cell based on polymer-coated carbon nanotube/platinum, Sci. Rep. 5 (2015) 12236.
[13] H. Hammache, L. Makhloufi, B. Saidani, Electrocatalytic oxidation of methanol on PPy electrode modified by gold using the cementation process, Synth. Met. 123 (3) (2001) 515–522. [14] A. Glüsen, F. Dionigi, P. Paciok, M. Heggen, M. Müller, L. Gan, et al., Dealloyed PtNi-Core–Shell nanocatalysts enable significant lowering of Pt electrode content in direct methanol fuel cells, ACS Catal. 9 (5) (2019) 3764–3772. [15] V. Johánek, A. Ostroverkh, R. Fiala, Vapor-feed low temperature direct methanol fuel cell with Pt and PtRu electrodes: chemistry insight, Renew. Energy 138 (2019) 409–415. [16] J.C. Abrego-Martínez, Y. Wang, A. Moreno-Zuria, Q. Wei, F.M. Cuevas-Muniz, L.G. Arriaga, et al., Nanostructured Mn2O3/Pt/CNTs selective electrode for oxygen reduction reaction and methanol tolerance in mixed-reactant membraneless microDMFC, Electrochim. Acta 297 (2019) 230–239. [17] A.B. Yousaf, M. Imran, S.J. Zaidi, P. Kasak, Engineering and understanding of synergistic effects in the interfaces of rGO-CNTs/PtPd nanocomposite revealed fast electro-oxidation of methanol, J. Electroanal. Chem. 832 (2019) 343–352. [18] W. Gong, Z. Jiang, R. Wu, Y. Liu, L. Huang, N. Hu, et al., Cross-double dumbbell-like Pt–Ni nanostructures with enhanced catalytic performance toward the reactions of oxygen reduction and methanol oxidation, Appl. Catal. B Environ. 246 (2019) 277–283.
6
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electrochemical oxidation of methanol on highly dispersed Pt nanoparticles supported on NiAl layered double hydroxide/carbon nanotubes composite
T
⁎
D. Yazdania, , Atefeh Yassemib a b
Institute of Nano Science and Nano Technology, Razi University, Kermanshah, 67149, Iran Faculty of Chemistry, Sensor and Biosensor Research Center (SBRC) & Nanoscience and Nanotechnology Research Center (NNRC), Razi University, Kermanshah, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Direct methanol fuel cell Platinum catalysts Electrodeposition Ni–Al layered double hydroxide Nanostructured materials Methanol electro-oxidation
Platinum catalysts play a major role in the large scale commercialization of direct methanol fuel cells (DMFCs). Here, we present a procedure to create a nanostructural NiAl Layered Double Hydroxide/Carbon Nanotubes composite (NiAl-LDH/CNTs) containing a small amount (2.6 μg) of platinum nanoparticles (Pt NPs). It is found that Pt/NiAl-LDH/CNTs electrocatalyst exhibits higher electrocatalytic activity, better anti-poisoning ability and stability for methanol oxidation comparing to pure Pt and commercial Pt/ CNTs catalyst. Such an enhancement is probably attributed to the synergistic effect of Ni- NiAl LDH and the uniform loading of Pt NPs on the LDH layers. The morphology and structure of composites were characterized by scanning electron microscopy, atomic force microscopy, energy dispersive spectroscopy and electrochemical methods. It was found that the Pt NPs are formed on the surface of NiAl-LDH/CNTs with small particle size, high loading density, and uniform dispersion.
1. Introduction Layered double hydroxides (LDHs) are a class of 2D inorganic layered matrices, which consist of positively charged layers, with anions and water molecules intercalated in the interlayer region. They are expressed by the general formula, [MII1-xMIIIx(OH)2]x+(An− x/n) mH2O, where M2+ and M3+ represent divalent and trivalent metal cations respectively, and An− is the interlayer anions [1–3]. Among the wide variety of transition metal LDHs, Ni-containing LDHs are capable of undergoing an inner redox reaction within a limited potential range in alkaline medium, have been exploited for electrochemical application [4,5]. Howsoever, the weak conductivity of LDH itself is unfavorable to electron transfer, which deteriorates the electrocatalytic performance of the material. Therefore, it is desirable to develop active and easy-to-make catalysts on the basis of LDH hybrid materials [6–9]. Direct-methanol fuel cells (DMFCs) have been regarded as a broad prospect for development in portable devices because of high conversion efficiency, lower or non-pollution, simple fabrication process, convenient transport and storage. Because the use of methanol as fuel has several advantages in comparison to hydrogen: it is a cheap liquid fuel, easily handled, transported, stored and high theoretical energy density, this is the main motivation for studying the of direct methanol oxidation in a fuel cell [10–13].
