The hydrogen evolution reaction on Ni-Sn alloys and intermetallics

Surface and Coatings Technology, 28 (1986) 93

-

111

93

THE HYDROGEN EVOLUTION REACTION ON Ni-Sn ALLOYS AND INTERMETALLICS ANDRÉ BELANGER and ASHOK K. VIJH Institut de Recherche d ‘Hydro-Quebec, Varennes, Québec JUL 2P0 (Canada) (Received November 18, 1985)

Summary Ni—Sn alloys containing 0%, 1.1%, 11.6%, 25.5%, 40.3%, 63.5%, 77.9%, 83.5%, 98% and 100% tin were prepared and examined regarding their activity towards the hydrogen evolution reaction in sulphuric acid solutions. The composition of these alloys and intermetallics was determined from the initial weight of the constituents used in the alloy preparation. The identification of various phases was carried out by X-ray diffraction analysis using ASTM standard microfiles. The surface composition of these alloys was determined by Auger electron spectroscopy for several typical cases, both before and after the cathodic polarization. The electrochemical measurements consisted of steady state potentiostatic polarization curves at various temperatures and potentiodynamic profiles. The electrochemical data deduced include Tafel slopes, exchange current densities, apparent heats of activation and potentiodynamic behaviour. On nickel and nickel-rich intermetallics, electrochemical desorption is indicated as the rate-determining step whereas the hydrogen evolution reaction appears to proceed by the initial discharge mechanism on tin and tin-rich alloys. Also the activity of nickel-rich intermetallics approaches that of nickel whereas the tin-rich alloys tend to exhibit activity similar to that of tin.

1. Introduction In a previous paper [1], we reported work on the hydrogen evolution reaction (HER) on an alloy system that forms a solid solution in its entire composition range, namely Ag—Pd. In the present investigation, the HER was studied on a very different alloy system that does not form solid solutions: the Ni—Sn couple. The Ni—Sn phase diagram is quite complex [2] and reveals at least three known intermetallic compounds, namely Ni3Sn, Ni3Sn2 and Ni3Sn4 (Fig. 1). 0257-8972/86/$3.50

© Elsevier Sequoia/Printed in The Netherlands

94

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232 100

0 Ni

25

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-

50 Composition ot the alloy (at

___________

75

0/0

Sn)

_______

100 Sn

Fig. 1. Phase diagram of the Ni—Sn system.

The HER has been examined on eight Ni—Sn alloys and on pure nickel and tin. The alloys had nickel contents of 1.1%, 11.6%, 25.5%, 40.3%, 63.5%, 77.9%, 83.5% and 98.0%, as indicated on the composition axis of the phase diagram shown in Fig. 1.

2. Experimental details 2.1. Preparation and analysis of the alloys The alloys were prepared starting from nickel (purity, 99.97%) obtained from the International Nickel Corporation and tin (purity, 99.999%) purchased from A. D. Mackay, New York. The main impurities contained in the nickel were carbon at 200 ppm or less, iron at 50 ppm or less and sulphur, silicon, copper, chromium, titanium, cobalt and magnesium, all at less than 10 ppm. The alloys (about 10 g of each) were melted in a vacuum “Minivac” furnace (Vacuum Industries Ltd.) in alumina crucibles, and then annealed in two groups. Alloys containing less than 40.3 at.% Sn were annealed for 36 h at 900 °C while those having a tin content higher than 40 at.% were annealed at 350 °Ceven if the c phase (tin) is liquid at that temperature. The alloys were then allowed to cool slowly (1 °Cmin1 or less) to room temperature. Micrographic examination was used to verify the homogeneity of the sample. For all the alloys, we used the etching solution suggested for tin by the Material Research Corporation [31. This solution contains a 1:1 mixture of concentrated HF and HNO 3 solutions. A dip for a few seconds proved to be sufficient to reveal different phases in the various samples. Figure 2 shows four micrographs of Ni—Sn alloys containing 1.1, 78, 84 and 98 at.% Sn in which the different phases are revealed. The

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th~alloys was determimd by weighing the initial constitu-

ents. ld.ontifieation of various l~hases~~‘ascarried out by X-ray diffraction analysis using .AS’lM standard microfil~s( I’ig. :31.

