Synthesis and electrochemical characterization of stabilized nickel nanoparticles

international journal of hydrogen energy 34 (2009) 1664–1676

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Synthesis and electrochemical characterization of stabilized nickel nanoparticles M.A. Domı´nguez-Crespoa, E. Ramı´rez-Menesesa,*, V. Montiel-Palmab, A.M. Torres Huertaa, H. Dorantes Rosalesc a

Centro de Investigacio´n en Ciencia Aplicada y Tecnologı´a Avanzada, CICATA-IPN Unidad Altamira, Carretera Tampico-Puerto Industrial, C.P. 89600 Altamira, Tamaulipas, Mexico b Centro de Investigaciones Quı´micas, Universidad Auto´noma del Estado de Morelos, Av. Universidad 1001, Colonia Chamilpa, C.P.62201 Cuernavaca, Morelos, Mexico c Departamento de Metalurgia, Escuela Superior de Ingenierı´a Quı´mica e Industrias Extractivas – IPN, C.P. 07300, D.F., Mexico

article info

abstract

Article history:

Nickel stabilized nanoparticles produced by an organometallic approach (Chaudret’s

Received 21 October 2008

method) starting from the complex Ni(1,5-COD)2 were used as electrode materials for

Received in revised form

hydrogen evolution in NaOH at two temperatures (298 and 323 K). The synthesis of the

5 December 2008

nickel nanoparticles was performed in the presence of two different stabilizers, 1,3-dia-

Accepted 5 December 2008

minopropane (DAP) and anthranilic acid (AA), by varying the molar ratios (1:1, 1:2 and 1:5

Available online 11 January 2009

metal:ligand) in order to evaluate their influence on the shape, dispersion, size and electrocatalytic activity of the metallic particles. The presence of an appropriate amount of

Keywords:

stabilizer is an effective alternative to the synthesis of small monodispersed metal nano-

Ni nanoparticles

particles with diameters around 5 and 8 nm for DAP and AA, respectively. The results are

Amines

discussed in terms of morphology and the surface state of the nanoparticles. The impor-

Electrocatalysts

tance of developing a well-controlled synthetic method which results in higher perfor-

HER

mances of the resulting nanoparticles is highlighted. Herein we found that the

EIS

performance with respect to the HER of the Ni electrodes dispersed on a carbon black Vulcan substrate is active and comparable to that reported in the literature for the state-ofthe-art electrocatalysts. Appreciable cathodic current densities of w240 mA cm2 were measured with highly dispersed nickel particles (Ni-5DAP). This work demonstrates that the aforementioned method can be extended to the preparation of highly active stabilized metal particles without inhibiting the electron transfer for the HER reaction, and it could also be applied to the synthesis of bimetallic nanoparticles. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The hydrogen evolution reaction (HER) is technologically important for processes such as water electrolysis and chlorine production. At the same time, the HER is a relatively simple electrochemical reaction. For these reasons the HER ranks among

the most extensively studied electrode reactions [1–5]. Although platinum presents the highest activity for the HER, new electrode materials have been investigated, aiming at the reduction of the cost associated with the electrocatalyst development. Among these materials, nickel and its alloys show a high initial electrocatalytic activity toward the HER [6–8]. However, in order to

* Corresponding author. Tel.: þ52 55 57296000x87515; fax: þ52 55 833 264930. E-mail address: [email protected] (E. Ramı´rez-Meneses). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.12.012

international journal of hydrogen energy 34 (2009) 1664–1676

improve its activity, resulting in reduced overpotentials for the HER, the real surface area and/or the intrinsic activity of the electrode material must be increased. For this reason, there is continuous interest in the synthesis and application of nanometric particles of metals or metal oxides [9]. The physical and chemical properties of these metal nanoparticles are strongly depending on their size and shape [10,11]. Nickel nanoparticles below 100 nm have attracted considerable interest in the last years due to their applications as catalysis superconductors, electronic, optical, mechanical devices and magnetic recording media, etc [12–14]. Metal nanoparticles have generally been produced by both gas and liquid phase processes. Among the gas phase methods, hydrogen reduction of the metal chloride, laserassisted gas phase photonucleation [15], and hydrogen plasma metal reaction [16] have been used. Alternatively, g-ray irradiation [17], borohydride reduction of metal salts [18], sonochemical and thermal decomposition of metal complexes [19] are the most useful techniques to synthesize metal nanostructures in liquid phase. More recently, an organometallic approach proposed by Chaudret and J. S. Bradley using olefinic complexes as a source of metal atoms has shown to be an interesting alternative to control the shape, size and dispersion of nano-objects [20–24]. These authors found that the method leads to the production to monodispersed particles with very small sizes (1–2 nm) and to perform coordination chemistry on their surface. Therefore, the use of organometallic precursors is now well established as a method to obtain size and surface state controlled nanoparticles in mild conditions (room temperature and 1–3 bar of a reactive gas). Additionally, they state that depending on their skeleton and functional groups, the stabilizers can interact with the surface of the particles and then favour the growth of the particles in a preferential direction. As part of our interest to obtain materials with a well-controlled size, shape and dispersion, in order to improve their chemical and catalytic properties, we have previously reported, the synthesis of Pt nanoparticles from the Pt2(dba)3 precursor using hexadecylamine as stabilizer [25]. In that work, the important role of the amine ligand in the nanostructure shape was demonstrated. As a continuation of such study, in this paper, we report that the use of different stabilizing agents and concentrations not only affects size, shape or dispersion but also the catalytic activity toward the HER when an organometallic approach is used. Specifically, it is shown that the nature of the stabilizer (anthranilic acid and 1,3-diaminopropane) and its concentration in the reaction media (molar ratios metal:stabilizer 1:1, 1:2 and 1:5) exert a strong influence on electrochemical properties of the material. The characterization was followed by Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron Microscopy (HRTEM). The electrochemical measurements were performed by Cyclic Voltammetry (CV), Tafel plots and AC impedance techniques.

