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Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell Van Vinh Pham a,*, Van-Thao Ta a, Cho Sunglae b a b
Hanoi National University of Education, Viet Nam The University of Ulsan, South Korea
article info
abstract
Article history:
The alloy of NiPt nanoparticles was successfully synthesized by galvanic replacement
Received 9 November 2016
method in which Ni nanoparticles used as the templates and H2PtCl6 solution as additional
Received in revised form
reagent. The preparation conditions of Ni nanoparticle were optimized. The effect of
28 December 2016
platinum contents on the structure, morphology, magnetic and electrocatalyst of NiPt was
Accepted 16 January 2017
investigated. The phase analysis by XRD showed the presence of Ni and Pt crystalline
Available online xxx
phases on the alloy. The TEM images indicated that the NiPt nanoparticles had porous crystalline structure with grain size in the range of 25 nme30 nm. Besides, composition
Keywords:
analysis by EDX showed that the ratios of Ni and Pt were changed with a change of the
NiPt alloy nanoparticles
amount of H2PtCl6 using for the galvanic reaction. The magnetic properties of NiPt nano-
Electrocatalyst
particles change significantly with a change of Pt composition. The NiPt nanoparticles
Galvanic replacement
exhibit ferromagnetic behavior depending on the amount of Pt composition. In particular,
Direct ethanol fuel cell
saturation magnetization decreases from 6.5 emu/g to 4.0 emu/g with the decrease of Ni:Pt
Ethanol oxidation
ratio from 57.0:3.6 to 57.0:8.1 respectively. With lower Ni:Pt ratio (57.0:18.0), the NiPt nanoparticles exhibits superparamagnetic properties. The magnetic properties were attributed to the formation of NiPt alloy in which the electrons transfer from Pt atoms to d band of Ni. The cyclic voltammetry measurement showed that NiPt nanoparticles exhibit better ethanol oxidation in alkali medium comparing with pure Platinum. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the recent years, with accelerating depletion of fossil fuels (petroleum, coal, ...), there has been a great deal of interest in the development of new technologies using alternative energy sources, including solar, wind, tide or chemical fuel related energies. Amongst these, the technology related to development of fuel cells has been attracted particularly attention from researchers. Fuel cells (FC) produce electricity by converting chemical energy to electrical energy directly without
heat-to-mechanical conversion so their efficiency is not affected by limiting efficiency of Carnot cycle. Thus, fuel cell operation is quiet, high performative and environment friendly. One of the most attractive fuel cells that have been studied widely is direct ethanol fuel cell (DEFC) because ethanol is an excellent electroactive organic fuel which is possible to provide 12 e in alkaline ethanol oxidation [1,2]. Moreover, ethanol is non-toxic and easy to be synthesized from agriculture productions or bioproductions. Up to now, DEFCs, however, are not yet to be utilized popularly due to their high cost. The main reason for this is that Pt
* Corresponding author. Faculty of Physics, Hanoi National University of Education, 136 Xuan Thuy, Cau Giay, Hanoi, Viet Nam. E-mail address: [email protected] (V.V. Pham). http://dx.doi.org/10.1016/j.ijhydene.2017.01.236 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
nanoparticles have widely been used as electrocatalysts for ethanol oxidation in DEFCs [3]. This electrocatalyst is not only expensive but also poisoned by strongly adsorbed CO intermediates during ORR [4]. To find solutions for these issues, current tendency is towards searching new materials or the materials with unique structures in order to reduce the amount of Pt used and improve the efficiency of the FCs. Recent studies showed that hollow structures exhibited great enhancement of the catalytic activity over the solid nanocatalysts, and thus cut down the costs of the fuel cells. Indeed, because of the lower densities and higher specific surface areas compared with that of solid ones, hollow nanostructures exhibited steadily improved catalytic performance [5e9]. Besides, control shape and size of nanoparticles are also studied for this purpose [10] e.g. N.V. Long et al. [11,12] have demonstrated that sharp polyhedral Pt exhibited better electrocatalyst activity. However, such approaches are limited by not solving the carbonaceous poison problem. Alloyed nano-catalysts exhibited significantly enhanced electrocatalytic activity and durability over the pure Pt nanocatalysts, and thereby reduce the costs of the catalysts [5,13]. According to this trend, the Pt-based alloy materials, combining of Pt and a much cheaper metal such as, Fe, Co, Ni or Cu nowadays, have been also studied widely [6,7]. Among them, NiPt exhibits the noble properties which can be utilized for electrochemical activities. Theoretical and experimental studies revealed that NiePt catalysts with Pt-rich surfaces and Ni-rich subsurfaces are very active towards the ORR [8,14e16]. Indeed, in alkaline media, the formation of nickel hydroxide such as NiOH, Ni(OH)2 and NiOOH [9] during ORR can remove the carbonaceous intermediate. Moreover, charges transfer from Ni to Pt modified the electronic structure of Pt, resulting in weakened CO adsorption on NiPt binary clusters [17]. Therefore, this study is focused on the synthesis and evaluation electrocatalyst ability of the porous NiPt alloy nanoparticles for ethanol oxidation reaction (EOR). The main purpose of the study is to improve the electrocatalytic activity of Pt-based catalyst as well as reduce the Pt content used for the production of ethanol fuel cells.
