Influence of Fe contamination and temperature on mechanically alloyed Co–Ni–Mo electrodes for hydrogen evolution reaction in alkaline water

Materials Characterization 56 (2006) 138 – 146

Influence of Fe contamination and temperature on mechanically alloyed Co–Ni–Mo electrodes for hydrogen evolution reaction in alkaline water M.A. Domínguez-Crespo a,b,⁎, M. Plata-Torres b , A.M. Torres-Huerta b , I.A. Ortiz-Rodríguez b , C. Ramírez-Rodríguez b , E.M. Arce-Estrada b a

Universidad Nacional Autónoma de México, Edificio D Facultad de Química, Departamento de Metalurgia. Ciudad Universitaria, C.P.04510 México D.F., México b Instituto Politécnico Nacional, Departamento de Metalurgia y Materiales, A.P. 75-874, 07300 México, D.F. Mexico Received 27 September 2005; received in revised form 7 October 2005; accepted 10 October 2005

Abstract Ni–Co–Mo–Fe solid solutions, such as Co30Ni70, Co30Mo70, Ni30Mo70, Co10Ni20Mo70, Fe10Co25Ni65, Fe20Co20Ni60 and Fe30Co15Ni55 wt.% alloys, have been produced by mechanical alloying using commercial Ni, Co, Mo and Fe powders. The electrocatalytic properties of these nanocrystalline materials have been studied for hydrogen evolution reaction (HER) in 30 wt.% KOH aqueous solution at three different temperatures (308, 323 and 343 K) to determine the effect of Fe contamination. The methods employed were cyclic voltammetry, steady-state polarization (Tafel) techniques and ac impedance. For comparison, it was found that iron improved electrocatalytic activity for the hydrogen evolution reaction. Important changes in the activity were obtained when the temperature was increased. The electrocatalytic effect of Mo became important at high overvoltages and temperatures. The MA Fe30Co15Ni55 and Co10Ni20Mo70 alloyed powders showed the best catalytic activities for HER. © 2005 Elsevier Inc. All rights reserved. Keywords: Mechanical alloying; Electrocatalysts; Hydrogen evolution reaction; Ac impedance; Fe contamination

1. Introduction The field of nanocrystalline materials has been extremely active over the past one or two decades in view of its new and exciting properties, which will both test our scientific understanding of the behavior of materials and offer new scope for applications [1]. This interest ⁎ Corresponding author. Universidad Nacional Autónoma de México, Edificio D Facultad de Química, Departamento de Metalurgia. Ciudad Universitaria, C.P.04510 México D.F., México. E-mail address: [email protected] (M.A. Domínguez-Crespo). 1044-5803/$ - see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2005.10.003

was simulated by the research of Gleiter [2] using a gascondensation/vacuum compaction method, although other synthesis techniques have also been used, i.e. electrodeposition [3], sputtering [4] and thermo-chemical [5]. A method that has received significant attention is mechanical alloying (MA). The MA process has been applied widely in binary and ternary systems, enabling us to obtain intermetallic compounds, supersaturated solid solutions and also amorphous alloys [6]. These alloys have the characteristic of forming nanostructures, which are expected to exhibit better physicochemical properties [7]. For example, some authors have shown that a mechanically alloyed combination of Ni or Co

M.A. Domínguez-Crespo et al. / Materials Characterization 56 (2006) 138–146 Table 1 Chemical composition of MA powders Material

Ni (wt.%)

Co (wt.%)

Mo (wt.%)

Fe (wt.%)

Co30Ni70 Co30Mo70 Ni30Mo70 Co10Ni20Mo70 Fe10Co25Ni65 Fe20Co20Ni60 Fe30Co15Ni55

68.795 – 29.345 29.150 64.960 59.340 54.670

29.945 28.896 – 11.384 22.150 18.530 14,580

– 69.564 70.487 59.35 – – –

1.26 1.54 0.168 0.116 12.890 22.130 30.750

% Fe contamination has been found to be normally present in most of the milled powders; this could have an appreciable effect on the electrochemical properties of the materials [11,12]. In previous work [13], the electrocatalytic behavior at room temperature on the HER of nanocrystalline Co30Ni70, Co30Mo70, Ni30Mo70, and Co10Ni20Mo70 wt.% materials was studied. These materials were prepared by low-energy ball-milling starting with pure crystalline nickel, cobalt and molybdenum powders. The current paper reports the influence of Fe contamination and temperature on these materials. In order to determine this influence, mechanically alloyed Fe10Co25Ni65, Fe20Co20Ni60 and Fe30Co15Ni55 ternary combinations were also studied. The electrocatalytic properties of the alloy powders were investigated in deaerated 30% KOH aqueous solution at 308, 323 and 343 K by cyclic voltammetry, Tafel and ac impedance methods.

