Kinetic analysis of hydrogen evolution at Ni–Mo alloy electrodes

Electrochimica Acta 45 (2000) 4151– 4158 www.elsevier.nl/locate/electacta

Kinetic analysis of hydrogen evolution at Ni–Mo alloy electrodes J.M. Jaksˇic´ a,*, M.V. Vojnovic´ b, N.V. Krstajic´ b b

a Faculty of Agriculture, Uni6ersity of Belgrade, Karnegie6a 4, 11000 Belgrade, Yugosla6ia Faculty of Technology and Metallurgy, Uni6ersity of Belgrade, Karnegie6a 4, 11000 Belgrade, Yugosla6ia

Received 2 December 1999; received in revised form 30 March 2000

Abstract The hydrogen evolution reaction (HER) on Ni, Mo and MoNi intermetallic compound was investigated with ac impedance and dc polarization measurements in 1.0 mol dm − 3 NaOH solution at 25°C. The rate constants of the forward and backward reactions of Volmer, Heyrovsky and Tafel steps were estimated by a nonlinear fitting method. MoNi alloy electrode provides a much lower overpotential for the HER than Ni and Mo electrodes, sometimes which exhibit similar catalytic activity. The main pathway for the HER at above mentioned electrode materials is Volmer–Heyrovsky with Heyrovsky as the rate determining step. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Hydrogen evolution reation; Mehanism Mo–Ni alloys; ac Impedance

1. Introduction The hydrogen evolution reaction (HER) is one of the most frequently investigated reactions. The reason is that the reaction proceeds through a limited number of steps with only one type of intermediate. The kinetics of H2 evolution from bases have been widely investigated at Ni [1–3], owing to the relatively good catalytic activity and high corrosion stability. From the theory of electrocatalysis [4] the electrocatalytic acivity depends on the heat of adsorption of the intermediate on the electrode surface, in a way giving arise to the well known ‘vulcano’ curve. It is clear that beside the precious metals, there is no way to find new materials among pure metals, which would possess a high catalytic activity for HER. Practically there are two possibility to enhance the activity of bare Ni, (i) to increase the surface area by various methods or (ii) from a catalytic point of view, to combine Ni with other pure metals to obtain alloys

* Corresponding author.

with optimized adsorption characteristics. Miles [5] suggested that a combination of two metals from the two brunches of vulcano curve could results in enhanced activity. Jaksic [6] showed that a combination of Ni or Co with Mo could result in a substantial enhancement of HER. Numerous studies thoroughly investigated the kinetics and mechanism of HER at Ni– Mo [7 – 16] alloys of various compositions, obtained by electrodeposition from suitable baths. All these electrodes are characterized by having a high roughness factor. Chialvo et al. [17] investigated the dependence of the electrocatalytic activity of bulk Ni– Mo alloys on composition for HER, varying atomic percentage from 0 to 25% in molybdenum. In this study, we report the results regarding the HER on some metallurgically prepared Ni– Mo alloy electrodes. The electrodes were mechanically polished to avoid the problem of ‘geometric’ effect and to find out eventually presented true catalytic effect for HER. The general mechanism of HER in alkaline solution is based on the following three steps:

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J.M. Jaksˇic´ et al. / Electrochimica Acta 45 (2000) 4151–4158

4152 k1

M+ H2O+e ? MHads +OH− k−1

(rate 61)

k2

MHads +H2O+e ? H2 +M+ OH− k−2 k3

MHads +MHads ? H2 +2M k−3

(1)

(rate 62)

(2)

(rate 63)

(3)

HER starts with the proton discharge electrosorption (Volmer reaction, Eq. (1), and follows either or both electrodesorption step (Heyrovsky reaction, Eq. (2)), and/or H recombination step (Tafel reaction, Eq. (3)). The relation between the coverage ([H) of the adsorbed H intermediate and cathodic overpotential for HER was investigated on Mo, Ni and MoNi intermetallic compounds especially. The rate constants for forward and backward reactions in the three steps of HER were calculated from dc current densities, charge transfer resistance (Rct) and relaxation elements (Rp and Cp) obtained from impedance spectra, based on the analysis of Armstrong and Hendersons [18]. The reaction rates of three steps are given by