2.

Experimental

2.1.

Synthesis of Ni nanoparticles

Ni(COD)2 (COD ¼ cycloocta-1,5-diene), 99% Aldrich and two different stabilizers 1,3-diaminopropane (DAP), H2N(CH2)3NH2 (99%, Aldrich) or anthranilic acid (AA), 2-(H2N)C6H4CO2H (99.5%, Aldrich) were dissolved in THF under argon at 70  C

1665

without stirring using a Fisher–Porter bottle. Then, the mixture was pressurised under dihydrogen (3 bars) for 20 h [26–28] and the one step synthesis of nickel nanoparticles was achieved according to Eq. (1). H2 NiðCODÞ2 þstabilizer ƒƒƒƒƒƒƒƒƒ! stabilized Ni – colloid 3 bar H2 ; 70 C stabilizer ¼ 1; 3-diaminopropane; H2 NðCH2 Þ3 NH2 or

(1)

anthranilic acid; 2  ðH2 ÞNC6 H4 CO2 H The reactions were carried out in the presence of 1, 3 and 5 equivalents of stabilizer in order to determine their effect on the shape, size and dispersion of the nanoparticles. After 20 h, the colloidal solutions become dark brown and homogenous. A drop of each crude colloidal solution was deposited on a holey carbon covered copper grid for TEM analysis. The colloidal solution was then concentrated to ca. 5 mL. This solution was washed with hexane (3  20 mL) and dried under a reduced pressure. The addition of hexane leads to homogenize the Ni colloid by redissolution before use. The TEM analyses were performed on a JEOL-2000 FX II electron microscope, operating at 200 kV. The HRTEM study was also carried out on a JEOL 2010 FasTem field emission transmission ˚. electron microscope with a high resolution of 2.1 A

2.2.

Preparation of catalysts

Different systems of Ni nanoparticles namely Ni-xDAP, and NiyAA (x ¼ y ¼ 1, 3 or 5 eq.) were prepared as electrode material. The support of the electrode was a glassy carbon rod (3.0 mm in diameter). The base of the rod was polished up to 1500 paper grid and thereafter with a cloth and alumina powder solution (ca. 0.3 mm). The working electrodes were prepared by attaching 3 mL of an ultrasonically redispersed catalyst (stabilized nickel nanoparticles) suspension containing a 4:1 ratio of carbon black Vulcan/total metal (20 wt.%) powders in deionized water onto the glassy carbon. After drying under a high purity argon flow at room temperature, the deposited catalyst layer was then covered with 4 mL of a diluted aqueous Nafion solution and finally, the electrode was immersed in a nitrogen purged electrolyte to record the electrochemical measurements. During the preparation of catalysts, the quantity of stabilized Ni nanoparticles on the working electrode (carbon black Vulcan þ stabilized Ni particles þ Nafion) was considered to be completely homogeneous; then in this case, the metallic area was about 20% of the geometric area in contact with the electrolyte and all the current density values are given with respect to this area. A thin film with a thickness of 0.3 mm was also estimated by taking into account the amount of electrocatalysts and the Nafion density. In addition, the electrodes were carefully prepared in order to obtain reproducible electrode surfaces and comparable electrocatalytic results. Cyclic voltammetry (CV), steady state potentiostatic methods and ac impedance were performed with a potentiostat/galvanostat Autolab PGSTAT 30 coupled to a personal computer. A three-compartment pyrex glass electrochemical cell designed to work at high temperatures was used. A saturated calomel electrode (SCE) in the upper section of a Lugging capillary was used as reference electrode. All voltages are reported with respect to the SCE reference electrode. The counter electrode was a large-area graphite bar.

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The electrochemical measurements were performed at two different temperatures (298 and 323 K) in a deaerated 1 M NaOH aqueous solution. The solution was prepared from NaOH analytical grade (98.5%, Aldrich) and Type I, 18 MU water. Under the experimental conditions, the working

electrodes displayed an equilibrium potential close to 800 and 850 mV for Ni-xDAP, and Ni-yAA, respectively. The CV measurements were conducted at a 20 mV s1 scan rate. The upper positive limit (Eþ), was 450 mV and the lower negative limit (E), was 1250 mV. After 100 potential cycles,

Fig. 1 – TEM micrographs of nickel nanoparticles synthesized in THF from Ni(1,5-COD)2 in the presence of 1,3diaminopropane (a–b) Ni-1DAP, (c–d) Ni-2DAP and (e–f) Ni-5DAP and their corresponding selected area electron diffraction (SAED).

international journal of hydrogen energy 34 (2009) 1664–1676

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the quasi steady state polarization curves were attained from low to high potential at a scan rate of 0.20 mV s1. The potential scanning was followed in the negative direction starting from the open circuit potential (Ei¼0) to 1500 mV and thereafter, the Tafel parameters were determined. No iR correction was applied. The solution resistance was determined by electrochemical impedance spectroscopy (EIS). The EIS measurements were carried out in the frequency region from 100,000 to 0.1 Hz (10 frequency points per decade) with an amplitude of 10 mV root mean square, at room temperature. The real (Z0 ) and imaginary (Z00 ) components of impedance spectra in the complex plane were analyzed using the nonlinear least squares (NLS) fitting program to estimate the parameters of the different resistances and capacitances.

