Kinetic study of hydrogen evolution reaction on Ni30 Mo70, Co30Mo70, Co30Ni70 and Co10Ni20Mo70 alloy electrodes

Materials Characterization 55 (2005) 83 – 91

Kinetic study of hydrogen evolution reaction on Ni30 Mo70, Co30Mo70, Co30Ni70 and Co10Ni20Mo70 alloy electrodes M.A. Domı´nguez-Crespoa,b, M. Plata-Torresa, A.M. Torres-Huertaa, E.M. Arce-Estradaa,*, J.M. Hallen-Lo´peza b

a Instituto Polite´cnico Nacional, Departamento de Metalurgia y Materiales, A.P. 75-876, 07300 Me´xico, D. F. Mexico Instituto Mexicano del Petro´leo, Programa de Tratamiento de Crudo Maya, Avenida Eje Central La´zaro Ca´rdenas No.152, Col. San Bartolo Atepehuacan, 07730 Me´xico, D. F. Mexico

Received 26 January 2005; accepted 11 March 2005

Abstract The hydrogen evolution reaction on nanocrystalline Ni30Mo70, Co30Mo70, Co30Ni70, and Co10Ni20Mo70, metallic powders prepared by mechanical alloying was investigated with linear polarization and ac impedance methods, in 30 wt.% KOH aqueous solution at room temperature. The formation process and structural properties of these nanocrystalline materials were characterized by X-ray diffraction and transmission electron microscopy. Alloyed powders showed the presence of two phases: an fcc solid solution and intermetallic compounds of Ni, Co and Mo. Based on polarization and ac impedance measurements, an improved electrocatalytic activity for hydrogen evolution reaction was observed in mechanically alloyed Co30Ni70 powders, which is slightly higher than milled metallic Ni powders. D 2005 Elsevier Inc. All rights reserved. Keywords: Mechanical alloying; Electrocatalysts; Hydrogen evolution reaction; Ac impedance

1. Introduction In recent years Ni–Co–Mo alloys prepared by mechanical alloying (MA) have been studied as suitable electrode materials for the hydrogen evolution reaction (HER) in alkaline media as an improved

* Corresponding author. Tel.: +52 55 57296000x54212; fax: +52 55 57296000x55270. E-mail address: [email protected] (E.M. Arce-Estrada). 1044-5803/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2005.03.003

alternative to traditional materials like mild steel or electrodeposited Ni–Co–Mo alloys [1–3]. Jaksic [4] reported that a combination of Ni or Co with Mo could result in a substantial enhancement of HER. MA offers two possibilities to enhance the activity of these materials. (i) increasing surface area by diminishing grain size and (ii) from a catalytic point of view, to combine an active metal with other pure metals to obtain alloys with optimized adsorption characteristics [5]. Performance of electrocatalytic materials have been improved by either increasing

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the ratio between effective and geometric surface area of an electrode or by synergetic combination of electrocatalytic components. Most studies have been carried out by recording current–potential curves and then analyzing the corresponding Tafel parameters and the dependence of potential with temperature or pH [1–3,6,7]. Obtained parameters have been compared with those predicted by classical electrochemical theories. This approach is very limited but led to the conclusion that improved performance of the material can be attributed to a real electrocatalytic effect, to a substantial increase in effective electrochemical area or both. Unless a detailed study of the reaction is carried out, it is not always obvious which is the most important characteristic of a given material. Thus, it is important to use more powerful techniques to study HER and to gather fundamental data that may lead to the design of new materials with improved performance. For example, impedance spectroscopy has proved to be a powerful tool to determine the kinetics and mechan-

ism of hydrogen evolution reaction and has helped substantially to the present knowledge of this process [7–10]. The objective of this work is evaluating the electrocatalytic behavior on the HER of nanocrystalline Ni30Mo70, Co30Mo70, Co30Ni70 and Co10Ni20Mo70 alloys, which were prepared by MA, starting from bulk metallic nickel, cobalt and molybdenum powders. Electrocatalytic properties of the alloyed powders were evaluated in a deaerated 30% KOH aqueous solution at room temperature. The electrode performance was evaluated by Tafel and ac impedance methods.