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Fig. 3. X-ray diffraction patterns obtained on Ni-Sn alloys using a Debye—Sherrer camera. Relative intensities are given on the lines of each spectrum. Asterisks indicate an ASTM standard spectrum.

2.2. Alloy compositions In view of the Ni—Sn phase diagram, tin is considered to be insoluble in nickel at temperatures lower than 400 °Cand the same is true for nickel in tin [2]. The various intermetallic compounds were identified and their lattice constants determined as follows: Ni3Sn possesses a hexagonal structure with a = 5.286 A, c = 4.242 A and c/a = 0.803. Ni3Sn2 also has a hexagonal structure with a = 4.145 A, c = 5.213 A and c/a = 1.258 (saturated in nickel) or a = 4.048 A, c = 5.123 A and c/a = 1.266 A (saturated in tin). Ni3Sn4 has a monoclinic structure with a 12.17 A, b 4.06 A, c = 5.16 A and i3 = 104.3° [2]. !3-Tin has a tetragonal structure with a 5.83 A and c = 3.182 A; nickel solidifies in the f.c.c. structure with a = 3.524 A. Since the ASTM X-ray file was not available, we have reconstituted the predicted spectra (Table 1) from the experimental results of Bandyopadhyay and Gupta [4] using the relation that expresses the lattice spacing “d” for a hexagonal system knowing the lattice parameters a and c and the plane indices [5], i.e. 2+hh+k2 12 1 ir h +— d2 3 a2 c2

97 TABLE 1

(A)

Plane

Intensity

d

110 200 002 201 112 211 202 220 203

10 20 100 100 5 5 20 10 20

2.645 2.291 2.12 2.015 1.654 1.603 1.560 1.323 1.203

a

=

5.29

A, c

=

4.24

A.

Figure 3 presents the detailed X-ray spectra obtained from the intermetallic compounds and metals together with the ASTM Spectra Standards for Ni3Sn2 and Ni3Sn4 and the calculated Ni3Sn spectrum. The sample containing 25.5 at.% Sn is almost pure Ni3Sn phase. The alloys with 1.1 and 11.6 at.% Sn contain Ni(c~)and Ni3Sn mixtures; the lever rule predicts that the former contains 1% Ni3Sn and 99% Ni(o1) while the latter consists of 72% Ni(a) and 28% Ni3Sn. Some traces of undissolved tin are present in these two samples as indicated by the weak diffraction lines at 2.7 ~ d ~ 2.9 A. The diffraction pattern of the alloy with 40.3 at.% Sn is also consistent with the ASTM Standard file for Ni3Sn2 (ASTM card 6-0414) with the exception of a line at 2.57 A having a relative intensity of 30. The samples containing 63.5%, 67.4%, 83.5% and 98% Sn are mixtures of Ni3Sn4(~i) and pure Sn(). A lever rule evaluation of the relative abundance of these two phases is given in Table 2. TABLE 2 Composition (at.%)

Relative abundance Ni3Sn4 phase

Sn phase

63.5 77.9 83.5 98.0

47% 14% 10% 1%

53% 86% 90% 99%

In these four high tin content alloys the lines corresponding to planes 200 and 101 of tin (ASTM card 4-0673) are slightly shifted to lower interplanar distances. This could be due partly to lattice distortion in the preparation of the powder for the X-ray analysis. Table 3 summarizes X-ray diffraction data and composition analysis of the prepared alloys.

98 TABLE 3 Alloy characterization by X-ray diffraction Alloys

a

Ni

3Sn

Ni3Sn2

Ni3Sn4

Remarks

(at.% Sn) 0 (Ni)

f.c.c.; a

—

1.1

1% 99% Ni(a)

11.6

28% 72%Ni(81)

3.524

A

Contains traces of undissolved Sn -—

—

25.5

100%

—

—

40.3

—

100%

—

63.5

—

-—

77.9

—

—

83.5

—

—

98.0

—

—

—

—

100% (Sn)

=

Contains traces of undissolved Sn Hexagonal Ni3Sn; a = 5.29 c = 4.24 A, c/a = 0.803

A,

Hexagonal Ni3Sn2a = 4.15 c = 5.21 A, c/a = 1.258

A,

47% 53% Sn(e)

Ni3Sn4monoclinic;a = 12.17 A, b = 4.06 A, c 5.16 A, = 104.3 A

14% 86% Sn(e)

Lines (200) and (101) slightly displaced

10% 90% Sn(e)

Lines (200) and (101) slightly displaced

1% 99% Sn(e)

Lines (200) and (101) slightly displaced

—

Tetragonal;a 3.18 A

=

5.83

A,

c

=

aThe composition is determined by weight and weight-loss methods.