3.

Results and discussion

The particle composition and crystallinity were studied for each nanostructure system and the results are shown in Figs. 1–3. It was found that the appropriate amount of DAP can drastically reduce the dispersion and Ni particle size. It seems that a higher concentration of stabilizer induces a homogeneous distribution of smaller particles. When 1 eq. of DAP is used, some zones of 10 nm mean-size-agglomerated nanoparticles with a semispherical shape (Fig. 1a) are observed. The Selected Area Electron Diffraction (SAED) shows that the small semispherical primary particles and the unshaped large agglomerates consist of face centered cubic (FCC) Ni (Fig. 1b). The monomorphic flower-like Ni nanoparticles (namely nanoflower or sponge-like) with an average dimension of 138 nm were obtained when 2 eq. of DAP were added to the reaction solution (Fig. 1c). This indicates that the as-synthesized Ni metallic particles with 2 eq. can be regarded as secondary structures, which are self-assemblies organized by hundreds of smaller primary nanoparticles with a semispherical shape. Precedents for this type of particles have previously been observed by Xu et al. [29]. This self-organization should be ascribed to the mutual attraction of magnetic dipoles as mentioned by the authors but, additionally, it could also be due to a fast exchange of the amine groups that have been observed in the case of Pt and Ru nanoparticles stabilized by alkylamines [22,25]. The corresponding SAED for this system is shown in Fig. 1d. The indexing of the ring diffraction pattern confirms the presence of FCC nickel with the following ˚ . These interplanar distances 2.00, 1.73, 1.11, 1.02 and 0.77 A distances correspond to the (111), (200), (220), (222) and (420) planes, respectively. The dimension of the Ni nanoparticles decreased with the increase of the stabilizer concentration (5 eq.) as shown in Fig. 1e. Although, the nanoparticles seem to appear more aggregated, their sizes ranged from 5 to 14 nm. Then, mean size and distribution of the formed nanoparticles strongly depend on the stabilizer concentration. It seems that higher concentrations of stabilizer induce a dispersion of primary particles. The electron diffraction patterns of small Ni particles also show crystalline lattice fringes (Fig. 1f). Even when all the nanoparticles correspond to face centered cubic (FCC) structures; the amount of stabilizer seems to affect the crystallographic orientation. To confirm the dispersion of the

Fig. 2 – HRTEM images of Ni-5DAP nanoparticles synthesized in THF from Ni(1,5-COD)2 at different magnifications.

Ni-5DAP system, high magnification TEM analyses were carried out and are shown in Fig. 2. These measurements distinctly demonstrate the well-dispersed Ni nanoparticles in which recognizable boundaries among those smaller primary nanoparticles can be clearly seen. The size of the Ni particles gradually decreased from 138 to 5 nm with low and high contents of ligand, respectively. This may be ascribed to the poor stabilization of the Ni nanoparticles due to the weak coordination of the amine group on the surface of the particles. The observed tendency indicates a better stabilization induced by a higher amount of ligand and the use of a coordinating solvent such as THF, which previously demonstrated its participation toward the stabilization of this type of nanoparticles [25]. In contrast, when the reactions are carried out using the alternative stabilizer (AA) under the same experimental conditions, the formation of spherical particles using 1 eq. of AA is observed (Fig. 3a). Apparently, these particles are dense, well dispersed and surrounded by fine nanocrystallites interconnecting the big ones. The mean size of the big particles ranges from 300 to 500 nm. The typical electron diffraction pattern of the particles is shown in Fig. 3b. Four fringe patterns with plane distances of 2.03, 1.76, 1.02 and ˚ were obtained. They are also related to the (111), (200), 1.23 A (220) and (311) planes of the pure FCC nickel. The influence of the stabilizer concentration on the particle size, shape and dispersion was also studied with 2 and 5 eq. (Fig. 3c, e). By increasing the concentration of AA up to 2 eq., the dispersion and particle size of Ni which can be nearly monodispersed in diameter are favored, most probably through the specific

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Fig. 3 – TEM micrographs of nickel nanoparticles synthesized in THF from Ni(1,5-COD)2 in the presence of anthranilic acid (a– b) Ni-1AA, (c–d) Ni-2AA and (e–f) Ni-5AA and their corresponding selected area electron diffraction (SAED).

coordination of the oxygen atoms on the surface of the metallic particles. The entities were small semispherical and spherical particles. These small particles were not agglomerated as in the case of 1 eq. However, the Ni nanoparticles in the presence of 5 eq. of AA, show organizations of agglomerated particles, which are again observed. This could perhaps

be related to the lower coordination ability of oxygen to nickel with respect to the nitrogen atom. It is noteworthy that the SAED analyses show that the Ni particles in the presence of 2 and 5 eq. are still crystalline (Fig. 3d, f). The cyclic voltammetries in 1 M NaOH aqueous solution at a sweep rate of 20 mV s1 of nanostructured nickel after 100

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international journal of hydrogen energy 34 (2009) 1664–1676

100

Ni-1DAP Ni-2DAP

80 60

A

Ni-5DAP

i (mA cm-2)

40

B

20 0 -20 C

-40 -60

D

-80 -100

T= 298K -1500

-1000

-500

0

500

E vs SCE (mV) Fig. 4 – Cyclic voltammetries curves of the Ni-stabilized nanoparticles with DAP using different concentrations of stabilizer: 1, 2 and 5 eq. at room temperature in 1 M NaOH aqueous solution. CVs were recorded up to 100 potential cycles at sweep rate 20 mV sL1.