⁎
The high price and limited supply of Pt constitute a high barrier to commercialization of DMFCs. At the present stage, most of the commercial DMFCs are employed Pt black and Pt/C as anode catalyst to oxidize methanol. Many supporting materials with high specific surface area, good electric conductivity, and electrochemical stability have been employed to improve the composites catalytic effect and stability, such as nano-carbon materials [14,15], metal oxides [16,17] and hybrid materials. In this work, the novel Pt/NiAl-LDH/CNTs electrocatalyst is firstly synthesized by a two-step facile process. The key aspects of this electrocatalyst are the formation of ultrathin Pt/NiAl-LDH/CNTs nanoparticles and association with the uniform loading of Pt. The prepared Pt/NiAl-LDH/CNTs electrocatalyst displayed excellent electrocatalytic activity with a high peak current density and also good stability for methanol oxidation. 2. Experimental 2.1. Materials and methods Al(NO3)3·9H₂O, Ni(NO3)2.6H2O and Na2CO3 were purchased from Merck. K2PtCl6, methanol and acetic acid were obtained from Aldrich. Functionalized CNTs (with COOH groups, > 95% purity) were purchased from Chengdu Organic Chemical Co.Ltd. (Chengdu, China),
Corresponding author. E-mail address: [email protected] (D. Yazdani).
https://doi.org/10.1016/j.synthmet.2019.116191 Received 29 July 2019; Received in revised form 20 September 2019; Accepted 30 September 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
Synthetic Metals 257 (2019) 116191
D. Yazdani and A. Yassemi
transferred to the Teflon lined stainless steel reactor and solvothermal reacted at 180 ℃ for 36 h. The suspension was aged at 70 C for 5 h. The solid precipitate was rinsed with deionized water and ethanol, respectively, and then dried at 60 C for 10 h, the black NiAl-LDH/CNTs powders were prepared.
2.3. In situ electrodeposition of Pt NPs onto the NiAl-LDH/CNTs composite The electrodeposition of Pt NPs onto the NiAl-LDH/CNTs composite was performed in a conventional electrochemical cell, consisting of three electrodes held at room temperature. Firstly, a 0.5-wt% NiAl-LDH solution was prepared by diluting the 5-wt% NiAl-LDH solution with 0.1 M phosphate buffer solution (pH 7), and then 10 mg CNTs were added in a 1-mL 0.5 wt% NiAl-LDH solution, and ultrasound agitation was performed for 1 h to form a homogeneous solution. Then, 3 μl of resultant suspension was spread on the surface of the pretreated glassy carbon electrode (GCE), and the coating was dried at room temperature. The Pt NPs are deposited onto the NiAl-LDH/CNTs composite by using a potentiostatic method in a 0.1 M phosphate buffer solution (pH 7) containing 1.0 Mm K2PtCl6 for 20.0 min. The electrodeposition of the Pt particles were performed at constant potential of −0.2 V vs. Ag/AgCl for fabrication of Pt/NiAl-LDH/CNTs/GCE. The Pt amount was estimated from the equation: m / μg = Qnet M / FZ = 0.5Qnet , where m was calculated by using the integral of current-time curve (Qnet / mC ) during the deposition process by assuming that the reduction efficiency of Pt (IV)–Pt (0) is 100% [18–21]. M is the atomic weight of Pt (195.078 g mol −1), F(96485 C mol−1) is the Faraday constant, and Z( = 4) is the number of electrons transferred. The Qnet (net charge) consumed for electrodeposition can be calculated by subtraction of charges obtained from chronoamperogram of the electrodeposition and blank solutions. The amount of deposited Pt was controlled about 2.6 μg for 0.017 cm2 (150 μg cm−2) which corresponds to a charge of 0.3 C /cm2. After platinum deposition, the electrode was removed from the deposition solution thoroughly rinsed with water. The 2 M CH3OH and 1 M H2SO4 solution was purged with N2 for 30 min to eliminate oxygen before methanol oxidation. The potential was scanned between 0 to 1 V versus Ag/AgCl at a scan rate of 50 mVs−1.