Experimental Alloy of NiPt nanoparticles were synthesized by galvanic replacement method using Ni nanoparticles as the templates. The experimental process is described as following:
Preparation of Ni nanoparticles 0.9 g Ni(NO3)2$6H2O (Sigma Aldrich, USA: Purification of 99.999%) was dissolved in 50 ml polyvinyl pyrrolidone (PVPSigma Aldrich, USA: Purification of 99.999%) solution. The concentrations of PVP were varied in tern 0.3, 0.5, 07 M in order to determine the effect of PVP on the structure of Ni nanoparticles. The solutions were bubbled for 60 min by pure nitrogen gas to remove the oxygen remain. After that, 0.4 g ml NaBH4 (BDH, UK: Purification of 98%) dissolved in 2 ml DI water was dropped slowly in the solution. NaBH4 reduced Ni(NO3)2$6H2O to form Ni nanoparticles due to following chemical reaction:
2Ni
2þ
þ 4BH 4 þ 12H2 O/2Ni þ 14H2 þ 4BðOHÞ3
(1)
The black solution containing Ni nanoparticles and unwanted materials that produced during the reaction were separated by a centrifuge to collect Ni nanoparticles. The materials thus obtained were washed in DI water many times. Amount of them was kept at room temperature and the others were annealed in hydrogen gas at 250 C, 300 C and 400 C to purge and to study their crystallization process.
Preparation of NiPt nanoparticles Selected Ni nanoparticles gained after washing were quickly redispersed in PVP solution 7 M. The solution containing Ni nanoparticles then separated equally in three parts. Different amount of H2PtCl6 (Alpha Aesar; Purification of 99.99%) solution 0.5 M was in tern dropped slowly in each part solution. Galvanic replacement reaction is described by equation: 2Ni þ H2 PtCl6 ¼ Pt þ 2NiCl2 þ 2HCl
(2)
After 2 h reaction, the productions were purged and centrifugally collected. The collected particles were dried and annealed in hydrogen gas at 300 C for 60 min.
Physical properties analysis The crystalline phases were determined by an X-ray diffractometer (Bruker, D8 Advance). The morphologies and chemical compositions were characterized by scanning electron microscopy (SEM, HitachiS-4800) with an energy dispersive Xray spectroscope (EDS), and transmission electron microscopy (TEM). Magnetic properties were studied by a vibrating sample magnetometer (VSM, Lakeshore 7400).
Preparation of electrode for cyclic voltammetry measurement 0.05 g of material had been ultrasonically mixed with 30 ml nafion® 5% (Alpha Aesar) until it started to condense. The working electrodes were prepared by stuffing the mixed material to a plastic tube (diameter of 0.2 mm) with a graphite electrode inside. Cyclic voltammetry were preformed at room temperature by an instrument using a three-electrode (Metrohm, 797 VA Computrace) in 1 M KOH þ 2 M ethanol solution. The measurements were carried out at the potential sweep rate of 50 mVs1 in the range from 0.8v to 0.6v for the alloy of NiPt nanoparticles and 0.4v to 0.8v for the Ni nanoparticles. The test solutions were purged with high-purity nitrogen gas before measurement.
Results and discussions Ni nanoparticles synthesized by a reduction method using the strong reductant of NaBH4 were used as the templates to prepare NiPt nanoparticle. To obtain fine NiPt nanoparticles, the templates should be pure and possible to disperse in PVP solution. However, unwanted elements such as nickel oxide and nickel hydroxide are able to be created during
Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
synthesizing process. The purification can be controlled by the heat treatment in hydrogen gas or varying the legend contents. The heat process always accompany with the crystalline growing resulting in the extended crystals size. Besides, water content in the materials diminishes whereby the particles become inert and not to redisperse in the solution. In
Table 1 e The composition analysis of Ni nanoparticles synthesized with different PVP concentrations. PVP concentrations 0.3 M 0.5 M 0.7 M
Ni at%
O at%
Totals %
74.27 75.06 97.02
25.73 24.94 2.98
100.00 100.00 100.00
Fig. 2 e XRD diffractogram of Ni particles annealing in hydrogen environment with different temperatures: (a) room temperature; (b) 250 C; (c) 300 C; (d) 400 C.
this study, the different concentration of PVP was used to determine the optimum condition to prepare the templates. Fig. 1 is FE-SEM images of Ni nanoparticles synthesized with PVP concentration of 3 M, 5 M and 7 M. Although PVP concentration was not significant effect on the morphology and the particle size, this influenced on concentration of oxygen in the samples. The composition analysis (in Table 1) showed that Ni nanoparticles synthesized with PVP solution 7 M exhibited lest oxygen. Therefore, this concentration was used for further studies. The XRD patterns in Fig. 2 show the influence of annealing temperatures on crystalline structure of Ni nanoparticles. No XRD peaks are found for the samples prepared without annealing. This means that the nanoparticles after centrifugal collection were formed in amorphous phase. The XRD patterns of Ni nanoparticles after annealing show the presence of peaks corresponding to face centered cubic structure of Ni crystal and the intensity of the peaks increase with an increase of the annealing temperatures. This result is used as a reference data for heating treatment of NiPt in further studies.
Table 2 e Atomic ratio of Ni:Pt nanoparticles synthesized with different amount of H2PtCl6. Sample name Fig. 1 e FE-SEM images of nickel particles with different PVP concentration (a) 3 M; (b) 5 M; (c) 7 M.