2.3. Auger electron spectroscopy The Auger electron spectroscopy (AES) technique was used to determine the surface composition of the Ni—Sn alloys in order to reveal any segregation that could have been brought about by sample polishing or by some electrochemical phenomena such as preferential dissolution of one of the constituents. Three samples were used. They were 1 at.% Sn, 25.5 at.% Sn and 100 at.% Sn. A thin slice of each freshly polished alloy was cut off and analysed. The electrodes were repolished and submitted to cathodic polarization for 1 h at 20 mA cm ~2; three further samples were cut from these polarized electrodes and analysed by AES. These samples were only rinsed with triply distilled water to remove any traces of electrolyte on the surface. Concentration profiles are shown in Fig. 4. A first observation is that the automatic profile tracing technique used in the present study at Ecole Polytechnique de Montréal indicates that the pure tin sample contains nickel concentrations up to 1%. However,

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Fig. 4. Concentration profiles obtained by AES on alloys containing (a) 1.1 at.%, (b) 25.5 at.% and (c) 100 at.% Sn. The spectra were taken before and after cathodic polarization on each sample.

an examination of the corresponding Auger spectra reveals that the nickel peak is indistinguishable from the background noise of the spectrum. The 1% is thus due to an error in the profiling technique and, in reality, the tin sample is absolutely free of any nickel. Furthermore, the atomic percentages shown on the profiles are only approximate and this is due to the lack of precision of the AES technique, the precision being of the order of a few per cent [6]. Preferential sputtering could also be responsible in part for the errors in the concentrations. The immediately sampled surface is far from being representative of the alloy composition. Important concentrations (5 30 at.%) of elements such as carbon, sulphur, oxygen and chlorine are found on the surface (Table 4). Carbon is particularly abundant on the surface of the 1.1 at.% Sn sample and may be due to the rather high carbon content of the nickel used. It could also arise from the adsorption of oxides of carbon on a surface containing a catalytic metal such as nickel. The surface concentration of oxygen is nearly the same for all the alloys and this oxygen content normally shows a decrease at depths greater than 5 A, except for the unpolarized 1.1 at.% Sn where the oxygen remains at 7 at.% at depths up to 1000 A. The sulphur content is always higher in the polarized samples and is indicative of a -

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slight cathodic reduction of the S042_ anion onto the electrode surface. The fact that the 1.1 at.% Sn, again, contains more sulphur and at greater depth could be due to the facility of nickel to form stable compounds with sulphur, as was shown for the Pd—S interactions by Haque [7]. Apart from the sulphur contamination, we can assume that most of the other impurities will be removed from the surface during the cathodic electrolysis at high current density under hydrogen bubbling. Thus the surface concentration of the alloy is only slightly different from the bulk composition of the alloy. It is difficult to quantify the effect of these impurities on the electrocatalytic properties of the alloys. It is well known, for example in gas-phase catalysis, that platinum can be totally poisoned by sulphur contamination. In electrochemistry little has been done so far to correlate the loss in electroactivity with the nature and amount of impurities on the catalytic surface. 2.4. Electrochemical techniques and apparatus All the electrochemical tests were performed in a three-compartment Pyrex cell which has been described recently together with details on the electrolytes and electrode mounting techniques [1]. Classical potentiostatic and voltammetric procedures were used [8, 9J. Corrections for ohmic drops were made through oscillographic interruption techniques. Freshly pre-electrolysed solutions (24 h, on a nickel foil at 20 mA cm2) consisting of mixtures of 1 N H2S04 and 1 M Na2SO4 in triply distilled water were used.