with Mo could result in a substantial enhancement of electrocatalytic activity on hydrogen reaction evolution (HER), increasing the surface area by diminishing grain size [8–10]. However, an important disadvantage is that during the MA process significant contamination may occur. This arises from several sources, namely gas adsorption/reaction by the metal with gas from the chamber and from the atmosphere, metallic contamination, often Fe from balls and the container, and organic contaminants; the latter contamination arises from organic additives often used during milling to avoid excessive welding of the powder particles to each other, to the container and to the balls [11,12]. The magnitude of contamination appears to depend on the milling time, intensity of the milling, the atmosphere and the difference in strength/hardness of the powders. Up to 20 wt.

(a)

20 h

2.1. Mechanical alloying Bulk metallic powders of Ni, Co, Mo and Fe with a high purity of 99.99% and 100 mesh were used to obtain mechanically alloyed Co30Ni70, Co30Mo70,

fcc solid solution Co (hcp)

*

*

100 h

* INTENSITY (A. U)

INTENSITY (A. U)

2. Experimental

(b)

*

*

fcc solid solution Fe (Ferrite) Co (hcp)

20 h

*

100 h

*

300 h

300 h 20

139

40

60

2θ (degrees)

80

100

20

* 40

60

80

100

2θ (degrees)

Fig. 1. X-ray diffraction patterns for MA a) Fe10Co25Ni65 and b) Fe30Co15Ni55 powders after 20, 100 and 300 h milling times.

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Ni30Mo70, Co10Ni20Mo70, Fe10Co25Ni65, Fe20Co20Ni60 and Fe30Co15Ni55 wt.% powders in an attrition ball mill of austenitic stainless steel under argon atmosphere. Before each process, powders were mixed ultrasonically. The MA process was carried out for 300 h at room temperature and 110 rpm. The ball-to-powder weight ratio was 50 : 1. Fe was added systematically (10, 20 and 30 wt.%), in order to determine its influence on the electrocatalytic behavior in Ni, Co and Mo alloys. 2.2. Characterization Elemental analyses were carried out using the ASTMD-1977 method and a Perkin-Elmer 5000 atomic absorp-

(a)

tion spectrophotometer. The samples were dissolved completely in a H2SO4 / HNO3 solution in a 2 : 1 volume ratio. The structure of the MA powders was determined by X-ray diffraction (XRD) using Co Kα radiation. 2.3. Electrocatalytic evaluation The electrode preparation is similar to work published elsewhere [13]. A steady-state potentiostatic technique was utilized in this study. All the electrochemical studies were performed with a AUTOLAB 30 potentiostat coupled to a personal computer. All the experiments were carried out in deaerated 30 wt.% KOH aqueous solution (at 308, 323 and 343 K), which was prepared from analytical reagent grade KOH and Type I, 18 MΩ

(b) 10

Ni A2

i (mA cm-2)

A1

C1

-2

-10 -1500

B2

D2

-2

D1

B1

-6

-1000

-500

0

-10 -1500

500

-1000

(c)

(d) 20

Fe E1

Fe10Co25Ni65

1 0 -1

F1

-2 -3

-1100

-600

-100

400

-1500

-1000

500

20

Fe20Co20Ni60

2

Fe30Co15Ni55

15

1

i (mA cm-2)

i (mA cm-2)

0

(f)

3

0 -1

10 5 0 -5

-2

-10

-3 -1500

-500 E (mV) vs SCE

E (mV) vs SCE

(e)

500

2 i (mA cm-2)

i (mA cm-2)

E2

-20 -1600

0

4 3

0 -10

-500 E (mV) vs ECS

E (mV) vs ECS

10

C2

2

i (mA cm-2)

2

-6

Co

6

6

-1000

-500 E (mV) vs SCE

0

500

-1500

-1000

-500

0

500

E (mV) vs SCE

Fig. 2. Cyclic voltammograms of nanocrystalline Fe, Co, Ni and MA Fe10Co25Ni65, Fe20Co20Ni60 and Fe30Co15Ni55 powders in 30 wt.% KOH at 308 K with a sweep rate / dE / dt / = 20 mV s− 1.