=61 =k%1(1−[) −k%− 1[



61 =k1(1− [)exp −



(1−iFp) iFp −k − 1[ exp RT RT



(4a)

(1−i)Fp iFp 62 =k2[ exp − −k − 2(1−[)exp RT RT =62 =k%2[−k%− 2(1−[) 63 =k3[ −k − 3(1− [) 2

2

n n

(4b) (4c)

where, ki and k − i are the rate constants of the forward and backward reactions. For convenience, the rate constant values have nominally been written also to include the OH−, H2O and H2 pressure. The charge balance (r0) under a constant current density ( j ), and the mass balance (r1) of the fractional surface coverage ([) are given by the following equations: j r0 = = −(61 +62) F q ([ =61 −62 −263 r1 = F (t

  

(5) (6)

Fig. 2. X-ray diffraction patterns of some Mo– Ni alloys. Compositions indicated in the figure.

Fig. 1. Equivalent circuit for single-adsorbate mechanism (Armstrong’s electric circuit).

where, j is the current density for HER and q is the maximum surface charge corresponding to a surface [ equal to 1. The steady-state H coverage, [, is given by setting r1 = 0, then

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Fig. 3. Tafel polarization curves on Mo–Ni electrodes in 1.0 mol dm − 3 NaOH at 25°C.

−(k%1 +k%− 1 +k%2)+[(k%1 +k%− 1 +k%2)2 +8k%1k3]1/2 [= 4k3 (7) The rate constants k − 2 and k − 3 for the backward reactions are not included in Eq. (7), because they have no significant influence on the reaction rates of the second (Eq. (4b)) and third step (Eq. (4c)). At the equilibrium condition (61 =62 =63 =0), the relationship between ki values can be represented by k−2 =

k1k2 , k−1

k−3 =

k 21k3 2 k− 1

(8)

Eq. (8) allows us to reduce the number of independent parameters in the ki values from six to four. In the impedance studies, HER mechanism is estimated by using the electric circuit of Armstrong [18], given in Fig. 1, which contains the ac impedance elements, R , Rp and Cp. Zf =R +

Rp (1+j…~p)

(9)

The double-layer capacitance (Cdl) is used as a circuit element of the smooth electrodes. The rate constants for forward and backward reactions in the three steps of the HER were calculated from dc current densities, charge transfer resistance (Rct) and relaxation elements (Rp and Cp) obtained from impedance spectra, based on Armstrong and Hendersons analysis [17,18].

Fig. 4. Effect of Ni content on the current density for HER at overpotential, p = −0.4 V, for Mo– Ni alloys measured in 1.0 mol dm − 3 NaOH at 25°C.

2. Experimental Mo– Ni alloy compounds with the following nominal composition, MoNi; MoNi2; MoNi3; and MoNi4 were metallurgically prepared by arc melting of the pure components (Ni, Mo) under argon atmosphere (Philips). The phase structure of the alloys was analyzed by X-ray diffraction method. Typical X-ray patterns for some Mo– Ni alloys are presented in Fig. 2. The results of X-ray analysis show that except MoNi4 compound all other alloys have a multiphase structure. For example, MoNi3 sample beside MoNi3 phase contains also Mo and MoNi4 phases. X-ray diagram for MoNi intermetallic compound was not analyzed in detail, since the reference diagram does not exist in the opened literature, but some peaks have been analyzed with reference to the other patterns. These analyze pointed out that MoNi sample contains MoNi and MoNi4 as separated phases. The polished Mo– Ni electrodes (0.05 mm finished) were mounted in Teflon. The electrochemical experiments were performed at 25°C in solutions of 1.0 mol dm − 3 NaOH. Solutions were made with spectrograde NaOH (Merck) and deionized water.