2. Experimental 2.1. Mechanical alloying Bulk metallic powders of Ni, Co and Mo with a high purity of 99.99% and 100 mesh were used to

Fig. 1. XRD patterns of metallic powders before and after 300 h milling time.

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obtain milled metallic powders in an attrition ball mill of austenitic stainless steel under argon atmosphere. In addition, four alloys, Co30Ni70, Co30Mo70, Ni30Mo70, and Co10Ni20Mo70 (wt.%) were mechanically alloyed from blended bulk metallic powders at the same conditions. Before each process, powders were mixed ultrasonically. Milling process was carried out for 300 h at room temperature and 110 rpm. The ballto-powder weight ratio was 50:1.

lysis were prepared by ultrasonically treating by suspending them in a 96 (v/v)-% ethyl alcohol aqueous solution. A drop of the suspension was put on a clean copper grid and dried in air. In order to get information about the overall distribution of milled metallic and MA powders a dark field imaging was employed.

2.2. Characterization

Milled metallic and MA powders were supported in graphite to prepare the working electrode by mixing 1.6 g of graphite powders-200 mesh, high purity 99.999% and 0.4 g of the milled powders in an agate mortar. A 1.6 ml of silicon oil was added to the resulting powder and carefully mixed to obtain an homogeneous paste. A standard threeelectrode set up and a pyrex electrochemical cell designed to work at room temperature under inert atmosphere were used. The counter electrode was a large-area graphite bar. The reference electrode was a saturated calomel electrode SCE (0.2415 V vs. RHE) [11]. All measurements were carried out in a deaerated 30 wt.% KOH aqueous solution at room temperature, which was prepared from analytical reagent grade KOH and Type I, 18 MV water. Before

The structure of milled metallic and MA powders was followed by X-ray diffraction (XRD) using radiation Ka of Cu and Co. JCPDS Co (FCC) 150806, Co (HCP) 05-0727, Ni 04-0850 and Mo 421120 cards were used to analyze the XRD pattern and to obtain the position of peaks and full width at half maximum (FWHM). Morphology of structure at nanometric scale of milled powders was observed by transmission electron microscopy (TEM) using a JEOL 100-CX-II Microscope. Scanning of samples was adjusted to observe in focus. TEM observation was under a 100 kV accelerating voltage and 200,000 of magnification. Milled metallic and MA powders for TEM ana-

2.3. Electrocatalytic evaluation

Fig. 2. XRD patterns of mechanically alloyed Ni30Mo70, Co30Mo70, Co30Ni70 and Co10Ni20Mo70 powders.

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Table 1 Lattice parameters and average grain size after 300 h milling time of crystalline materials Compound

Lattice parameter, a (nm)

Average grain size (nm)

Ni Mo Co Ni30Mo70 Co30Mo70 Co30Ni70 Co10Ni20Mo70

0.3537 0.3151 0.3551 0.3312 0.3286 0.3486 0.3367

11.3 10.4 9.5 10.3 14.7 9.8 15.2

each measurement, the electrode was held at 1300 mV vs. SCE for 2 h to reduce the oxide film on the electrode surface. Then, the electrode was held at its

open circuit potential until equilibrium was reached. Tafel parameters were calculated from linear polarization curves. These curves were obtained from a potential scanning in negative direction starting at the open circuit potential (Ei 0) to 1500 mV at a scan rate of 0.02 mV s 1, using an AUTOLAB 30 potentiostat. The working electrode (graphite-MA or milled metallic powders) was considered to be completely homogeneous, so that its active area was 20% of the geometric area in contact with the electrolyte solution. Therefore, current densities were calculated based on this active area (2.8  10 2 cm2). The solution resistance was determined by ac impedance spectroscopy. ac Impedance measurements were carried out in the frequency region of 0.05– 10,000 Hz (10 frequency points per decade). The

Fig. 3. TEM micrographs and its corresponding electron diffraction pattern of mechanically alloyed (a) Ni30Mo70, (b) Co30Ni70, (c) Co30Mo70 and (d) Co10Ni20Mo70 powders after 300 h milling time.