(a) (b) (c)
Ni:Pt ratios 57.0:3.6 57.0:8.1 57.0:18.0
Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236
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Fig. 3 e XRD diffractogram of NiPt nanoparticles with different Ni:Pt ratios: (a) 57.0:3.6; (b) 57.0:8.1 and (c) 57.0:18.0.
Fig. 4 e Hysteresis loop of Ni and NiPt nanoparticles with different Ni:Pt ratios: (a) pure Ni; (b) 57.0:3.6; (c) 57.0:8.1 and (d) 57.0:18.0.
The compositional analysis (EDX) was used to determine the atomic ratios of Ni:Pt in the NiPt alloy nanoparticles after annealing at 300 C in hydrogen gas. The ratio decreases with the increase of the amount of H2PtCl6 (Table 2).
Only a few percent of remaining oxygen found confirmed the purification of the nanoparticles. The crystal analysis was characterized by X-ray powder diffraction method. The XRD pattern in Fig. 3 shows that two crystal phases corresponding to Pt and Ni crystals presented for the sample with Ni:Pt ratio of 57.0:3.6. Further increase of Pt contents, the XRD peaks of Ni were disappeared while the intensity of Pt peaks increased and shifted toward the high 2q angle. In general, the peak shift toward the higher angle is believed to originate from the incorporation of Ni into Pt lattice or substitution of Ni to Pt in the Pt lattice due to smaller Ni atom comparing to Pt atom. Beside, theoretical study of NiPt alloy shows that the separation between the majority spin s- and d-band centers of Pt atoms caused by interaction between Ni and Pt lowered the S potential leading to a reduce of the lattice [18,19]. According to Bragg's Law, the reduced lattice shifted the XRD peaks toward higher 2q angle. This confirmed that the nanoparticles were formed as NiPt alloy. The supposition agrees with argument of and some others authors [20,21]. Fig. 4 shows the hysteresis loops of the Ni and NiPt nanoparticle measured at 300 K. Ni and NiPt nanoparticles with low Pt contents exhibited ferromagnetic behaviors. The magnetizations decreased with the increase of Pt contents. With higher Pt content (curve (d)), NiPt nanoparticles showed superparamagnetic behavior. Theoretical studies [22] showed that NiPt alloy exhibits ferromagnetic behavior and magnetic properties were strong influence by contributions of Pt atoms occupied the surface of Ni particles and Pt changes d band structure of Ni resulting in the changes of magnetic properties. In detail, the decrease of magnetization was attributed to the transfer of electrons from Pt to Ni d band resulting in the decrease of the number of nonpaired electrons [23]. On the other hand, platinum is a paramagnetic material and Nickel is a ferromagnetic material but no paramagnetic phase of Pt is observed in the hysteresis loop. This reconfirms that Pt incorporated with Ni to form NiPt alloy. Fig. 5 is TEM images of NiPt nanoparticles with different Ni:Pt ratios. Most of the nanoparticles had polyhedron shapes with the sizes ranging from 20 to 30 nm. The polyhedron shapes revealed the crystalline structure of the nanoparticles. It is interesting to find that a part of nanoparticles were formed as the hollow shapes for the samples with Ni:Pt ratio of 57.0:18.0 (Fig. 5c). The cyclic voltammetry measurement was used to determine the ethanol oxidation activity of the materials in 1.0 M
Fig. 5 e TEM images of NiPt nanoparticles with different Ni:Pt ratios: (a) 57.0:3.6; (b) 57.0:8.1 and (c) 57.0:18.0. Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
5
KOH and 2.0 M ethanol. The measurement was first carried out with Ni nanoparticles and then with NiPt nanoparticles. Fig. 6 shows the cyclic voltammogram (CV) of the Ni nanoparticles. The present peak at the applied voltage of 0.37 V was attributed to the oxidation of Ni(II) to Ni(III) due to changing of Ni(OH)2 to NiOOH [24]. The oxidation reaction process is described as following equations [9]: Ni þ OH /Ni OHads þ e
(3)
Ni OHads þ OH /NiðOHÞ2 þ e
(4)
NiðOHÞ2 /NiOOH
(5)
The peaks that relate with the oxidation of Ni(0) to Ni(I) did not present in the range of measurement because of its occurrence at low negative potential (about 0.8 V [1]). Fig. 7 (a) is the CV of pure Pt wire. A current peak in forward sweep (If) is the result of ethanol oxidation reaction and the current peak in backward sweep (Ib) is related with the carbonaceous intermediate in which the oxidation of COads is not complete oxidized in anodic sweep [1]. Fig. 7 (b, c and d) show the CV of NiPt nanoparticles with different Ni:Pt ratio. The first peak (marked by “I”) is related to ethanol oxidation due to equation: C2 H5 OH þ 12OH /2CO2 þ 9H2 O þ 12e
(6)
and the second peak agrees with the oxidation of Ni(II) to Ni(III) as showing in Fig. 6. The ethanol oxidation peak (I) increases with the increase of Pt content. The increase of the peak is believed to the contribution of Pt in NiPt alloy and the removal of the carbonaceous adsorption. Indeed, platinum, an active metal for Ce C bond activation has been known as an excellent electrocatalyst material for ethanol oxidation. Therefore, increasing amount of Pt improves its ability to activate the CeC bond resulting in increasing the oxidation reactions. In addition, the present of NiOOH is expected to remove the carbonaceous intermediates according to reaction:
Fig. 7 e The cyclic voltammogram of the NiPt nanoparticles with different Ni:Pt ratios: (a) pure Pt wire; (b) 57.0:3.6; (c) 57.0:8.1 and (d) 57.0:18.0 in KOH 1 M þ C2H5OH 2 M.