100 Ni-1AA Ni-2AA Ni-5AA

E

0 C

30

F

-50

-100

B

10 0

C'

-10

T=298 K -1500

20

-2

i ( mA cm-2)

50

i (mA cm )

a

-600 -400 -200 0 200 400 600 E vs SCE (mV)

-1000

-500

0

500

E vs SCE (mV)

b

100

T= 323 K

B A

50

i (mA cm-2)

potential cycles and using DAP as stabilizer are presented in Fig. 4. The anodic peaks formed at 752 (A) and 354 (B) mV vs SCE and their corresponding reduction are related to the reversible process Ni þ 2OH 4 aNi(OH)2 þ 2e [30] and to the electron process one Ni(OH)2 þ OH / NiOOH þ H2O þ e which can give two varieties of oxyhydroxides b and a, forming different reduction peaks. From these CVs, it may be seen that not for all the samples the two oxidation peaks are clearly observed. In the case of Ni-5DAP there is only the oxidation peak at 752 while Ni-2DAP does not display evidence of oxidation peaks. Due to the fact that the respective magnitude of these peaks depends on the aging of the surface and thus on the mutual proportion of hydroxides in each phase and also on the crystallographic orientation, the stabilizer may provoke a preferential orientation toward some planes as it was observed in the indexing of ring diffraction patterns, which masks the signal of these oxidation peaks. In addition, the peak close to the oxygen evolution reaction is sometimes hidden due to the current density increase. Fig. 5 a, b also shows typical cyclic voltammetry curves obtained after several potential cycles at two different temperatures. Fig. 5a highlights two characteristics, a redox system consisting of an oxidation peak at 150 mV vs SCE (E ) and two reduction peaks close to 200 (C ) and 342 (F ) mV vs SCE. These oxidation– reduction peaks are shifted to negative values as the concentration of the stabilizer increases. According to the conditions imposed to the electrode surface, the reduction peak C could be correlated to the formation and subsequent reduction of few monolayers of Ni(OH)2 or NiOOH or both. Even though, in CV diagrams recorded from 1400 to 500 mV vs SCE, the Ni(OH)2 or NiOOH formation characteristic peaks seem to be missing, but the electrodes were recorded at less negative and positive values, the oxidation and reduction peaks of NiOOH are clearly observed (see inset Fig. 5), confirming the presence of these Ni species. The charge associated with the anodic E and cathodic F peaks can be attributable to the stabilizer response as the presence and position of the hydroxyl group exert remarkable effects on the

0 C -50

D

-100 -150

Ni-1AA -1200

-800

-400

0

400

E vs SCE (mV) Fig. 5 – Cyclic voltammetries curves of the Ni-stabilized nanoparticles with AA (a) using different concentrations of stabilizer: 1, 2 and 5 eq. at room temperature and (b) with 1 eq. at 323 K. CVs were recorded in 1 M NaOH aqueous solution up to 100 potential cycles at sweep rate 20 mV sL1.

redox behavior of the electrode [31]. The organization of individual nanoparticles into superstructures appears to be more important with AA as ligand than that observed for the DAP system, which may be due to the coordination of the amine group in the DAP on the surface of the particles and to a supposed mobility that could make them less stable. Nevertheless, the apparent mobility of the amine group when DAP is used does not interfere with the electron transfer behavior from the particles as indicated by the cyclic voltammograms of both types of Ni nanoclusters. From Figs. 4 and 5a it can be deduced that the concentration and nature of the stabilizer during the synthesis of the nanoparticles (chemical surface composition) as well as the accessibility of the surface atoms have an important influence on the shape of the voltammograms and probably on the amount of adsorbed hydrogen. The large charge associated with the Ni-xDAP electrodes (redox systems) seems to indicate that the hydrogen evolution reaction takes place more easily on them than on Ni-yAA. In general, it was observed that the charge associated with the active–passive transition increases during the initial period of potential cycling and then decreases to lower constant values due to the fact that the surface becomes stable. The final shape of these CVs may be