M.A. Domínguez-Crespo et al. / Materials Characterization 56 (2006) 138–146

iron concentration only. These diffractograms for MA Fe10Co25Ni65 and Fe30Co15Ni55 powders after different milling times are shown in Fig. 1(a–b). As was obtained for MA Co30Ni70, Co30Mo70, Ni30Mo70 and Co10Ni20Mo70 powder diffraction patterns, previously reported [13], it is observed that the intensity of the peaks decreases with milling time and become wider and shift to higher angles. These indicate that individual grains are broken into smaller particles during the process, which can cause an expansion in lattice parameters. It appears that the grain size diminished with milling time. After 20 h milling time, an fcc solid solution appears and only two minor peaks are detected that correspond to Co (hcp), see Fig. 1a. For MA Fe30Co15Ni55 powders it can be observed that the individual peaks for Fe, Co, and Ni almost disappeared; only the fcc solid solution was identified along with minor low intensity peaks of Co (hcp) and Fe (ferrite), see Fig. 1b. As the intensity of the X-ray elemental lines is reduced, the diffraction

(a)

0.4

E1

0.2

i (mA cm-2)

water. A standard three-electrode set up and a Pyrex electrochemical cell designed to work at these temperatures was used. The counter electrode was a large-area graphite bar. The reference electrode was a saturated calomel electrode SCE (0.2415 V vs RHE). Before 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 work electrode was held at its open circuit potential until equilibrium was reached. It was considered convenient to carry out an electrochemical characterization of the electrodes using cyclic voltammetry (CV). The scan rate in CV experiments was 20 mV s− 1, the positive limit, E+, was 450 mV, and the negative limit E−, was − 1400 mV, starting at open circuit potential (Ei = 0). Tafel parameters were calculated from linear polarization curves. These curves were obtained from a potential scanning in the negative direction starting at the open circuit potential (Ei = 0) to − 1500 mV at a scan rate of 0.02 mV s− 1. The working electrode (graphite + MA or milled metallic powders) was considered to be completely homogeneous, so that its active area was around 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. The ac impedance measurements were carried out in the frequency region of 0.05 to 10,000 Hz (ten frequency points per decade) at 308, 323 and 343 K. The real (Z′) and imaginary (Z″) components of the impedance spectra in the complex plane were analyzed using a nonlinear least squares (NLS) fitting program to estimate the parameters of solution resistance (Rs), charge transfer resistance (Rct) and double layer capacitance (Cdl).

0.0 -0.2 -0.4 -0.6 -0.8 -1500

-1000

-500

(b)

0.6

C2

0.4 i (mA cm-2)

3.1. Characterization

3.1.2. XRD In order to determine the alloy Fe–Co–Ni structures, X-ray diffraction patterns were analyzed for low and high

0

E (mV) vs SCE

3. Results and discussion

3.1.1. Elemental analysis The chemical composition of the MA alloy powders and milled pure powders are shown in Table 1. The presence of a small iron concentration can be observed for all MA powders because of contamination during the grinding process. A maximum contamination value of 2.89 wt.% was obtained for MA Fe10Co25Ni65 powders.

141

0.2 0.0 -0.2 -0.4 -400

-200

0

200

400

E (mV) vs SCE

Fig. 3. Voltammograms recorded at a) less positive and b) negative values for MA Fe30Co15Ni55 powders at 308 K with a rate / dE / dt/ = 20 mV s− 1.