Fig. 1. Raman spectra of CNTs.
where the materials were prepared by chemical vapor deposition (CVD), and used without further purification. Fig. 1 shows the Raman Spectrum of CNTs. As can be observed in Raman spectrum, the characteristic peaks of CNTs, named the D band at 1338 cm−1 and the G band at 1585 cm−1, approximately, are identified. The ratio between the intensity of the D band and the G band, noted ID/G, is related to the degree of disorder of the nanotube. The ID/G it was achieved 0.2. Nafion solution (5%) was provided by DuPont Co. (USA). Fuel cell grade commercial Pt/ C (Platinum on Vulcan xc 72) was acquired from Sainergy (USA). The surface morphologies of NiAl-LDH/CNTs/GCE and Pt/NiAlLDH/CNTs/GCE were observed with a scanning electron microscopy (SEM) (Philips X1-40 FEG) equipped with an energy dispersive spectroscopy (EDS). AFM images were obtained on a FEI QUANTA 200 F.
2.2. Preparation of NiAl-LDH/CNTs composite The preparation process of the NiAl-LDH/CNTs composite is schematically shown in Fig. 2. 0.3 g of functionalized CNTs was added to 50 ml of an alkali solution of NaOH (0.3 M) and Na2CO3 (0.15 M) and placed in an ultrasonication bath for 60 min. Subsequently, the solution was added to 50 ml of a salt solution of Ni(NO3)2.6H2O (0.20 M) and Al(NO3) 3·9H2O (0.0.1 M) under vigorous stirring at room temperature. The mixtures were
Fig. 2. The schematic diagram of preparation of Pt/NiAl-LDH/CNTs composite. 2
Synthetic Metals 257 (2019) 116191
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Fig. 3. AFM images of GCE, NiAl-LDH/CNTs /GCE and Pt/NiAl-LDH/CNTs/GCE.
3. Results and discussion 3.1. Morphologies and composition of Pt NPs supported electrodes The surface morphology of GCE, NiAl-LDH/CNTs/GCE and Pt/NiAlLDH/CNTs/GCE was investigated by AFM and SEM, the results are shown in Fig. 3 and Fig. 4. The AFM images of the NiAl-LDH/CNTs/GCE exhibited a regular and homogeneous structure. The Pt/NiAl-LDH/ CNTs/GCE have slightly different structure, since the surface showed a strong increase in heterogeneities. This change of topography is due to the Pt NPs incorporation on surface of the composite film (Fig.3). Fig. 4 shows the SEM images of the NiAl-LDH/CNTs/GCE before and after eletroreduction of Pt NPs. It was found that Pt NPs were deposited to the surface of NiAl-LDH/CNTs, which could from a passageway for electrons to carry out the reaction of electrodeposition. The images of Pt/LDH/CNTs/GCE shows Pt NPs deposited onto the composite surface homogenously covered with them. From the data obtained by SEM micrographs, the Pt NPs size histogram was drawn and the average particle diameter was determined (inset Fig. 5). The size distribution was small and narrow in and the average particle diameter was about 50 nm. Fig. 6 shows EDS spectrum of elemental analysis data of the modified electrode. From the EDS results Pt was major element, carbon with 73.69% content in product was derived from GCE and CNTs and O, Ni and Al elements were derived from functionalized CNTs and NiAl–LDH. The specimen is usually coated with an ultra-thin coating of electrically conducting material Gold is the most common conductive material in current use for specimen coating. Also, the weight ratio of CNTs / NiAlLDH prepared according to the recipe described in the Experimental
Fig. 5. Pt NPs size histogram of NiO-NPs obtained from TEM.
Section was about 3/5. 3.2. Electrochemical characterization After deposition of Pt NPs, modified electrodes were characterized in 0.5 M H2SO4 aqueous solution using cyclic voltammetry. No hydrogen adsorption and desorption peaks are observed at the NiAl-LDH/ CNTs/GCE, on the contrary, the hydrogen adsorption and desorption peaks can be seen at the Pt/NiAl-LDH/CNTs/GCE (Fig. 7). The sharp anodic peak at around -0.20 V versus Ag/AgCl after incorporation of Pt
Fig. 4. SEM images of GCE, NiAl-LDH/CNTs/GCE and Pt/NiAl-LDH/CNTs/GCE. 3
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Fig. 6. EDS pattern of Pt/NiAl-LDH/CNTs/GCE.