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international journal of hydrogen energy 34 (2009) 1664–1676

consistent with the stabilizer concentration and dispersion of the Ni nanoparticles. The physical, chemical, and electrochemical stabilities of the electrode material are the most important characteristics that must be established. Thus, the electrodes were tested at 323 K during 100 potential cycles. Fig. 5b shows a typical voltammogram of Ni-1AA as a reference. In all the samples, at this temperature (323 K), the oxidation corresponding to the Ni(OH)2 and NiOOH species and their corresponding reduction are clearly observed; in addition, an interesting increase in the current density was obtained in the evaluated specimens; such increase was more pronounced with 5 eq. of DAP. The variation of the current density can perhaps be correlated with the higher coordination of DAP with respect to the AA stabilizer. In coordination chemistry, the preference of Ni (II) centers to coordinate to diamines such as ethylenediamine and isopropylenediamine in the presence of anthranilic acid which is left uncoordinated in the reaction media is well-documented [32]. Furthermore, under the experimental conditions of this work, the AA is expected to interact with the metal through the two oxygen atoms as shown in Fig. 6a. Although, for both stabilizers, it is the chelate effect that prevents their donor atoms from leaving the metal surface at once. According to the aforementioned, AA could perhaps be more susceptible to leave exposed larger Ni surface areas and thus form bigger aggregates than DAP due to the lower coordination ability of the oxygen to nickel with respect to the nitrogen atoms (see Fig. 6b). In the later case fine nanocrystallites were observed.

In contrast, the use of AA as stabilizer displayed some agglomeration and exhibited lower electroactivity. This suggests that the charge transfer is influenced by the nature of the stabilizer donor atoms present on the surface of the metallic particles. The apparent activities of the electrodes were studied by steady state polarization Tafel curves. The cathodic polarization behavior of the Ni nanoparticles having different stabilizer contents is compared in Fig. 7a, b and the major Tafel parameters are listed in Table 1. Two distinct regions of steady state behavior with different Tafel slopes can be observed in the evaluated electrodes. In the low HER overpotential region from 130 to 300 mV the Tafel slopes (bl) are higher than 120 mV dec1, which may indicate the presence of some oxides on the surface of the Ni particles while in the high overpotential range (320 to 500 mV) the Tafel slope (bh) varied between 80 and 120 mV dec1; although any trend can be observed, the lower Tafel slopes are displayed for Ni-5DAP in comparison with that showed by the other samples. These results also indicate that the electrodes have better electrocatalytic behavior at relatively high overpotentials (bh), which is desirable in commercial applications. The existence of two Tafel slopes might be an indication of a change in the reaction mechanism; moreover the Tafel slopes intercept a similar overpotential for most of the electrode materials, which implies potential-dependent surface hydrogen coverage. The small differences can be correlated with the amount of stabilizer and dispersion of the metallic particles.

a +

H3N -

O O

O O-

+

H3N

Ni O

+ -

O

ONH3+

O

+

O-

+

H3N

H3N

NH3+

H3N

-

O

O

O

O-

NH2 H2N NH2

NH2 H N 2

NH3+

O

+

H3N -

O

+

H3N O

O

O -O

Ni

O

O O-

O-

NH3+

NH3+

NH3+

O

NH3+

b

Ni

O

O-

NH3+

NH2

-

Ni

O

Ni

Ni O

O

-

-

T=323 K

O

H 2N

O

O

H3N

O O

-

O-

-

Ni O

H3N

+

H3N

+

+

H2N H2N

Ni NH2

H2N

NH2

H2N

NH2 T=323K

NH2 NH2

H2N Ni NH2

H2N

NH2

H2N Ni

H2N H2N Ni

Ni NH2

H2N H2N

H2N

H2N

Fig. 6 – Schematic representation of stabilized Ni nanoparticles and the influence of the temperature of the electrolytic cell (a) AA and (b) DAP.

international journal of hydrogen energy 34 (2009) 1664–1676

ηH2 (mV)

a

-500

accepted mechanism for nickel electrodes in alkaline solutions for the hydrogen evolution reaction is given in Eqs. (2)– (4), where any of them could be the rate determining step [35].

-400

H2 O þ e þ M5M  Hads þ OH

-600

Ni-1DAP

-300

2M  Hads 5H2 þ 2MðrecombinationÞ

Ni-5DAP

-200

(3)

or M  Hads þ H2 O þ e 5H2 þ OH

0 100

þ Mðelectrochemical desorptionÞ -5

-4

-3

-2

-1

0

1

2

3

4

Log i (A cm-2) -600 Ni-5AA

-500 Ni-1AA

-400

ηH2 (mV)

(2)

Followed by

Ni-2DAP

-100

b

1671

Ni-2AA

-300 -200 -100 0 -5

-4

-3

-2

-1

0

1

2

3

4

Log i (A cm-2) Fig. 7 – Tafel plots obtained in 1 M NaOH aqueous solution at room temperature, after 100 cyclic voltammetries (a)NixDAP and (b) Ni-yAA.

The kinetic parameters indicate that the stabilized Ni nanoparticles using higher DAP concentrations have appreciable cathodic current densities during HER, followed by Ni1DAP and Ni-2AA electrodes. It was also found that the Tafel slopes are quite different and even higher than that reported elsewhere at low overpotentials [33–36]. The commonly

(4)