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lines of the solid solutions are slightly displaced compared to Bragg peaks of pure Fe, Co, Ni, thus indicating true alloying. After 100 h, the peaks for the fcc solid solution, Co (hcp) and Fe (ferrite) are broadened and decrease in intensity with milling time. Although, the Co (hcp) and Fe (ferrite) peaks have decreased considerably, they remain clearly visible. A similar process is observed for both iron compositions. Finally, after 300 h, a significant widening and intensity decrease is seen for the fcc solid solution because of the diminishing grain size for both samples. At this milling time, only a slight peak of Co (hcp) is seen for Fe30Co15Ni55 alloy powders; the Fe (ferrite) was integrated with the fcc solid solution. As a result of this, in the Fig. 1(b) a new peak formation can be identified. As an approximation, Scherrer's equation was used to calculate average grain size. The average grain size was about 13 and 10 nm for MA Fe10Co25Ni65 and Fe30Co15Ni55, respectively. These values are within the interval previously reported [13]. 3.2. Electrochemical measurements 3.2.1. Cyclic voltammetry For comparison and characterization of electrochemical behavior of these nanocrystalline compounds metals, cyclic voltammograms were run between the potential range of the hydrogen evolution and oxygen

evolution reactions. Fig. 2 shows the cyclic voltammograms of nanocrystalline Ni, Co, Fe and the mechanically alloyed powders (Fe10Co25Ni65, Fe20Co20Ni60, Fe30Co15Ni55) as recorded after several cycles when the voltammograms became reproducibly stable. The characteristic peak structures of the pure metals correspond to the well-known formation and reduction of the corresponding metal hydroxides and oxides [14–18]. In particular, the formation of Ni(OH)2 and NiOOH [17,18] corresponds to the peaks A1 and A2 in Ni, the formation of Co(OH)2 and CoOOH [15,16] to the peaks C1 and C2 in Co and Fe(OH)2 and γ-Fe2O3 corresponds to the peaks E1 and E2 in Fe [18,19]. The voltamperometric response of nanocrystalline metal alloys cannot be considered as a simple superposition of the peak structures of the corresponding metal components. However, in some cases the overall shapes of the voltammograms reflect the oxide formation and reduction of main metal components. For example, the voltammograms exhibit Ni(OH)2 and NiOOH characteristic peaks, but may exist a superposition of Co(OH)2 and the peak CoOOH shifted towards higher potentials. Also, characteristic peaks for iron, Fe(OH)2, γ-Fe2O3, Co(OH)2 and CoOOH cannot be observed in the voltammograms due to overlap with Ni peaks; however, when the cyclic voltammograms were recorded at less positive and negative values, the oxidation and

Table 2 Kinetic parameters of hydrogen evolution reaction Materials

Co30Ni70

Co30Mo70

Ni30Mo70

Co10Ni20Mo70

Fe10Co25Ni65

Fe20Co20Ni60

Fe30Co15Ni55

Tafel slope [mV decade

−1

]

b1

b2

234 215 70 238 171 62 280 275 144 118 114 50 110 103 103 259 132 82 107 110 125

35 30 33 42 35 32 91 66 41 30 29 47 66 60 61 38 34 44 44 42 45

Log io [mA cm− 2]

i1(1100 mV) [mA cm− 2]

I2(1250 mV) [mA cm− 2]

i3(1300 mV)∞ [mA cm− 2]

i4(1400 mV)∞ [mA cm− 2]

i5(1500 mV)∞ [mA cm− 2]

T (K)

−3.45 −2.94 −1.10 −5.48 −3.89 −2.53 −5.65 −4.66 −2.79 −4.43 −2.01 −0.94 −4.49 −2.1 −1.90 −2.45 −1.58 −1.44 −1.57 −1.50 −0.99

1.05 0.95 2.14 0.53 0.80 0.94 0.49 0.76 1.78 0.36 1.64 3.77 0.83 2.27 2.28 1.33 2.08 2.39 1.87 2.24 241

2.18 2.56 6.67 0.75 1.56 2.89 0.67 1.17 3.49 0.67 2.67 6.67 1.32 4.07 4.09 2.65 3.85 4.18 3.26 4.11 4.95

3.87 4.20 9.91 1.02 2.09 3.91 0.74 1.02 3.84 1.23 3.18 7.80 1.55 4.70 4.73 3.13 4.52 5.36 3.61 4.66 6.35