M.A. Domı´nguez-Crespo et al. / Materials Characterization 55 (2005) 83–91 Table 2 Planes and phases analyzed in the diffraction patterns of mechanically alloyed powders Ring (a) Ni 30 Mo 70 1 2 3 4 5 (b) Co 30 Mo 70 1

2

(c) Co 30 Ni 70 1 2 3 4 5 6 7 8 9 10 11 (d) Co 10 Ni 20 Mo 70 1 2 3

Planes (h k l)

Phase

1 0 3 2 4

0 0 3 2 0

1 2 0 0 0

Ni3Mo Ni3Mo MoNi4 fcc solid solution fcc solid solution

1 2 1 2 2 3

00 00 0 10 11 20 09

fcc solid solution Co3Mo Co7Mo6 fcc solid solution Co3Mo Co7Mo6

100 111 200

220 311

002 211 103

Not identified Not identified Not identified fcc solid solution fcc solid solution fcc solid solution Not identified Not identified fcc solid solution fcc solid solution Not identified Ni3Mo fcc solid solution fcc solid solution

real (ZV) and imaginary (ZW) components of impedance spectra in the complex plane were analyzed using nonlinear least squares (NLS) fitting program to estimate the parameters of solution resistance (R s), charge transfer resistance (R ct) and double layer capacitance (C dl).

3. Results and discussion 3.1. Structural characterization XRD patterns of Co before grinding (Fig. 1) showed two types of elemental Co: cubic and hexagonal phases. After 300 h grinding, it was observed

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only an fcc phase. The lines of the XRD pattern are wide; may be due to the existence of numerous stacking faults in the two Co structures. After 300 h of milling time, it can be seen a significant reduction in intensity of the (100), (101) and (110) lines for hexagonal compact Co phase. In addition, it is observed a significant width of the (111) (200) and (220) lines of the cubic Co phase, this is for two reasons: the grinding decreases the crystallite size and cause hexagonal-fcc cubic transformation, which has been attributed by Corrias et al. [12] and Aymard et al. [13] to ABABCABAB stacking fault creation. Nevertheless, our hexagonal-fcc cubic transformation seems to be in higher degree in comparison with their observations, although, they used a Spex mill. In fact, after 300 h of grinding, hexagonal phase is no longer detectable by XRD. XRD patterns of Ni after 300 h of milling time showed an important widening and intensity decrease of (111), (200), (220), (311) and (222) main reflections due to the existence of numerous stacking faults and reduction of grain size. Similar behavior is presented by Mo in its main reflections (100), (200), (211) and (220). Fig. 2 shows the XRD patterns of Ni30Mo70, Co30Mo70, Co30Ni70 and Co10Ni20Mo70 crystalline solid solutions. These phases have an fcc structure, this suggest that the Co30Mo70, Co30Ni70 and Co10Ni20Mo70 alloys formation proceeds simultaneously with the formation of Co (hcp) into Co (fcc) and with a mutual diffusion of Co, Ni and Mo species into fcc phases. In addition, the following intermetallic compounds were identified: Ni3Mo for Ni30Mo70, Co3Mo and Co7Mo6 for Co30Mo70 and Ni3Mo for Co10Ni20Mo70 MA powders. There was not any intermetallic compound for MA Co30Ni70 powders. The presence of these phases seems to be in agreement with JCPDS cards 17-0572, 29-0488, 29-0489, 29-0490, 03-1036 and equilibrium phase diagrams [14]. Although, lattice parameters for these MA powders were quite different compared to milled metallic powders other characterization techniques besides XRD must be used to confirm alloys formation (see Table 1). In an initial approach, size of crystals was calculated from the average width of the X-ray peaks using Scherrer’s equation after a proper correction of instru-