2OH
PtðCOÞads !Pt þ CO3 2 þ 2Hþ
(7)
It is possible to see that in the backward sweep (Fig. 7bed), the oxidation peak of COads did not appear clearly like it did for pure Pt wire (Fig. 7a). This is an experimental evidence of NiPt alloy to demonstrate the removal of the carbonaceous adsorption. Besides carbonaceous removal, the improvement of the electrocatalyst should be concerned with the contribution of weakening PteCO bonding. Pt atoms adsorb CO by receiving the electrons from the 5s orbit of CO molecules and transferring electrons from their d band to the 2p* antibonding orbit of CO molecules [25]. The PteCO bonding will be weakened when Pt alloys with Ni because Pt atoms have to donate electrons to Ni atoms instead of to CO molecules. Thus, it limited the electrode poisoning effect.
Conclusions
Fig. 6 e The cyclic voltammogram of the Ni nanoparticles in KOH 1 M þ C2H5OH 2 M.
The alloy of NiPt nanoparticles were successfully synthesized by galvanic replacement method using Ni nanoparticles as the templates. The presence of the peaks of Ni and Pt in XRD pattern as well as polyhedron shapes in TEM demonstrated that NiPt alloys had crystalline structure with the particle sizes ranging from 25 nm to 30 nm. NiPt nanoparticles exhibited ferromagnetic behavior with the Ni:Pt ratio lesser than 57.0:18.0 and became superparamagnetic for further increase of Pt contents. The XRD peaks of Pt shift toward high 2q angle and the magnetic properties of NiPt nanoparticles revealed the formation of NiPt alloy. The result indicated that the electrocatalytic activities for ethanol oxidation reactions of NiPt nanoparticles were better than that of pure platinum. The electrocatalytic activities for ethanol oxidation of NiPt increased with increasing of Pt content. The improvements of the electrocatalytic activities were the results of the formation of NiPt alloy due to the weakening of PteCO bonding and the removal of carbonaceous intermediate.
Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236
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Acknowledgement This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number of 103.02-2013.50.
references
[1] Soundararajan D, Park JH, Kim KH, Ko JM. PteNi alloy nanoparticles supported on CNF as catalyst for direct ethanol fuel cells. Curr Appl Phys 2012;12:854e9. [2] Wang Y, Zou S, Cai W-B. Recent advances on electrooxidation of ethanol on Pt- and Pd-Based catalysts: from reaction mechanisms to catalytic materials. Catalysts 2015;5:1507e34. [3] Akhairi MAF, Kamarudin SK. Catalysts in direct ethanol fuel cell (DEFC): an overview. Int J Hydrogen Energy 2016;41:4214e28. [4] Huang W, Wang H, Zhou J, Wang J, Duchesne PN, Muir D, et al. Highly active and durable methanol oxidation electrocatalyst based on the synergy of platinumenickel hydroxideegraphene. Nat Commun 2015;6:1e8. [5] Zhou X-W, Zhang R-H, Zhou Z-Y, Sun S-G. Preparation of PtNi hollow nanospheres for the electrocatalytic oxidation of methanol. J Power Sources 2011;196:5844e8. [6] Stamenkovic VR, Fowler B, Mun BS, Wang G, Ross PN, Lucas CA, et al. Improved oxygen reduction activity on Pt3Ni(111) via increased surface site availability. Science 2007;315:493e7. [7] Stamenkovic VR, Mun BS, Arenz M, Mayrhofer KJ, Lucas CA, Wang G, et al. Trends in electrocatalysis on extended and nanoscale Pt-bimetallic alloy surfaces. Nat Mater 2007;6:241e7. [8] Hoover RR, Tolmachev YV. Electrochemical properties of Pt coatings on Ni prepared by atomic layer deposition. J Electrochem Soc 2009;156:A37e43. [9] Mukherjee P, Bagchi J, Dutta S, Bhattacharya SK. The nickel supported platinum catalyst for anodic oxidation of ethanol in alkaline medium. Appl Catal A General 2015;506:220e7. [10] Peng Z, Yang H. Designer platinum nanoparticles: control of shape, composition in alloy, nanostructure and electrocatalytic property. Nano Today 2009;4:143e64. [11] Long NV, Ohtaki M, Nogami M, Hien TD. Effects of heat treatment and poly(vinylpyrrolidone) (PVP) polymer on electrocatalytic activity of polyhedral Pt nanoparticles towards their methanol oxidation. Colloid Polym Sci 2011;289:1373e86.