Table 1 Kinetic parameters of the HER for Mo–Ni electrodes in 1.0 mol dm−3 NaOH at 25°C (from the Tafel curves) Electrode −b (mV) j0×106 (A cm−2)

Ni 121 3.3

Mo 126 5.4

MoNi

MoNi2

MoNi3

MoNi4

132 79

142 70

148 32

138 13

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A conventional three-compartment glass cell was used. A saturated calomel electrode (SCE) in the same

solution as that of the working electrode was used as the reference electrode. Platinum gauze, spot-welded onto a platinum wire sealed into a glass tube, was used as a counter electrode. Tafel curves were recorded potentiostatically in the current range of : 1 mA cm − 2 to 100 mA cm − 2, using a PAR273 potentiostat. The resistance of the solution was determined by ac impedance spectroscopy. The ac impedance measurements were carried out with the same potentiostat connected to a PAR5301 lock-in amplifier, controlled by a computer through a GPBI PC2A interface. The impedance measurements were carried out in the frequency region of 50 mHz to 80 kHz. There were ten frequency points per decade above 5 Hz. The fast Fourier transformation (FFT) technique was used in the impedance measurements in the frequency region below 5 Hz. The real (Z%) and imaginary (Z¦) components of the impedance spectra in the complex plane were analyzed using the nonlinear least squares (NLS) fitting program to estimate the parameters of R , Rp and Cp. The rate constants were calculated by minimizing the residual (S) of the sum of each experimental datum (the dc polarization measurement and ac impedance results).

3. Results and discussion

Fig. 5. Experimental (circled points) and solid lines calculated from the evaluated ki values complex plane diagrams for HER on Ni (S=3.45 cm2, p= − 0.272 V), Mo (S =1.65 cm2, p= − 0.345 V) and MoNi (S =0.84 cm2, p= −0.180 V) in 1.0 mol dm − 3 NaOH at 25°C.

Polarization curves for the HER, obtained by the polarization measurements are presented in Fig. 3. Polarization curves were recorded after holding the electrodes at a constant cathodic current density of 100 mA cm − 2 for about 1 h. A single-valued Tafel slope is observed at all electrodes. The values for Tafel slopes at Mo– Ni alloy electrodes are somewhat higher than − 0.12 V, probably due to the presence of the oxide films on the electrode surfaces, which causes additional potential drop. Table 1 presents the main kinetic parameters, the exchange current density for an apparent surface area of 1 cm2 and Tafel slope, b for those electrodes. All investigated Mo– Ni alloys possess higher catalytic activity of either Ni or Mo. For example, MoNi2 and MoNi4 have about three times the activity of base components. The highest activity for HER exhibits MoNi electrode, which is illustrated in Fig. 4. Conway et al. [7] obtained somewhat lower catalytic activity for the HER at bulk Ni(40)Mo(60) in the same solution and temperature (at j = 0.1 A cm − 2, p = 0.48 V) than MoNi electrode (at j = 0.1 A cm − 2, p = 0.42 V) in this study. In the same time, the catalytic activity of bulk Ni1 − x Mox (05 x 50.25) alloys, investigated by Chialvo [17] were much lower. The impedance analyzes were carried out only at Ni, Mo and MoNi electrodes, because the other Mo–

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Table 2 The fitted parameter values of the equivalent circuit for HER at Ni, Mo and MoNi in 1.0 mol dm−3 NaOH at 25°C Electrode

−p (V)

Ni

0.184 0.272 0.364 0.440 0.494 0.233 0.345 0.393 0.472 0.516 0.112 0.180 0.227 0.315 0.382

Mo

MoNi

R (V cm2)

Rp (V cm2)

567 80.7 13.7 4.20 2.10 37.5 13.1 5.62 1.56 1.14 46.2 8.90 2.43 0.93 0.51

Cp (mF cm−2)

1476 111 13.6 3.10 0.09 340 17.2 5.2 1.6 0.59 408 35.9 5.94 0.75 0.02

Ni alloys did not exhibit significantly better catalytic activity then Ni and Mo for the HER. Fig. 5 shows typical complex-plane impedance diagrams at certain overpotentials, p on Ni, Mo and MoNi electrodes. The experimental data are represented by the circled points. The impedance spectra consist of two overlapping semicircles. The experimental data are treated through the NLS fitting to estimate the elements of Armstrong’s equivalent circuit given in Fig. 1. The values of the impedance parameters, which are necessary to calculate the rate constants in the mechanism at Ni, Mo and MoNi electrodes, are listed in Table 2. The rate for HER was estimated from dc current density and fitted parameters values of Armstrong’s equivalent circuit using factorial fitting. The estimated values of the rate constants are presented in the Table 3, for the apparent surface area of 1 cm2. The complex-plane impedance diagrams calculated from the evaluated ki values are presented by the solid lines in Fig. 5. The potential dependence of the current density is illustrated in Fig. 6. The corresponding solid lines are obtained from the evaluated rate constants. The fit of polarization curve at low overpotential val-