7.45 7.59 12.88 2.94 4.06 7.50 1.45 2.34 6.61 3.96 6.08 10.00 3.09 6.24 6.27 4.55 6.67 8.99 4.93 6.89 12.88

2.18 2.56 6.67 6.35 8.53 11.48 3.87 5.37 10.72 8.47 10.86 13.18 8.26 8.43 8.43 10.00 10.72 18.20 9.20 12.02 25.12

308 323 343 308 323 343 308 323 343 308 323 343 308 323 343 308 323 343 308 323 343

M.A. Domínguez-Crespo et al. / Materials Characterization 56 (2006) 138–146

reduction peaks of Fe (Fig. 3a) and Co (Fig. 3b), respectively are clearly observed. The charge associated with the active–passive transition increases during the initial period of potential cycling and then decreases to lower constant values, consistent with the quantities of each material. At the beginning of the cycling process the surface becomes covered with oxide films; however, these disappear almost immediately and the surface becomes stable and free of oxide films. These results indicate that the formation and properties of oxide films on the crystalline materials may differ significantly from those of the pure metal components; this is indicated by a slight shift in the positive direction or by a broadening of the peaks. This positive shift increased with iron concentration. The Fe10Co25Ni65, Fe20Co20Ni60, and Fe30Co15Ni55 alloy powders show an interaction similar to that of a metallic compound, which is characteristic of solid solution formation as Jovic et al. have shown [19]. This was confirmed by the X-ray diffraction patterns.

3.3. Polarization measurements 3.3.1. Temperature and iron content effect Tafel parameters of pre-reduced work electrodes on the HER are summarized in Table 2. In addition, the steady-state current densities at various fixed potentials are also shown in Table 2 for a comparison of electrode activity. As can be observed, all MA powders continue showing two distinct Tafel regions that can be attributed to the change in the mechanism of the reaction. The higher Tafel slopes, b1 [50–280 mV decade− 1] were observed in the low overpotential region (1050–1250 mV vs SCE) and lower Tafel slopes, b2 [29–91 mV decade− 1] in the higher overpotential region (1250–1450 mV vs SCE). It is clear that the change in the inflection points of the slope depends on temperature, which indicates that when the temperature is increased the potential-dependent surface hydrogen coverage could also be increased [20].

T=343K

Co 30 Ni 70

Co 30 Mo 70

143

T=323 K

Co 30 Ni 70

-1100

-1100

E (mV) vs SCE

E (mV) vs SCE

Co 30 Mo 70

Co 10 Ni 20 Mo 70

-1300

-1300

Co 10 Ni 20 Mo 70

Ni 30 Mo 70

Ni 30 Mo 70

-1500 -0.50

0.00

0.50

1.00

-1500 -0.50

1.50

0.00

Log i (mA cm -2)

0.50

1.00

1.50

Log i (mA cm -2)

T=308 K Co 30 Ni 70

E (mV) vs SCE

-1100

-1300

Co 30 Mo 70 Co 10 Ni 20 Mo 70

Ni 30 Mo 70 -1500 -1.00

-0.50

0.00

0.50

1.00

1.50

Log i (m A cm -2)

Fig. 4. Tafel plots for the HER on MA Co30Ni70, Co30Mo70, Ni30Mo70, Co10Ni20Mo70 powder electrodes in 30 wt.% KOH at different temperatures. Scan rate: 2 mV s− 1.

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The effect of temperature on HER catalytic performance is clearly shown in Fig. 4. Although previous research at room temperature [13] reported that MA Co30Ni70 powders showed highest electrocatalytic effects on HER, when the temperature is increased the best catalytic performance was obtained by MA Co10Ni20Mo70 powders, which is not a surprising result if we take into account that other research has reported better electroactivities for ternary than binary alloys [21]. Kedzierzawski et al. [22] have observed a strong effect of kinetic parameters with temperature for cold and hot consolidated Ni–Mo alloys produced by mechanical alloying. This behavior was attributed to the electrocatalytic effect of Mo, which became important at higher temperatures. These results agree with our electrocatalytic performance, although it is not easy to estimate the role of Mo because of other effects that can occur, such as lattice expansion, diminished grain size, and other chemical effects such as the presence of oxygen. In general, it was observed that the exchange current densities (io) and Tafel slopes for the different electrodes at higher temperatures are greater than Tafel slopes