[12] Long NV, Ohtaki M, Uchida M, Jalem R, Hirata H, Chien ND, et al. Synthesis and characterization of polyhedral Pt nanoparticles: their catalytic property, surface attachment, self-aggregation and assembly. J Colloid Interface Sci 2011;359:339e50. [13] Long NV, Yang Y, Thi CM, Minh NV, Cao Y, Nogami M. The development of mixture, alloy, and core-shell nanocatalysts with nanomaterial supports for energy conversion in low-temperature fuel cells. Nano Energy 2013;2:636e76. [14] Menning CA, Hwu HH, Chen JG. Experimental and theoretical investigation of the stability of Pt3dPt(111) bimetallic surfaces under oxygen environment. J Phys Chem B 2006;110:15471e7. [15] Radillo-Diaz A, Coronado Y, Perez LA, Garzon IL. Structural and electronic properties of PtPd and PtNi nanoalloys. Eur Phys J D 2009;52:127e30. [16] Yang Z, Zhang Y, Wang J, Ma S. First-principles study on the Ni@Pt12 Ih coreeshell nanoparticles: a good catalyst for oxygen reduction reaction. Phys Lett A 2011;375:3142e8. [17] Jiang Q, Jiang L, Hou H, Qi J, Wang S, Sun G. Promoting effect of Ni in PtNi bimetallic electrocatalysts for the methanol oxidation reaction in alkaline media: experimental and density functional theory studies. J Phys Chem C 2010;114:19714e22. [18] Singh PP. Relativity and magnetism in NiePd and NiePt alloys. J Magn Magn Mater 2003;261:347e52. [19] Singh PP, Gonis A, Turchi PEA. Toward a unified approach to the study of metallic alloys: application to the phase stability of Ni-Pt. Phys Rev Lett 1993;71:1605. [20] Park K-W, Choi J-H, Kwon B-K, Lee S-A, Sung Y-E. Chemical and electronic effects of Ni in Pt/Ni and Pt/Ru/Ni alloy nanoparticles in methanol electrooxidation. J Phys Chem B 2002;106:1869e77. [21] Jiang S, Ma Y, Tao H, Jian G, Wang X, Fan Y, et al. Highly dispersed PteNi nanoparticles on nitrogen-doped carbon nanotubes for application in direct methanol fuel cells. J Nanosci Nanotechnol 2010;10:3895e900. [22] Wang L, Yang Y, Wang N, Huang S. Theoretical investigation of structural, electronic and magnetic properties for PtnNi55-n (n ¼ 0e55) nanoparticles. Comput Mater Sci 2016;117:15e23. [23] Benguedouar Y, Keghouche N, Belloni J. Structural and magnetic properties of NiePt nanoalloys supported on silica. Mater Sci Eng B 2012;177:27e33. [24] Riyanto, Othman MR, Salimon J. Analysis of ethanol using copper and nickel sheet electrodes by cyclic voltammetry. Malays J Anal Sci 2007;11:379e87. [25] Blyholder G. Molecular orbital view of chemisorbed carbon monoxide. J Phys Chem 1964;68:2772e7.
Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell Van Vinh Pham a,*, Van-Thao Ta a, Cho Sunglae b a b
Hanoi National University of Education, Viet Nam The University of Ulsan, South Korea
article info
abstract
Article history:
The alloy of NiPt nanoparticles was successfully synthesized by galvanic replacement
Received 9 November 2016
method in which Ni nanoparticles used as the templates and H2PtCl6 solution as additional
Received in revised form
reagent. The preparation conditions of Ni nanoparticle were optimized. The effect of
28 December 2016
platinum contents on the structure, morphology, magnetic and electrocatalyst of NiPt was
Accepted 16 January 2017
investigated. The phase analysis by XRD showed the presence of Ni and Pt crystalline
Available online xxx
phases on the alloy. The TEM images indicated that the NiPt nanoparticles had porous crystalline structure with grain size in the range of 25 nme30 nm. Besides, composition
Keywords:
analysis by EDX showed that the ratios of Ni and Pt were changed with a change of the
NiPt alloy nanoparticles
amount of H2PtCl6 using for the galvanic reaction. The magnetic properties of NiPt nano-
Electrocatalyst
particles change significantly with a change of Pt composition. The NiPt nanoparticles
Galvanic replacement
exhibit ferromagnetic behavior depending on the amount of Pt composition. In particular,
Direct ethanol fuel cell
saturation magnetization decreases from 6.5 emu/g to 4.0 emu/g with the decrease of Ni:Pt
Ethanol oxidation
ratio from 57.0:3.6 to 57.0:8.1 respectively. With lower Ni:Pt ratio (57.0:18.0), the NiPt nanoparticles exhibits superparamagnetic properties. The magnetic properties were attributed to the formation of NiPt alloy in which the electrons transfer from Pt atoms to d band of Ni. The cyclic voltammetry measurement showed that NiPt nanoparticles exhibit better ethanol oxidation in alkali medium comparing with pure Platinum. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In the recent years, with accelerating depletion of fossil fuels (petroleum, coal, ...), there has been a great deal of interest in the development of new technologies using alternative energy sources, including solar, wind, tide or chemical fuel related energies. Amongst these, the technology related to development of fuel cells has been attracted particularly attention from researchers. Fuel cells (FC) produce electricity by converting chemical energy to electrical energy directly without