Assuming the symmetry factor, b ¼ 0.5 a Tafel slope of 120 mV dec1 is expected if the reaction (2) is the determining step. Otherwise, if reactions (3) or (4) are the rate determining steps Tafel slopes of 30 or 40 mV dec1 are, respectively, expected. The Tafel slopes obtained on Ni-xDAP, and Ni-yAA nanoparticles, which ranged at low overpotentials from 145 to 172 mV dec1 and at high overpotentials from 80 to 120 mV dec1; with some of them close to 120 mV dec1, seem to indicate that Eq. (2) is likely to be the rate determining step, although the experimental values were in some cases somewhat larger than expected in comparable conditions [33–35]. Kedzierzawski et al. [36] previously mentioned about this case, where small slopes of the order of w60 mV were calculated from low overpotential parts of polarization curves, which might be misleading since a well-defined linear E vs log i dependence only can be measured for overvoltages larger than 100 mV. For lower values the E vs log i curves are steeper because of the overlap with the reverse reaction, which is also potential-dependent [37]. A comparison between the relevant Tafel parameters of the stabilized Ni nanoparticles revealed that the addition of higher quantities of stabilizer increases the Tafel slopes at low overpotentials, but at the same time, it could increase the exchange current densities. From the comparison of the polarization curves of Ni nanoparticles in the presence of 2 eq. of DAP at 298 and 323 K (not shown here), it was evidenced that the nanostructured electrodes increase their activity at higher temperatures and are still stable after 100 potential cycles. It is quite logical that as the temperature increases, the Ni nanoparticles tend to agglomerate as a consequence of the electrostatic forces apparently reducing the specific surface area, which suggests

Table 1 – Kinetic parameters obtained on studied electrodes toward HER in 1 M NaOH aqueous at 298 K. Electrode

Ni-1DAP Ni-2DAP Ni-5DAP Ni-1AA Ni-2AA Ni-5AA

bla (mV dec1)

bha (mV dec1)

log Job (mA cm2)

J1c (h ¼ 100 mV) (mA cm2)

J2c (h ¼ 200 mV) (mA cm2)

J3c (h ¼ 300 mV) (mA cm2)

J2c (h ¼ 400 mV) (mA cm2)

155 138 147 145 156 172

114 100 84 80 82 120

1.6 2.5 1.8 3.8 3.2 2.4

1.06 0.12 3.11 6.3E2 0.62 0.12

4.68 0.83 9.04 0.24 2.54 0.45

18.19 3.91 50.58 1.43 7.04 1.71

85.11 25.70 239.88 15.70 45.71 1.21

a Tafel slopes obtained from linear part of the Tafel curves at low and high overpotentials. b Hydrogen exchange current density extrapolated from the Tafel plots. c Different current densities obtained at different overpotentials.

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a

120

Ni-2DAP

100 80 60

0.41 Hz

51.8 mHz

40

400

300

Ni-5DAP

Z" (Ω cm2)

Z' (Ω cm2)

b

Ni-1DAP

η=-100

the temperature seems to be more important than the apparent weak agglomeration of the Ni particles; however, further studies are required to confirm this assumption. In order to characterize the electrocatalytic activity of the electrodes, the EIS analysis was carried out at different overpotential values (h ¼ 100, 200 and 400 mV). Fig. 8a–f shows the diagrams of the Ni-xDAP and Ni-yAA electrodes as functions of the overpotential, in the HER region. The complex plane plots display two semicircles, a small one in the high frequency domain that depends on the potential and it is also modified with the concentration of the stabilizer, which can be attributed to the textural properties of the electrode and/or

50

Z" (ohm cm2)

a perceptible contradiction with the observed catalytic activity at 323 K; however, it is necessary to consider that with temperature, the kinetics of the electrocatalytic reactions is more sensitive to diminishing the transfer charge resistance [38]. Thus, in our case the higher interaction of metallic particles among each other, with increasing temperature, is not strong enough to cause agglomeration and rather it appears to provoke a renewal of catalytic sites. In addition, it is necessary to recall that the temperature of electrochemical experiments is below the reaction temperature and in the former case, we can assume the nanoparticles to be thermally stable [39]. Then, the electronic transfer due to the increase in

1.46 Hz

0

25 Z' (ohm cm2)

50

0.41 Hz

100

Ni-1AA Ni-2AA Ni-5AA

20 0

16.0 Hz 0

20

40

60

0

80

0

100

Z' (Ω cm2) 60

η=-200

Ni-2DAP

Ni-1AA

η=-200

75

Ni-2AA Ni-5AA

Z" (Ω cm2)

40 0.91 Hz 10 mHz 10

20

Z" (ohm cm2)

Z" (Ω cm2)

300

d

Ni-1DAP

100 mHz

Ni-5DAP

0.72 Hz 0

0

25

5

0

e

0

40

η=-400

5

2

Z' (ohm cm )

80

9.10 Hz

10

0

120

0

40

Ni-1DAP

0.37 Hz

f

Ni-2DAP

40 Ni-1AA

Z' (Ω cm2)

60

2

Z" (Ω cm2) 40

20

Ni-5AA

1.27 Hz

20

120

η=-400

Ni-2AA

20

0

80

Z' (Ω cm2)

Ni-5DAP

0

2.22 Hz

1.01 kHz

50

Z' (Ω cm2)

Z" (Ω cm2)

200

Z' (Ω cm2)

Z" (ohm cm )

c

10 mHz

25

0

200

η=-100

0.52 Hz

20

9.10 Hz 0.83 kHz 9.10 Hz

0

0

0

20

0

10 2 Z' (ohm cm )

20

40

Z' (Ω cm2)

Fig. 8 – Nyquist plots of the impedance of Ni-xDAP and Ni-yAA polarized at (a–b) L100 mV, (c–d) L200 mV and (e–f) L400 mV, respectively, in 1 M NaOH aqueous solution at room temperature.