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Fig. 4. Tafel plots for the HER on milled metallic and mechanically alloyed powders in 30 wt.% KOH aqueous solution at room temperature (scan rate 0.02 mV s 1).

mental broadening. After 300 h of milling time, average crystallites size for metallic powders were 11.3, 10.4 and 9.5 nm for Co, Ni and Mo, respectively (see Table 1). The nanometric grain size is explained by a classical mechanism of crystallite size reduction by grinding effect. The grinding introduces faults such as dislocations. The relaxation of this type of fault takes place with reduction of the coherent domains when the dislocations move and approach each other to form new crystallites. This process stops when the material is not easy to amorphize by grinding. The crystallite size then reaches a critical value. It seems to be about 10.3 nm for Ni30Mo70, 14.7 nm for Co30Mo70, 9.8 nm for Co30Ni70 and 15.2 nm for Co10Ni20Mo70 MA powders.

In order to further confirm grain size and phases of MA Ni30Mo70, Co30Mo70, Co30Ni70 and Co10Ni20Mo70 powders transmission electronic microscopy has been used. TEM micrographs for MA powders are shown in Fig. 3 (a–d), respectively. Diffraction patterns confirm nanometric average grain size for MA alloy powders. The average values are 12, 15, 8, 10 for MA Ni30Mo70, Co30Mo70, Co30Ni70 and Co10Ni20Mo70 powders, respectively. This grain size is in good agreement with the crystallite size found with Scherrer’s formula. The following intermetallic compounds were detected by ring diffraction patterns: Ni3Mo and MoNi4 for Ni30Mo70, Co7Mo6 and Co3Mo for Co30Mo70, Ni3Mo for Co10Ni20Mo70 MA powders.

Table 3 Tafel parameters of HER on crystalline alloys and pure materials Materials

Ni Co Mo Co30Ni70 Co30Mo70 Ni30Mo70 Co10Ni20Mo70

Tafel slope [mV decade b1

b2

160 169 132 150 180 158 177

115 524 318 137 128 128 134

1

]

Log i o [mA cm 4.90 5.57 5.32 4.82 5.83 5.38 5.50

E 1 = 1250 mV, E 2 = 1300 mV, E 3 = 1400 mV, E 4 = 1500 mV.

2

]

i1 [mA cm 1.58 1.30 1.12 1.80 0.12 0.64 0.45

2

]

i2 [mA cm 1.65 1.40 1.37 2.13 0.16 0.70 0.51

2

]

i3 [mA cm 3.40 2.11 3.20 4.08 0.39 1.24 0.83

2

]

i4 [mA cm 8.96 7.41 8.66 10.23 1.72 3.34 1.98

2

]

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region (from 1100 to 1350 mV vs. SCE) defined by a slope b 1 (mV decade 1) and a high overpotential region (from 1350 to 1500 mV vs. SCE) with a slope defined as b 2 (mV decade 1). All samples showed these two regions. Ni and its alloys Co30Ni70, Ni30Mo70, Co10Ni20Mo70 showed greater slopes (b 1) at low overpotential region than at high overpotential region (b 2). These results indicate that electrodes have better electrocatalytic behavior at relatively high overpotentials (b 2), which is desirable in a commercial application [15]. Although, Tafel behavior at low overpotentials is less commonly reported in the literature than that at high overpoten-

Finally, no intermetallic compound was detected for MA Co30Ni70 powders (Table 2). Then, TEM results for these materials show the presence of: (i) no uniform agglomerates, (ii) intermetallic compounds determined by patterns diffraction, and (iii) formation of crystalline fcc solid solution. 3.2. Electrocatalytic activity Typical Tafel plots HER for milled metallic and MA powders are shown in Fig. 4. These plots can be divided into two Tafel regions: a low overpotential