at low temperatures. This behavior is in good agreement with data reported by Kreysa et al. [23], and Alemu and Jüttner [14]. At low overpotentials, the Tafel slopes obtained in this work are significantly higher than those reported by other authors [14,24]. One reason for this is that here, a numerical correction was made to generate the largest possible active surface area; another reason is the aqueous solution used, which can generate a larger resistance in the pores and in a larger uncompensated IR drop. These IR drops cannot be fully compensated without destabilizing potential. Finally, the preparation method of the electrodes was different, which could affect mechanism. However, excellent Tafel slopes and exchange currents were obtained for higher overpotentials. The kinetic values for MA Co10Ni20Mo70 and Fe30Co15Ni55 powders, particularly in the range 308– 343 K, are greater than those for Ni and close to those for Pt [14,21,22,25], although these values were reported using a different synthesis method. The Tafel lines for the hydrogen evolution in Fig. 5 indicate that the electrocatalytic activity of the different Fe30Co 15Ni 55

-1100

E (mV) vs SCE

298 343 308 -1300 323

-1500 -0.50

0.00

0.50

1.00

1.50

Log i (mA cm -2)

Fe 20 Co 20 Ni60 -1100

-1100

343

298

-1300

E (mV) vs SCE

E (mV) vs SCE

Fe 10 Co 25 Ni65

323

308

-1500 -0.50

0.00

0.50 Log i (mA c m -2)

1.00

1.50

343 298 308

-1300

-1500 -1.00

323

-0.50

0.00

0.50

1.00

-2

Log i (mA cm )

Fig. 5. Tafel plots for the HER on Fe–Co–Ni electrodes in 30 wt.% KOH at different temperatures. Scan rate: 2 mV s− 1.

1.50

M.A. Domínguez-Crespo et al. / Materials Characterization 56 (2006) 138–146

(a) 10000

343 K

-Z" / Ω cm-2

8000 Ni10Co20Mo70

6000 Fe30Co15Ni55 4000

145

could be attributed to synergetic combination during ternary alloy formation. However, the slope values are greater for the MA Co10Ni20Mo70 than for the Fe30Co15Ni55 powders. This indicates that the role of Mo became slightly more important at higher temperatures than Ni and Fe. This caused a decrease in the apparent energy of activation for the HER, as some authors have reported [21,22].

2000

3.4. Ac impedance measurements

0 0

10000

20000

30000

40000

50000

60000

-2

Z' / Ωcm

(b) 40000

Ni10Co20Mo70

Rct Rs

35000

CPE

-Z" / Ω cm-2

30000 25000

308 K

20000 15000 323 K

10000 5000

343 K

0 0

10000 20000 30000 40000 50000 60000 70000 80000 Z' / Ωcm

-2

Fig. 6. Impedance diagrams for (a) MA Fe30Co15Ni55 and Co10Ni20Mo70 powders at 343 K and (b) Co10Ni20Mo70 electrode at three different temperatures 308, 323 and 343 K. These measurements were carried out under an overpotential of − 350 mV in a 30 wt.% KOH aqueous solution.

iron-containing nanocrystalline metals is much better at higher temperatures and higher iron content. Comparing Fe10Co25Ni65, Fe20Co20Ni60, Fe30Co15Ni55 (Fig. 5) alloy powders with MA Co30Ni70 powders (Fig. 4), it is observed that iron additions to MA Co–Ni powders decreased the overpotential at the temperatures evaluated. This means that nanocrystalline metals containing iron showed higher activities for the hydrogen evolution reaction than MA Co30Ni70 powders at similar conditions. This series of experiments was initiated to analyze the role of iron contamination during the MA process and its effects in the electrocatalytic behavior on HER; these results show that the HER current density is increased with higher iron contents. This could be due to the synergetic interaction between iron and Co–Ni grain structures comprising the ternary alloys. Current densities at defined potentials are shown in Table 2. Fe30Co15Ni55 and Co10Ni20Mo70 displayed the highest current densities at similar potentials, which