international journal of hydrogen energy 34 (2009) 1664–1676

the shape of the stabilized nickel nanoparticles; and a bigger one in the middle and low frequency domain, which varies with the potential as well as the stabilizer concentration [40]. This part of the spectra has to be ascribed to the faradic impedance, which through high pseudocapacitance and strong potential dependence, the low frequency resistance dominates over a broad region from medium to low frequencies [35,41]. All the measured impedance values are relatively low and comparable with those reported in the literature [4], indicating a high extent double layer charging and high faradaic current densities. Similar results were already observed for impedance data measured on plated platinum, rhodium and nickel using a different technique [4,40], and explained in terms of the surface roughness effect, however, in our case it can only be ascribed to the nature and concentration of the stabilizer [42]. As a reference, Fig. 9a, b displays phase angle diagrams at h ¼ 100 for both systems. In general, the formation of two different frequency regions can also be observed in Bode spectra over the total range examined, but they are more evident with higher stabilizer/ metal molar ratios.

a phase angle (degrees)

η=-100

Ni-2DAP

60

Rs 40

CPE1

CPE2

R2

R3

Ni-1DAP

20

Ni-5DAP

0

-1

0

1

2

3

4

5

Log F (Hz)

b

η=-100

Phase angle (degrees)

60 Ni-1AA

Rs

Ni-5AA

40

CPE1

CPE2

R2

R3 Ni-2AA

20

0 -1

0

1

2

3

4

5

Log F (Hz) Fig. 9 – Bode plots at L100 mV in the HER region of Ni stabilized nanoparticles using different concentrations of (a) DAP and (b) AA.

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In general, at low and intermediate overpotentials, the electrocatalytic activity of the stabilized Ni particles was found to be high when the stabilizer content was 2 and 5 eq. for AA and DAP, respectively. Then, the addition of proper ligand concentrations during the synthesis can increase the activities of the electrode toward the HER. From these spectra the influence of the overpotential on the electrochemical behavior is also appreciated. It is clear that at higher overpotentials the electrodes display a similar resistance transfer charge for the Ni-xDAP metallic particles. Even the Ni particles with 1 eq. of AA show a completely different performance (the lowest Rct) than that observed at low overpotentials. Then, the overpotential seems to provoke the primary particle dispersion observed with low ligand concentrations. The formation of Ni nanoparticles from the redox reactions (reduction of the COD-ligand to cyclooctane) in the present systems could be a complex process. The process begins with a rapid nucleation and growth of the nuclei into smaller clusters. In this sense, it is generally accepted that the amine groups (DAP) are reversibly coordinated to surface metals. The process then reached its growth limit [27,43]. For the AA containing systems the presence and deprotonation of the acidic COOH group and the corresponding protonation of the –NH2 moiety result in the decrease of the coordination ability of the latter. However, as the overpotential is increased, the NHþ 3 group can presumably suffer a deprotonation, gaining back its coordination ability. This process is favored at low stabilizer concentrations, increasing the metallic dispersion, which makes hydrogen evolution more facile. In addition, changes on the surface of the Ni nanoparticles occur as soon as it is in contact with the solution, even under potentiostatic control. The nickel surface can be covered with the typical hydroxides (a / bNi(OH)2 or NiOOH) increasing the hydroxyl environment [44], which may explain the variations in the EIS results in the evaluated overpotential range. The obtained data were analyzed according to Reza and coworkers [40, 45] in comparison with an equivalent circuit consisting of an electrolyte resistance (Rs) in series with two parallel R-CPE elements (see inset figure). The impedance of the CPE has the form of ZCPE ¼ 1=TðjuÞf and was used to simulate the depression of the semicircle in the complex plane at various overpotentials; where T is the capacitance relative to the average double layer parameter 1 1f Þ; 4, is the CPE exponent (the value of 4 ðT ¼ Cfdl ðR1 s þ Rct Þ ranges from 0 to 1, and is equal to 1 for a complete smooth electrode); R2 and R3 are the high and low frequency resistances, respectively. In general, the curves fitted with the model for the faradaic impedance of HER do not differ much and are in good agreement with the measured spectra over the high and medium frequency ranges, however, in some cases, small differences were obtained from medium to low frequencies pointing to differences that exist between supposed steps and follow the formation of the surface fraction of the adsorbed hydrogen. The fitted values are shown as a function of the overpotential in Table 2. From these values, the high frequency resistance (R2) displays a weaker dependence on the overpotential compared with the low or intermediate frequency resistance (R3), which decreases in some cases up to an order of magnitude when the potential in the HER region increases, which is in good agreement with the

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international journal of hydrogen energy 34 (2009) 1664–1676

Table 2 – Comparison of the electrochemical impedance parameters at different overpotentials of Ni electrodes in NaOH 1 M by approximation of EIS experimental data using CNLS method. Ni-1DAP

Ni-2DAP

Ni-5DAP

Overpotential (mV)

Overpotential (mV)

Overpotential (mV)