3. Results 3.1. Potentiostatic measurements Potentiostatic Tafel curves obtained in the mixture of 1 N H2S04 and 1 M Na2SO4 (pH 0.9) are presented in Fig. 5. To obtain reproducible results, it is important to take a few precautions as follows. The electrode must be under applied potential before it enters the electrolyte; this impedes any preferential dissolution of the less noble constituent which would modify the surface composition of the alloy. For similar reasons, only the experimental points obtained by going from the higher to the lower cathodic potentials were retained; in fact, curves recorded in the opposite direction give much higher Tafel slopes (140 170 mV) that can be attributed to the presence of an oxide film on the electrode surface [10]. This oxide film could have formed at the lower starting potentials where the corrosion reaction of tin is likely to occur. A freshly pre-electrolysed solution on a sacrificial nickel electrode was used for every run. A rigorous analysis of the results is difficult to achieve since each of the tested alloys contains at least two constituents that possess very different electrocatalytic properties. As a first hypothesis we shall assume a simple additivity rule for each phase within the alloy electrode. -

102

0%Sn

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-

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-

C

io~

io-~

to~

io°

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to_i

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Fig. 5. Plots of the electrode potential i~against Iog(current density) for the various Ni-Sn alloys (solution, 1 N H 2S04 with 1 M Na2SO4 pH 0.7; T = 25°C).

The results obtained pure slope nickel of give an mV. exchange density 2 and on a Tafel 110 Levincurrent and Rotinyan of X 10-6a value A cm of i [11]2.5found 2 in a sulphuric acid medium 0 = 2.7 x 10-6 i A cm while Conway et al. [121 obtained 2 in 0.1 N HC1 0 = 6.3 X 106 cm solutions at 38 °C. Shamsul Huq and Rosenberg [131Aobtained i 0 = 2 X 10-6 2 with a slope of 125 mV in 1 M HC1O A cm 4 (pH 0.04). Thus our results compare rather well with the published results on nickel. It is thus possible to assume that the nickel used was of a similar purity and that the techniques used to prepare the electrode (mounting, polishing and cleaning) were adequate. In the case of of tin,120 wemV. obtained, a value i0 of 2 with a slope On tin,atthe25i~°C, versus log i for relation 5.6 X 1O~ A cm exhibits an inflection of some 250 mV in the current density range from 5 X 10~ to 5 X iO~~ A cm2. This behaviour is common to metals such as tin, bismuth, gallium and antimony and corresponds to the crossing of the potential of zero charge; the lower potential region (small values of i) is related to a positively charged surface while in the upper potential region (high values of i) the HER would proceed on a negatively charged surface. At 25°C,Kiimnik and Rotinyan [14] report a jump of 230 mV between the two types of surface while we obtain a value of 160 mV. However, as we note, the maximum jump only occurs when the transition region is scanned very slowly (a few hours); in our case, this period never exceeded 30 mm and this can explain the smaller jump. However, it was also shown that the jump height depended strongly on temperature, being highest at 5 °Cand reaching a complete extinction at 65 °C(Fig. 6).

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Trasatti [15] has interpreted this jump as the result of the change in water molecule orientation at the electrode surface. From this model, Trasatti has shown that the jump could attain 400 mV from a water dipole parallel to the surface to a dipole perpendicular to the surface. For pure 2 tin, Kilimnik and Rotinyan [14]i have reported i0 2 1.8 X 10_b A cm (negatively charged surface) and (positively charged 0 = 1.8 x 108 A cm surface) for 2 N H 2S04 solutions. Webut have i0 = 5.6any X io~ A 2 for a negatively charged surface we determined have not observed linear cm Tafel region at low overpotentials. Qualitatively, if we assume a Tafel slope of 120 mV at 10~A cm2, the extrapolation at zero overpotential would give i 2 which is comparable with the results of Kilimnik 0 = 4 X 10~ and Rotinyan [14].A cm All the Ni—Sn alloys give n—log i curves which are found to lie in between those of the parent metals. Of these, three alloys exhibit a potential jump, like tin, around their point of zero charge (PZC); they are 1.1, 11.6 and 40.3 at.% Sn. It thus seems that the Ni(cx) phase and the Ni 3Sn2 phase, even though the former contains little tin, behave like tin with respect to the potential jump at the PZC. It could also mean that at low overpotential some tin could dissolve and redeposit on the surface, masking most of the initial catalytic activity. In fact, it was often observed that an electrode behaviour similar to that of tin could be artificially provoked only by leaving some electrodes at low overpotentials, thus producing dissolved tin that was redeposited in the following cathodic sweep. From this it can be seen that the following precautions become very important: (i) prepolarization of the electrode before immersion in the solution; (ii) polishing the electrode before each run; (iii) use of freshly pre-electrolysed solutions

