Synthesis of nipt alloy nanoparticles by galvanic replacement method for direct ethanol fuel cell

The alloy of NiPt nanoparticles was successfully synthesized by galvanic replacement method in which Ni nanoparticles used as the templates and H2PtCl6 solution as additional reagent. The preparation conditions of Ni nanoparticle were optimized. The effect of platinum contents on the structure, morphology, magnetic and electrocatalyst of NiPt was investigated. The phase analysis by XRD showed the presence of Ni and Pt crystalline 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 analysis by EDX showed that the ratios of Ni and Pt were changed with a change of the amount of H2PtCl6 using for the galvanic reaction. The magnetic properties of NiPt nanoparticles change significantly with a change of Pt composition. The NiPt nanoparticles exhibit ferromagnetic behavior depending on the amount of Pt composition. In particular, saturation magnetization decreases from 6.5 emug to 4.0 emug with the decrease of Ni:Pt 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.

Synthesis of NiPt alloy nanoparticles by galvanic

replacement method for direct ethanol fuel cell

Van Vinh Phama,*, Van-Thao Taa, Cho Sunglae b

aHanoi National University of Education, Viet Nam

bThe University of Ulsan, South Korea

a r t i c l e i n f o

Article history:

Received 9 November 2016

Received in revised form

28 December 2016

Accepted 16 January 2017

Available online 26 April 2017

Keywords:

NiPt alloy nanoparticles

Electrocatalyst

Galvanic replacement

Direct ethanol fuel cell

Ethanol oxidation

a b s t r a c t

The alloy of NiPt nanoparticles was successfully synthesized by galvanic replacement method in which Ni nanoparticles used as the templates and H2PtCl6solution as additional reagent The preparation conditions of Ni nanoparticle were optimized The effect of platinum contents on the structure, morphology, magnetic and electrocatalyst of NiPt was investigated The phase analysis by XRD showed the presence of Ni and Pt crystalline 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 analysis by EDX showed that the ratios of Ni and Pt were changed with a change of the amount of H2PtCl6using for the galvanic reaction The magnetic properties of NiPt nano-particles change significantly with a change of Pt composition The NiPt nanonano-particles exhibit ferromagnetic behavior depending on the amount of Pt composition In particular, saturation magnetization decreases from 6.5 emu/g to 4.0 emu/g with the decrease of Ni:Pt 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

develop-ment of fuel cells has been attracted particularly attention

from researchers Fuel cells (FC) produce electricity by

con-verting 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:vinhpv@hnue.edu.vn(V.V Pham)

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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

in-termediates 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

nano-catalysts, 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

perfor-mance[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

electro-catalytic activity and durability over the pure Pt

nano-catalysts, 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)2and 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

(PVP-Sigma 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

nano-particles The solutions were bubbled for 60 min by pure

ni-trogen 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:

2Ni2þþ 4BH

4þ 12H2O/2Ni þ 14H2þ 4BðOHÞ3 (1) The black solution containing Ni nanoparticles and un-wanted 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 250C, 300C and 400C 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%) solu-tion 0.5 M was in tern dropped slowly in each part solusolu-tion Galvanic replacement reaction is described by equation: 2Niþ H2PtCl6¼ Pt þ 2NiCl2þ 2HCl (2) After 2 h reaction, the productions were purged and centrifugally collected The collected particles were dried and annealed in hydrogen gas at 300C for 60 min

Physical properties analysis

The crystalline phases were determined by an X-ray diffrac-tometer (Bruker, D8 Advance) The morphologies and chemi-cal compositions were characterized by scanning electron microscopy (SEM, HitachiS-4800) with an energy dispersive X-ray 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 ma-terial 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 (Met-rohm, 797 VA Computrace) in 1 M KOHþ 2 M ethanol solution The measurements were carried out at the potential sweep rate of 50 mVs
1in 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

synthesizing process The purification can be controlled by the

heat treatment in hydrogen gas or varying the legend

con-tents The heat process always accompany with the

crystal-line growing resulting in the extended crystals size Besides,

water content in the materials diminishes whereby the

par-ticles become inert and not to redisperse in the solution In

this study, the different concentration of PVP was used to determine the optimum condition to prepare the templates Fig 1is FE-SEM images of Ni nanoparticles synthesized with PVP concentration of 3 M, 5 M and 7 M Although PVP con-centration 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 inFig 2show 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 pat-terns 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 in-crease of the annealing temperatures This result is used as a reference data for heating treatment of NiPt in further studies

Fig 1e FE-SEM images of nickel particles with different

PVP concentration (a) 3 M; (b) 5 M; (c) 7 M

Table 1e The composition analysis of Ni nanoparticles synthesized with different PVP concentrations

PVP concentrations Ni at% O at% Totals %

0.3 M 74.27 25.73 100.00 0.5 M 75.06 24.94 100.00 0.7 M 97.02 2.98 100.00

Fig 2e XRD diffractogram of Ni particles annealing in hydrogen environment with different temperatures: (a) room temperature; (b) 250C; (c) 300C; (d) 400C

Table 2e Atomic ratio of Ni:Pt nanoparticles synthesized with different amount of H2PtCl6

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

de-creases 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 inFig 3shows that two crystal phases corre-sponding 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 4shows the hysteresis loops of the Ni and NiPt nano-particle measured at 300 K Ni and NiPt nanonano-particles with low

Pt contents exhibited ferromagnetic behaviors The magneti-zations 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 non-paired electrons[23] On the other hand, platinum is a para-magnetic material and Nickel is a ferropara-magnetic 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 5is 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 deter-mine the ethanol oxidation activity of the materials in 1.0 M

Fig 3e 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 4e 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

Fig 5e 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

KOH and 2.0 M ethanol The measurement was first carried

out with Ni nanoparticles and then with NiPt nanoparticles

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
OHadsþ OH
/NiðOHÞ2þ e
(4)

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 COadsis

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:

C2H5OHþ 12OH
/2CO2þ 9H2Oþ 12e
(6)

and the second peak agrees with the oxidation of Ni(II) to

Ni(III) as showing inFig 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

carbo-naceous adsorption Indeed, platinum, an active metal for Ce

C bond activation has been known as an excellent

electro-catalyst 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:

PtðCOÞadsƒ!2OH

It is possible to see that in the backward sweep (Fig 7bed), the oxidation peak of COadsdid 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 trans-ferring electrons from their d band to the 2p* antibonding orbit of CO molecules[25] The PteCO bonding will be weak-ened 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

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 in-crease 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

Fig 6e The cyclic voltammogram of the Ni nanoparticles

in KOH 1 Mþ CH OH 2 M

Fig 7e 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 Mþ C2H5OH 2 M

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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