Parameters

2

Rs (U cm ) R2 (U cm2) Chf (mF cm2) 41 R3 (U cm2) Clf (mF cm2) 42 c2

h ¼ 100

h ¼ 200

h ¼ 400

h ¼ 100

h ¼ 200

h ¼ 400

h ¼ 100

h ¼ 200

h ¼ 400

1.8 11.1 0.018 0.58 219.5 0.500 0.70 4.56E2

2.8 0.5 0.161 0.72 64.2 1.052 0.76 1.69E2

3.1 0.8 0.139 0.65 47.3 1.374 0.75 1.44E2

1.5 0.9 0.047 0.58 302.3 4.834 0.82 4.48E2

3.0 2.9 0.045 0.60 127.1 5.639 0.85 2.23E2

2.9 3.5 0.033 0.63 45.1 6.538 0.86 2.8E2

1.7 25.5 0.241 0.85 202.7 0.463 0.58 1.94E2

2.9 8.5 1.283 0.74 52.3 12.669 0.72 7.77E2

3.0 0.78 0.843 0.68 43.5 22.260 0.83 0.77E2

Parameters

Rs (U cm2) R2 (U cm2) Chf (mF cm2) 41 R3 (U cm2) Clf (mF cm2) 42 c2

Ni-1AA

Ni-2AA

Ni-5AA

Overpotential (mV)

Overpotential (mV)

Overpotential (mV)

h ¼ 100

h ¼ 200

h ¼ 400

h ¼ 100

h ¼ 200

h ¼ 400

h ¼ 100

h ¼ 200

h ¼ 400

2.1 4.1 0.377 0.70 409.0 6.931 0.87 1.15E2

3.2 2.1 0.594 0.59 114.8 13.932 0.84 2.64E2

3.1 27.2 0.619 0.88 37.3 18.268 0.82 2.66E2

2.3 16.2 2.11E3 0.62 228.0 7.438 0.84 0.18E2

2.9 10.8 5.163E3 0.60 98.9 9.138 0.81 0.12E2

2.8 12.2 6.781E3 0.62 46.0 13.063 0.85 0.36E2

1.93 9.0 5.021 0.63 254.5 6.929 0.88 0.55E2

3.4 20.9 5.483 0.63 207.0 8.311 0.88 0.76E2

2.7 13.8 7.889 0.61 67.6 11.892 0.90 1.18E2

behavior reported elsewhere [35]. The average of the Chf and Clf determined by the EIS measurements are in some cases higher than that reported in the literature for electrodeposited Ni (0.036 mF cm2) [40]. Hitz and Lasia [45] found capacitances in the range of 120–170 mF cm2, which were identified as double layer capacities and ascribed to porosity, however, in our case, Chf is only influenced by the dispersion of the nanoparticles. Therefore, Chf could reasonably be ascribed to the double layer capacity of the textural properties and shape of the particles in the electrodes. On the other hand, Clf can be associated with hydrogen adsorption. An apparent linear variation was also observed for log (1/Rct) vs h with slopes close to the kinetic parameters obtained from the Tafel plots, which indicates a Volmer–Heyrovsky´ reaction mechanism [46–48] for the HER on the Ni-stabilized nanoparticles. Therefore, the kind and suitable concentration of stabilizer does not change the reaction mechanism but, generates a high dispersion and can modulate the morphology of the nanoparticles which could enhance the electrochemical behavior of electrode materials. The reaction conditions such as reaction temperature, time, solvent and the applied overpotential on the HER might also have significant effects on the shape and behavior of the Ni particles.

4.

Conclusions

An organometallic approach to the synthesis of sizecontrolled Ni nanoparticles has been used and their catalytic performance on HER has been evaluated. The amine group containing DAP and AA stabilizers was used to control the shape and size of the particle. The presence of the appropriate amount of ligand is an effective alternative to obtain small

monodispersed metal nanoparticles. Then, the size, morphology and electrochemical properties of the nanoparticles can be correlated with the nature and the quantity of the ligand added, which can influence the equilibrium present on the surface of the particles, i.e. the nature and concentration of the stabilizer does not change the reaction mechanism but, influences the dispersion and can modulate the textural properties of the metallic particles. The important role of the amine ligand in the control morphology and size of the Ni nanoparticles appears at two different levels: in the first place, a higher electrostatic coordination on the surface metallic particles; secondly; a phenomenon related to the chelate effect which prevents donor atoms of the stabilizer molecule from leaving the metal surface at once. The Ni electrodes followed a typical Volmer–Heyrovsky´ reaction mechanism. Based on activity and stability data at elevated temperatures and several potential cycles a ranking of the different Ni electrodes may be established. The Ni-5DAP electrode seems to be the most effective system studied for the HER as revealed by the kinetic parameters and current densities in the evaluated overpotential range. The activity of the Ni electrodes is enhanced as the temperature is increased in the presence of DAP due to the coordinated amine groups on the surface of the Ni nanoparticles; on the other hand, AA favors the Ni electroactivity at low concentrations of ligand and high overpotentials The parameters for the textural properties and shape of the particles (R2 and Chf) and proton adsorption on the electrode surface (R3 and Clf) quantitatively determined by the ratio of these parameters also are in good agreement with the Tafel parameters. Finally, this work demonstrates that Chaudret’s method for synthesizing nanoparticles can be extended to the preparation of

international journal of hydrogen energy 34 (2009) 1664–1676

stabilizer-metal and possibly to bimetallic nanoparticles with interesting catalytic activities.

Acknowledgements The authors wish to acknowledge the financial support provided by CONACYT through the 59921 and 61354 projects, SIP-IPN 2008-1136, 2008-0838 and SNI. The authors would like to thank Ms. Cynthia Carolina Villanueva-Alvarado for her technical support.

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