104 TABLE 5 Experimental parameters derived from the potentiostatic curves in Fig. 5 Alloy

Alloy

composition (at.% Sn)

composition (wt.% Sn)

0

0 1.1 11.6 25.5 40.3 63.5 77.9 83.5 98.0 100

2.2 21.0 40.9 57.7 77.9 87.7 91.1 99.0 100

(—log io)

2y°~

5.6 5.9 6.3 5.4 6.4 6.5 6.5 6.2 6.3 8.3

(—log i)~=—0.5

-

v

1.0 2.4 2.14 14a 07a 1.24 2.37 2.78 2.73 3.70

b (mV)

110 145 120 125 90 118 120 135 140 120

(kca) moY’) 4.2 11.2 9.2 4.6 5.7 5.0 12.2 13.0 8.7 11.9

i0 and i are in amperes per centimetre squared. a Extrapolated.

to eliminate dissolved tin. Table 5 presents the log i0 and Tafel slope values derived from the curves of Fig. 5. To obtain the apparent heat of activation ~H* of the HER on Ni—Sn alloys, we obtained for each alloy a series of potentiostatic curves at various temperatures between 5 and 65 °C. Figure 6 shows the temperature dependence for tin while Fig. 7 depicts that observed for the Ni3Sn2 phase (40.3 at.% Sn). -0.5 15°C 30°C

I

1 to

to~

to_s ~ Current density (A cm2(

t0~°

t02

Fig. 7. Plots of the electrode potential i~against log(current density) for an alloy contaming 40.3 at.% Sn at various temperatures (solution, 1 N H 2SO4 with 1 M Na2SO4 pH 0.7).

105

3.2. Cyclic voltammetry Voltammetric studies on the Ni—Sn alloys were carried out in order to establish the electrochemical stability of the alloy in the H2S04—Na2SO4 medium. The method can also indicate whether the alloy tends to passivate or to dissolve actively. It may also reveal qualitatively the type of modifications undergone by the electrode surface during consecutive potential sweeps. Figures 8(a) 8(g) depict the observed behaviour for the different alloys in the order of increasing tin content. Analysis of the voltammograms reveals that all the alloy compositions between pure tin and the 63.5 at.% Sn have a similar behaviour and their electrochemical stability domain compares with that of pure tin. Thus we have to conclude that the phase Ni3Sn4, which constitutes 47% of the 63.5 at.% Sn alloys, possesses electrochemical characteristics that approach those of tin. These alloys are also similar with respect to their open-circuit potential which stands in an active corrosion region —0.22 V> E00~> —0.26 V (Fig. 9). The Ni3Sn2 phase (40.3 at.% Sn) exhibits a very different behaviour, however. For instance, this intermetallic compound remains passive far into the anodic region: +0.6 V measured against a normal hydrogen electrode (NHE). The cathodic side of the voltammogram reveals a shoulder located at about —0.4 V(NHE) and, since it shows up only on returning from the anodic sweep, it is associated with the deposition of a species that has been dissolved during the previous anodic cycle, i.e. the small inflexion at +O~3V. This combined dissolution—deposition process improves the catalytic properties of this alloy as can be observed on the following cycles. Part of this improvement could be due, however, to an increase in the real surface area of the electrode. The voltammogram of the compound Ni3Sn (Fig. 8(g)) is quite comparable with that of Ni3Sn2 in regard to its electroactivity domain. No visible redeposition wave can be seen on the cathodic sweep even though it could be masked by the concurrent HER. The fact that the catalytic activity is improved when the sweep extends to more anodic potentials supports this interpretation. Figure 8(h) shows the voltammogram of the 11.6 at.% Sn which contains 72% Ni(a) and 28% Ni3Sn. This alloy shows an appreciable anodic activity centred around 0.4 V with a small shoulder at 0.5 V, followed by a passivation region at more anodic potentials. The 1.1 at.% Sn alloy (99% Ni(c~)and 1% Ni3Sn) has a behaviour similar to the previous alloy, particularly on the anodic side (Fig. 8(i)). It shows a first maximum at about 0.5 V that moves with cycling to 0.4 V, i.e. at the same position as the previous alloy in Fig. 8(h). In the case of nickel, even if its immunity potential is around —0.4 V [16], we found experimentally that it remains noble as far as +100 mV where a generalized active corrosion begins. Like earlier researchers [16], we found that it was almost impossible to induce a passivation on nickel even at potentials exceeding 1 V anodic, in sulphuric acid solutions. -