heat-to-mechanical conversion so their efficiency is not affected by limiting efficiency of Carnot cycle. Thus, fuel cell operation is quiet, high performative and environment friendly. One of the most attractive fuel cells that have been studied widely is direct ethanol fuel cell (DEFC) because ethanol is an excellent electroactive organic fuel which is possible to provide 12 e in alkaline ethanol oxidation [1,2]. Moreover, ethanol is non-toxic and easy to be synthesized from agriculture productions or bioproductions. Up to now, DEFCs, however, are not yet to be utilized popularly due to their high cost. The main reason for this is that Pt
* Corresponding author. Faculty of Physics, Hanoi National University of Education, 136 Xuan Thuy, Cau Giay, Hanoi, Viet Nam. E-mail address: [email protected] (V.V. Pham). http://dx.doi.org/10.1016/j.ijhydene.2017.01.236 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e6
nanoparticles have widely been used as electrocatalysts for ethanol oxidation in DEFCs [3]. This electrocatalyst is not only expensive but also poisoned by strongly adsorbed CO intermediates during ORR [4]. To find solutions for these issues, current tendency is towards searching new materials or the materials with unique structures in order to reduce the amount of Pt used and improve the efficiency of the FCs. Recent studies showed that hollow structures exhibited great enhancement of the catalytic activity over the solid nanocatalysts, and thus cut down the costs of the fuel cells. Indeed, because of the lower densities and higher specific surface areas compared with that of solid ones, hollow nanostructures exhibited steadily improved catalytic performance [5e9]. Besides, control shape and size of nanoparticles are also studied for this purpose [10] e.g. N.V. Long et al. [11,12] have demonstrated that sharp polyhedral Pt exhibited better electrocatalyst activity. However, such approaches are limited by not solving the carbonaceous poison problem. Alloyed nano-catalysts exhibited significantly enhanced electrocatalytic activity and durability over the pure Pt nanocatalysts, and thereby reduce the costs of the catalysts [5,13]. According to this trend, the Pt-based alloy materials, combining of Pt and a much cheaper metal such as, Fe, Co, Ni or Cu nowadays, have been also studied widely [6,7]. Among them, NiPt exhibits the noble properties which can be utilized for electrochemical activities. Theoretical and experimental studies revealed that NiePt catalysts with Pt-rich surfaces and Ni-rich subsurfaces are very active towards the ORR [8,14e16]. Indeed, in alkaline media, the formation of nickel hydroxide such as NiOH, Ni(OH)2 and NiOOH [9] during ORR can remove the carbonaceous intermediate. Moreover, charges transfer from Ni to Pt modified the electronic structure of Pt, resulting in weakened CO adsorption on NiPt binary clusters [17]. Therefore, this study is focused on the synthesis and evaluation electrocatalyst ability of the porous NiPt alloy nanoparticles for ethanol oxidation reaction (EOR). The main purpose of the study is to improve the electrocatalytic activity of Pt-based catalyst as well as reduce the Pt content used for the production of ethanol fuel cells.
Experimental Alloy of NiPt nanoparticles were synthesized by galvanic replacement method using Ni nanoparticles as the templates. The experimental process is described as following:
Preparation of Ni nanoparticles 0.9 g Ni(NO3)2$6H2O (Sigma Aldrich, USA: Purification of 99.999%) was dissolved in 50 ml polyvinyl pyrrolidone (PVPSigma Aldrich, USA: Purification of 99.999%) solution. The concentrations of PVP were varied in tern 0.3, 0.5, 07 M in order to determine the effect of PVP on the structure of Ni nanoparticles. The solutions were bubbled for 60 min by pure nitrogen gas to remove the oxygen remain. After that, 0.4 g ml NaBH4 (BDH, UK: Purification of 98%) dissolved in 2 ml DI water was dropped slowly in the solution. NaBH4 reduced Ni(NO3)2$6H2O to form Ni nanoparticles due to following chemical reaction:
2Ni
2þ
þ 4BH 4 þ 12H2 O/2Ni þ 14H2 þ 4BðOHÞ3
(1)
The black solution containing Ni nanoparticles and unwanted materials that produced during the reaction were separated by a centrifuge to collect Ni nanoparticles. The materials thus obtained were washed in DI water many times. Amount of them was kept at room temperature and the others were annealed in hydrogen gas at 250 C, 300 C and 400 C to purge and to study their crystallization process.
Preparation of NiPt nanoparticles Selected Ni nanoparticles gained after washing were quickly redispersed in PVP solution 7 M. The solution containing Ni nanoparticles then separated equally in three parts. Different amount of H2PtCl6 (Alpha Aesar; Purification of 99.99%) solution 0.5 M was in tern dropped slowly in each part solution. Galvanic replacement reaction is described by equation: 2Ni þ H2 PtCl6 ¼ Pt þ 2NiCl2 þ 2HCl
(2)
After 2 h reaction, the productions were purged and centrifugally collected. The collected particles were dried and annealed in hydrogen gas at 300 C for 60 min.
Physical properties analysis The crystalline phases were determined by an X-ray diffractometer (Bruker, D8 Advance). The morphologies and chemical compositions were characterized by scanning electron microscopy (SEM, HitachiS-4800) with an energy dispersive Xray spectroscope (EDS), and transmission electron microscopy (TEM). Magnetic properties were studied by a vibrating sample magnetometer (VSM, Lakeshore 7400).