3.3. Application of Pt/NiAl-LDH/CNTs/GCE for methanol oxidation The electrochemical performance of the Pt NPs with different supports toward methanol oxidation was investigated by CV technique. For comparison, three controlled experiments at the Pt/CNTs/GCE, Pt/ NiAl-LDH/GCE and Pt/NiAl-LDH/CNTs/GCE were performed. Fig. 8A shows the cyclic voltammograms recorded at the Pt/CNTs/GCE, Pt/ NiAl-LDH/GCE and Pt/NiAl-LDH/CNTs/GCE. The methanol oxidation current of Pt/NiAl-LDH/CNTs/GCE is much larger than Pt/CNTs/GCE and Pt/NiAl-LDH/GCE. This significant improvement in the catalytic performance can be attributed the high level of dispersion of Pt NPs on NiAl-LDH/CNTs nanocomposite. Besides, the ratio of the forward anodic peak current (If) to the reverse anodic peak current (Ib) was considered to describe the catalyst tolerance to accumulation of carbonaceous species in some degree. Fig. 8B displays the steady-state CV curves of methanol oxidation at Pt/NiAl-LDH/GCE and commercial 20% Pt/C. Clearly, the Clearly, the Pt/NiAl-LDH/CNTs/GCE exhibits much larger current density and more negative onset potential (-0.55 V) compared to commercial Pt/C. Whole, compared with Yang et al. Report [24] performance of these catalysts a little less. Fig. 9A shows the effect of methanol concentration on the anodic current of methanol oxidation at the Pt/NiAl-LDH/CNTs/GCE. It is clearly observed that the anodic current increases with increasing methanol concentration and levels off at concentration higher than 2. M. In accordance with these results, the optimum concentration of
Fig. 7. Cyclic voltammograms of NiAl-LDH/CNTs/GCE and Pt/NiAl-LDH/ CNTs/GCE in 0.5 M H2SO4 solution with a scan rate of 50 mVs−1.
NPs onto the NiAl-LDH/CNTs/GCE is attributed to the hydrogen desorption on the surface of NiAl-LDH/CNTs. These data are in agreement with the results reported in the literature [22,23].
Fig. 8. A) Cyclic voltammograms recorded in 2 M CH3OH and 1 M H2SO4 solution at Pt/NiAl-LDH/GCE, Pt/CNTs/GCE, Pt/NiAl-LDH/CNTs/GCE and B) commercial Pt/C in the potential range from 0 to1.0 V at a scan rate of 50 mVs−1. 4
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Fig. 9. Cyclic voltammograms for methanol oxidation on the Pt/NiAl-LDH/CNTs/GCE with different concentrations of methanol (A) and different scan rate (B) in 2 M CH3OH and 1 M H2SO4 solution.
during the scanning in aqueous solutions, especially in the presence of organic compounds. 4. Conclusions Nanostructured NiAl-LDH/CNTs composite was successfully synthesized by a simple and facile hydrothermal method, then, Pt nanoparticles were successfully electrodeposited in situ, for the first time, on NiAl-LDH/CNTs nanocomposite. The Pt/NiAl-LDH/CNTs hybrid displayed the excellent methanol oxidation reaction activity. Compared to Pt/CNTs/GCE and Pt/NiAl-LDH/GCE, Pt/NiAl-LDH/CNTs/GCE displayed better electrocatalytic activity towards methanol oxidation in acidic medium. This result shows that the 2D ultrathin NiAl-LDH/CNTs can be used as promising support electrode in direct methanol fuel cell. References
Fig. 10. Chronoamperometry for methanol oxidation at the Pt/CNTs/GCE and Pt/NiAl-LDH/CNTs /GCE in 2.0 M methanol and 1 M H2SO4.
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methanol to obtain a higher current density may be considered 2 M. The effect of potential sweep rates was studied on the electrocatalytic behavior of methanol onto Pt/NiAl-LDH/CNTs/GCE as can be seen in Fig. 9B. The linear relationship between the anodic peak current and square rate of potential sweep rates can be observed. This implied that the electrooxidation of methanol at the surface of the modified electrode may be controlled by diffusion process. 3.4. Chronoamperometry studies comparison of stability In order to evaluate the electrocatalytic activity of the catalyst and the poisoning of the active surface under continuous operation conditions, long-term chronoamperometry experiments were performed (Fig. 10). In two current-time curve for Pt/CNTs/GCE and Pt/NiAlLDH/CNTs/GCE, there was an initial current slower decay, but the current values obtained for the Pt/NiAl-LDH/GCE were higher than those obtained for Pt/NiAl-LDH/CNTs/GCE. The data indicated that the Pt/NiAl-LDH/CNTs/GCE have high activity for methanol oxidation than Pt/CNTs/GCE. The stability of the Pt/NiAl-LDH/CNTs/GCE was examined by using cyclic voltammetry technique (figure not shown). The anodic peak current decreases gradually with potential cycling. In general, the loss of catalytic activity with successive scans of potential may result from the consumption of methanol during the potential scanning in cyclic voltammetry. It also perhaps due to poisoning and the structure changes of the Pt particles as a result of the potentials perturbation 5
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