The impedance for MA Co10Ni20Mo70 and Fe10Co25Ni65, Fe20Co20Ni60, Fe30Co15Ni55 powders ball milled for 300 h was measured after polarization at − 250, − 350, − 450, and − 550 mV overpotentials vs SCE from 0.05 to 10,000 Hz at 308, 323 and 343 K. Typical complex plane plots were obtained for these materials. Only one semi-circle was found for the electrodes at these overpotentials. In Fig. 6, data for only the alloys with the best electrocatalytic performance are shown. The experimental data are well fitted by the NLS approximation, using the model in place by the constant phase element (CPE). The model of equivalent circuit, includes the constant phase element (CPE) in parallel with the charge transfer resistance (Rct). In general, it was found that double-layer capacities decrease with iron content and temperature. These results indicate that the most active electrodes are those containing high Fe concentrations and Mo over the temperature interval tested (Fig. 6b). This may be attributed to the inherent porosity and roughness characteristics of the fcc solid solution electrodes. Table 3 gives values obtained from the impedance spectra. As we can observe, these results are in good agreement with the kinetics parameters obtained by Tafel plots, although ac impedance measurements show better electrocatalytic behavior for Fe30Co15Ni55 than Co10Ni20Mo70; however, the performance in both electrodes is close in terms of the HER. Table 3 Parameters from impedance spectroscopy fitting Materials

Rs (Ω cm2)

Rct (Ω cm2)

Cdl (μF) cm− 2

Temperature

Fe30Co15Ni55 Co10Ni20Mo70 (343) Co10Ni20Mo70 (323) Co10Ni20Mo70 (308)

3812 20690

15652 52450

19.74 23.56

343 343

13450

39971

27.93

323

9071

71830

60.71

308

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4. Conclusions Appropriate kinetic values can be obtained from 298 to 348 K for the hydrogen evolution reaction (HER) using alloy electrodes synthesized by mechanical alloying. Fe contamination could increase substantially the electrocatalytic performance of alloy electrodes. The electrocatalytic effect of Mo became important at high overvoltages and at higher temperatures. Tafel plots and ac impedance measurements showed an enhancement in the electrocatalytic activity on HER for MA Fe30Co15Ni55 and Co10Ni20Mo70 electrodes, both of which are much better than elemental Ni and are very close to Pt kinetic parameters reported in the literature. Finally, mechanical alloying can produce metastable alloys with important physical and chemical properties, which could increase their electrocatalytic performance. Acknowledgements Authors wish to thanks the financial support from CGPI-IPN, DGPA-UNAM, and CONACYT. References [1] Morris DG. Mechanical behavior of nanostructured materials. In: Magini M, Wöhlbier H, editors. Mater Sci Foundations, Trans Tec. Pubs.Uetikon-Zurich Trans Tech Publications; 1998. p. 21. [2] Gleiter H. Nanocrystalline materials. Prog Mater Sci 1990;33:223–315. [3] Hughes RO, Smith SD, Pande CS, Johnson HR, Armstrong RW. Hall–Petch strengthening the microhardness of twelve nanometer grain diameter electrodeposited nickel. Scr Metall 1986;20:93–7. [4] Li ZG, Smith DJ. Observations of nanocrystals in thin Tb–Fe– Co Films. Appl Phys Lett 1989;55:919–21. [5] Mc Candlish LE, Kear BH, Kim BK. Processing and properties of nanostructured WC–Co. Nanostruct Mater 1992;1:119–24. [6] Koch CC. In: Cahn RW, Haasen P, Kramer ES, editors. Mat. Sci. and Tech., vol. 15. Weinheim: VCH; 1991. p. 583. [7] Koch CC. The synthesis and structure of nanocrystalline material produced by mechanical attrition. Nanostruct Mater 1993;2:109–29. [8] Arce-Estrada EM, López-Hirata VM, Martínez-López L, Dorantes-Rosales HJM, Saucedo-Muñoz L, Hernández-Santiago F. Electrocatalytic properties of mechanically alloyed Co-20wt.% Ni-10wt.%Mo and Co-70wt.%Ni-10wt.%Mo alloy powders. J Mater Sci 2003;38:275–8.

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