129 99 94 109 87 495 474 434 435 266 248 206 267 327 357

Cdl (mF cm−2) 62 62 61 53 58 64 65 63 62 60 66 64 63 65 64

ues (near the reversible hydrogen potential) is worst. In our opinion the positive deviation of the experimental curves from the calculated Tafel plots is probably caused by the presence of some other electrochemical reactions such as the reduction of the oxide eventually presented at the surface and/or electrochemical absorption of the hydrogen. Now, it is also possible to evaluate the reaction rates of the three steps, and the surface coverage of atomic hydrogen from the estimated rate constants. The surface coverage, [, of adsorbed hydrogen atoms as function of overpotential for the investigated electrodes are illustrated in Fig. 7. In all cases, the value of [ increases at more negative potentials and reaches its saturation value ([ “ 1) at higher overpotentials. The reaction rates of the Volmer, Heyrovsky and Tafel processes in the hydrogen evolution mechanism and the resulting total current value are all independently shown in Fig. 8, as calculated from the estimated ki values, in a wide potential region. In this figure, all the rate values, 6i (mol cm − 2 s − 1) are shown in terms of current, F×6i (A cm − 2).

Table 3 The estimated values for the rate constant for HER at Ni, Mo and MoNi electrodes in 1.0 mol dm−3 NaOH solution at 25°C (ki, mol cm−2 s−1)

Ni Mo MoNi

k1

k−1

k2

k−2

k3

k−3

5×10−10 8×10−10 3×10−9

6×10−9 9×10−9 1.5×10−7

1.5×10−11 1.3×10−11 1.5×10−10

4.2×10−12 1.2×10−12 3×10−12

3×10−10 4×10−10 2.2×10−10

6.2×10−12 3.2×10−12 8.8×10−13

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From Fig. 8, one can see that at overpotentials lower than approximately −0.15 V, the reaction mechanism is a consecutive combination of Volmer step and parallel Tafel and Heyrovsky steps. At Ni electrode Heyrovsky step prevails over Tafel step, at Mo electrode prevails the Tafel step and at MoNi

Fig. 7. Potential dependence of the calculated values of the surface coverage by adsorbed hydrogen ([) on Ni, Mo and MoNi electrodes in 1.0 mol dm − 3 NaOH at 25°C.

electrode, these two steps are of equal rates. At overpotentials more negative of approximately − 0.25 V, at almost full coverage of electrodes with Hads, the mechanism of the HER is consecutive combination of Volmer and Heyrovsky steps of equal rates and rate of Tafel step negligible, at all investigated electrode materials. The reaction rate is controlled by the Heyrovsky reaction because of the much smaller k2 value as compared with the k1 and k3 values. The evaluated ki values correspond to apparent surface area of 1 cm2, but if we take into consideration the fact that corresponding values for the double-layer capacity of Ni, Mo and MoNi electrodes are very similar (see Table 1), which means that the roughness factors are also similar; one can conclude that true catalytic effect is obtained at MoNi electrode for HER.

4. Conclusions

Fig. 6. Tafel polarization curves on Ni, Mo and MoNi electrodes in 1.0 mol dm − 3 NaOH at 25°C. The continuous lines were calculated from the evaluated ki values.

Among investigated Mo– Ni alloys, such as MoNi, MoNi2, MoNi3 and MoNi4 intermetallic compounds only MoNi exhibits considerably higher true catalytic activity then Mo and Ni electrodes for HER in 1.0 mol dm − 3 NaOH solution at 25°C. The rate constants of the forward and backward reactions of Volmer, Heyrovsky and Tafel steps were estimated by a nonlinear fitting method. The main pathway is Volmer– Heyrovsky with ratedetermining step of Heyrovsky at all investigated electrodes.

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Fig. 8. Potential dependence of the current density ( j ) and the reaction rates for HER on Ni, Mo and MoNi electrodes in 1.0 mol dm − 3 NaOH at 25°C.

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