Preparation of electrode for cyclic voltammetry measurement 0.05 g of material had been ultrasonically mixed with 30 ml nafion® 5% (Alpha Aesar) until it started to condense. The working electrodes were prepared by stuffing the mixed material to a plastic tube (diameter of 0.2 mm) with a graphite electrode inside. Cyclic voltammetry were preformed at room temperature by an instrument using a three-electrode (Metrohm, 797 VA Computrace) in 1 M KOH þ 2 M ethanol solution. The measurements were carried out at the potential sweep rate of 50 mVs1 in the range from 0.8v to 0.6v for the alloy of NiPt nanoparticles and 0.4v to 0.8v for the Ni nanoparticles. The test solutions were purged with high-purity nitrogen gas before measurement.
Results and discussions Ni nanoparticles synthesized by a reduction method using the strong reductant of NaBH4 were used as the templates to prepare NiPt nanoparticle. To obtain fine NiPt nanoparticles, the templates should be pure and possible to disperse in PVP solution. However, unwanted elements such as nickel oxide and nickel hydroxide are able to be created during
Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236
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synthesizing process. The purification can be controlled by the heat treatment in hydrogen gas or varying the legend contents. The heat process always accompany with the crystalline growing resulting in the extended crystals size. Besides, water content in the materials diminishes whereby the particles become inert and not to redisperse in the solution. In
Table 1 e The composition analysis of Ni nanoparticles synthesized with different PVP concentrations. PVP concentrations 0.3 M 0.5 M 0.7 M
Ni at%
O at%
Totals %
74.27 75.06 97.02
25.73 24.94 2.98
100.00 100.00 100.00
Fig. 2 e XRD diffractogram of Ni particles annealing in hydrogen environment with different temperatures: (a) room temperature; (b) 250 C; (c) 300 C; (d) 400 C.
this study, the different concentration of PVP was used to determine the optimum condition to prepare the templates. Fig. 1 is FE-SEM images of Ni nanoparticles synthesized with PVP concentration of 3 M, 5 M and 7 M. Although PVP concentration was not significant effect on the morphology and the particle size, this influenced on concentration of oxygen in the samples. The composition analysis (in Table 1) showed that Ni nanoparticles synthesized with PVP solution 7 M exhibited lest oxygen. Therefore, this concentration was used for further studies. The XRD patterns in Fig. 2 show the influence of annealing temperatures on crystalline structure of Ni nanoparticles. No XRD peaks are found for the samples prepared without annealing. This means that the nanoparticles after centrifugal collection were formed in amorphous phase. The XRD patterns of Ni nanoparticles after annealing show the presence of peaks corresponding to face centered cubic structure of Ni crystal and the intensity of the peaks increase with an increase of the annealing temperatures. This result is used as a reference data for heating treatment of NiPt in further studies.
Table 2 e Atomic ratio of Ni:Pt nanoparticles synthesized with different amount of H2PtCl6. Sample name Fig. 1 e FE-SEM images of nickel particles with different PVP concentration (a) 3 M; (b) 5 M; (c) 7 M.
(a) (b) (c)
Ni:Pt ratios 57.0:3.6 57.0:8.1 57.0:18.0
Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236
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Fig. 3 e XRD diffractogram of NiPt nanoparticles with different Ni:Pt ratios: (a) 57.0:3.6; (b) 57.0:8.1 and (c) 57.0:18.0.
Fig. 4 e Hysteresis loop of Ni and NiPt nanoparticles with different Ni:Pt ratios: (a) pure Ni; (b) 57.0:3.6; (c) 57.0:8.1 and (d) 57.0:18.0.
The compositional analysis (EDX) was used to determine the atomic ratios of Ni:Pt in the NiPt alloy nanoparticles after annealing at 300 C in hydrogen gas. The ratio decreases with the increase of the amount of H2PtCl6 (Table 2).
Only a few percent of remaining oxygen found confirmed the purification of the nanoparticles. The crystal analysis was characterized by X-ray powder diffraction method. The XRD pattern in Fig. 3 shows that two crystal phases corresponding to Pt and Ni crystals presented for the sample with Ni:Pt ratio of 57.0:3.6. Further increase of Pt contents, the XRD peaks of Ni were disappeared while the intensity of Pt peaks increased and shifted toward the high 2q angle. In general, the peak shift toward the higher angle is believed to originate from the incorporation of Ni into Pt lattice or substitution of Ni to Pt in the Pt lattice due to smaller Ni atom comparing to Pt atom. Beside, theoretical study of NiPt alloy shows that the separation between the majority spin s- and d-band centers of Pt atoms caused by interaction between Ni and Pt lowered the S potential leading to a reduce of the lattice [18,19]. According to Bragg's Law, the reduced lattice shifted the XRD peaks toward higher 2q angle. This confirmed that the nanoparticles were formed as NiPt alloy. The supposition agrees with argument of and some others authors [20,21]. Fig. 4 shows the hysteresis loops of the Ni and NiPt nanoparticle measured at 300 K. Ni and NiPt nanoparticles with low Pt contents exhibited ferromagnetic behaviors. The magnetizations decreased with the increase of Pt contents. With higher Pt content (curve (d)), NiPt nanoparticles showed superparamagnetic behavior. Theoretical studies [22] showed that NiPt alloy exhibits ferromagnetic behavior and magnetic properties were strong influence by contributions of Pt