(a) 200

Rct

Ni

Rs CPE

-Z" (Ω cm2)

150

100

50

0 0

50

100

150

200

Z' (Ω

250

300

350

cm2)

(b) 200

Rct

Co30Ni70

Rs CPE

-Z" (Ω cm2)

150

100

50

0 0

50

100

150

200

Z' (Ω cm2) Fig. 5. Impedance spectra for (a) Co30Ni70 and (b) Ni, electrodes measured under an overpotential of solution at room temperature.

450 mV in 30 wt.% KOH aqueous

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Table 4 Parameters from impedance spectroscopy fitting Materials

R s (V cm2)a

R ct (V cm2)a

C dl (AF cm

Ni Co30Ni70

94.6 5.66

212.8 138.6

436 346

a

Impedance measurements carried out at

2 a

)

were found on the Co30Ni70 and Ni electrodes. This decrease in Rct from Co30Ni70 and Ni electrodes is in agreement with an enhancement of current density obtained by polarization measurements that are shown in Table 4.

450 mV overpotential.

tials, it is important to analyze surface effects at the beginning of the reaction, thus existence of two Tafel slopes is attributed to the change in mechanism of the reaction [15]. Most of electrode materials synthesized in this study showed a change in slope at similar potential, which could be indicative of a potentialdependent surface hydrogen coverage, as some authors have reported [16,17]. Electrokinetic parameters for electrode materials are listed in Table 3. Current densities at defined potentials have also been shown for practical interest. Co30Ni70 and Ni displayed highest current densities at similar potentials, which could be attributed to their higher surface area caused by a diminishing in grain size and synergetic effect. The slope values of MA Co30Ni70 powders are similar at low and high overpotentials. In addition, current densities decrease as Tafel slope increases. Furthermore, exchange current densities increased in the following order, Co30Ni70 N Ni Mo N Ni30Mo70 N Co10Ni20Mo70 N Co N Co30Mo70. Thus, a better electrocatalytical performance may be associated to MA Co30Ni70 powders after 300 h milling. Results are in agreement with Lian et al. [15], who evaluated electrocatalytic behavior in Nibased materials.

4. Conclusions In summary, based on the polarization and ac impedance measurements an enhancement in electrocatalytic activity for the HER was observed in the MA Co30Ni70 powders, which is higher than milled Ni powders. This behavior is attributed to the increase in effective surface area of electrode by a diminishing in the grain size and synergetic combination. Synthesized electrodes have better electrocatalytic activity at relatively high overpotentials which is important for commercial application. Co alloys formation consists of two steps of hexagonal-cubic phase transformation of Co and the mutual diffusion of Co into cubic Ni or Mo. XRD and TEM patterns indicated two phases present in the MA, Co30Mo70, Ni30Mo70, and Co10Ni20Mo70 powders; an fcc solid solution and intermetallic compounds. For Co30Ni70 alloy was detected only one phase, an fcc solid solution. Acknowledgements The authors wish to thank CGPI-IPN, CONACYT, SNI and Instituto Mexicano del Petroleo (IMP) for financial support.

3.3. ac Impedance spectroscopy ac Impedance measurements were carried out at an overpotential (g) in the range of 100 to 450 mV vs. SCE. The complex-plane plots, ZW imaginary impedance against ZV real impedance, measured at an overpotential of 450 mV vs. SCE on Co30Ni70 and Ni electrodes are shown in Fig. 5. The experimental data in the figure are presented as circle points. The model of equivalent circuit (insertion in Fig. 5), includes the constant phase element (cpe) in parallel with charge transfer resistance (Rct). This equivalent circuit was determined by NLS fitting. Table 4 gives the values of parameters obtained from the impedance spectra, the lowest Rct, 5.66 V cm2 and 94.6 V cm2

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