atoms occupied the surface of Ni particles and Pt changes d band structure of Ni resulting in the changes of magnetic properties. In detail, the decrease of magnetization was attributed to the transfer of electrons from Pt to Ni d band resulting in the decrease of the number of nonpaired electrons [23]. On the other hand, platinum is a paramagnetic material and Nickel is a ferromagnetic material but no paramagnetic phase of Pt is observed in the hysteresis loop. This reconfirms that Pt incorporated with Ni to form NiPt alloy. Fig. 5 is TEM images of NiPt nanoparticles with different Ni:Pt ratios. Most of the nanoparticles had polyhedron shapes with the sizes ranging from 20 to 30 nm. The polyhedron shapes revealed the crystalline structure of the nanoparticles. It is interesting to find that a part of nanoparticles were formed as the hollow shapes for the samples with Ni:Pt ratio of 57.0:18.0 (Fig. 5c). The cyclic voltammetry measurement was used to determine the ethanol oxidation activity of the materials in 1.0 M
Fig. 5 e TEM images of NiPt nanoparticles with different Ni:Pt ratios: (a) 57.0:3.6; (b) 57.0:8.1 and (c) 57.0:18.0. Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236
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KOH and 2.0 M ethanol. The measurement was first carried out with Ni nanoparticles and then with NiPt nanoparticles. Fig. 6 shows the cyclic voltammogram (CV) of the Ni nanoparticles. The present peak at the applied voltage of 0.37 V was attributed to the oxidation of Ni(II) to Ni(III) due to changing of Ni(OH)2 to NiOOH [24]. The oxidation reaction process is described as following equations [9]: Ni þ OH /Ni OHads þ e
(3)
Ni OHads þ OH /NiðOHÞ2 þ e
(4)
NiðOHÞ2 /NiOOH
(5)
The peaks that relate with the oxidation of Ni(0) to Ni(I) did not present in the range of measurement because of its occurrence at low negative potential (about 0.8 V [1]). Fig. 7 (a) is the CV of pure Pt wire. A current peak in forward sweep (If) is the result of ethanol oxidation reaction and the current peak in backward sweep (Ib) is related with the carbonaceous intermediate in which the oxidation of COads is not complete oxidized in anodic sweep [1]. Fig. 7 (b, c and d) show the CV of NiPt nanoparticles with different Ni:Pt ratio. The first peak (marked by “I”) is related to ethanol oxidation due to equation: C2 H5 OH þ 12OH /2CO2 þ 9H2 O þ 12e
(6)
and the second peak agrees with the oxidation of Ni(II) to Ni(III) as showing in Fig. 6. The ethanol oxidation peak (I) increases with the increase of Pt content. The increase of the peak is believed to the contribution of Pt in NiPt alloy and the removal of the carbonaceous adsorption. Indeed, platinum, an active metal for Ce C bond activation has been known as an excellent electrocatalyst material for ethanol oxidation. Therefore, increasing amount of Pt improves its ability to activate the CeC bond resulting in increasing the oxidation reactions. In addition, the present of NiOOH is expected to remove the carbonaceous intermediates according to reaction:
Fig. 7 e The cyclic voltammogram of the NiPt nanoparticles with different Ni:Pt ratios: (a) pure Pt wire; (b) 57.0:3.6; (c) 57.0:8.1 and (d) 57.0:18.0 in KOH 1 M þ C2H5OH 2 M.
2OH
PtðCOÞads !Pt þ CO3 2 þ 2Hþ
(7)
It is possible to see that in the backward sweep (Fig. 7bed), the oxidation peak of COads did not appear clearly like it did for pure Pt wire (Fig. 7a). This is an experimental evidence of NiPt alloy to demonstrate the removal of the carbonaceous adsorption. Besides carbonaceous removal, the improvement of the electrocatalyst should be concerned with the contribution of weakening PteCO bonding. Pt atoms adsorb CO by receiving the electrons from the 5s orbit of CO molecules and transferring electrons from their d band to the 2p* antibonding orbit of CO molecules [25]. The PteCO bonding will be weakened when Pt alloys with Ni because Pt atoms have to donate electrons to Ni atoms instead of to CO molecules. Thus, it limited the electrode poisoning effect.
Conclusions
Fig. 6 e The cyclic voltammogram of the Ni nanoparticles in KOH 1 M þ C2H5OH 2 M.
The alloy of NiPt nanoparticles were successfully synthesized by galvanic replacement method using Ni nanoparticles as the templates. The presence of the peaks of Ni and Pt in XRD pattern as well as polyhedron shapes in TEM demonstrated that NiPt alloys had crystalline structure with the particle sizes ranging from 25 nm to 30 nm. NiPt nanoparticles exhibited ferromagnetic behavior with the Ni:Pt ratio lesser than 57.0:18.0 and became superparamagnetic for further increase of Pt contents. The XRD peaks of Pt shift toward high 2q angle and the magnetic properties of NiPt nanoparticles revealed the formation of NiPt alloy. The result indicated that the electrocatalytic activities for ethanol oxidation reactions of NiPt nanoparticles were better than that of pure platinum. The electrocatalytic activities for ethanol oxidation of NiPt increased with increasing of Pt content. The improvements of the electrocatalytic activities were the results of the formation of NiPt alloy due to the weakening of PteCO bonding and the removal of carbonaceous intermediate.
Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236
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Acknowledgement This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number of 103.02-2013.50.
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Please cite this article in press as: Pham VV, et al., Synthesis of NiPt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.236