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—

—

~

WIn CI) 0

_____

i~~_ ~

III 0

C

C

108 -3OC S

-200

Active dissolution

-

Active dissolution

200

-

300 0

I 10

20

30

40

I......L~L...._ 50

Alloy composition (at

60 °/

70

I 80

90

100

Sri)

Fig. 9. Plot of open-circuit potentials of Ni--Sn alloys in solutions of 1 N H Na2SO4 (pH 0.7; T

2S04 with 1 M 25 °C). Active dissolution and passivation regions are shown.

4. Discussion 4.1. Mechanisms of the hydrogen evolution reaction The experimental results indicate that the electroactivity of the various Ni—Sn alloys does not follow a smooth relationship with composition, as is the case with metals forming completely miscible solid solutions such as the Cu—Ni [121 and Ag—Pd [11 systems. This observation seems quite normal, however, when we consider that the intermetallic compounds formed between nickel and tin possess chemical and physical properties very different from those of their parent metals. Thus, it would be logical to consider these compounds as individual constituents, which would have their own electrochemical behaviour; the following table contains the experimental results on nickel, tin, Ni3Sn and Ni3Sn2 relevant to the discussion of the mechanisms of HER on these materials (Table 6). 4.1.1. Nickel The Tafel slope for the HER in acid media is near 120 mV (2RT/F) in agreement with previously published results [11, 12]. This value indicates that the rate-determining step will be the proton discharge mechanism (Volmer) at nil hydrogen coverage (0 0) or the electrochemical desorption mechanism (Heyrovsky) at full coverage (0 1). The fact that in the gas phase hydrogen is adsorbed strongly by nickel would favour the second mechanism; this was also the conclusion of Mannan [18] in a study of the isotope separation factor on nickel. —~-

—~

109 TABLE 6 Parameter

Ni

Ni

Tafel slope (mV)

110

125

Exchange current2)density (Acm Open-circuit potential (mV)

2.5

3Sn

x

10—6

4

x

10—6

Ni3Sn2

Sn

90

120

4

x

i0~

5

x

i0~

4 -7

—110

+120

—280

Apparent heat of activation (kcal molt)

4.2

4.6

5.7

11.9

Hydrogen adsorption (kcal mol’)

In gas phase [17] heat of adsorption is 64

Probable

Probable

Nil

Electroactivity domain (V)

From 0 to cathodic

From —0.1 to cathodic

From +0.15 to cathodic

From —0.25 to cathodic

Proposed mechanism

Electrochemical desorption

Electrochemical desorption

Electrochemical desorption

Proton discharge (Volmer)

4.1.2. Intermetallic compounds: Ni 3Sn, Ni3Sn2, Ni3Sn4 No results on either the HER or the hydrogen adsorption in the gas phase are available for these compounds. For Ni3Sn, the Tafel slope of 125 mV favours either the Volmer step at 0 0 or the Heyrovsky step at 0 1 as the rate-determining step.with Thean rather high exchange current density 2) combined apparent heat of activation similar (i0 = 4 X 10-6 A cm to that of nickel (i.~H*= 4.2 kcal mol’) favours the Heyrovsky step. For the same reasons, for Ni 3Sn2 and Ni3Sn4 the rate-determining step is also the electrochemical desorption of hydrogen. —~

—~

4.1.3. Tin The Tafel slope on tin is also of the order of 2RT/F (120 mV). However, it is well known that tin2)does not adsorb hydrogen gas phase. makes tin a rather poorfrom HERthecatalyst, in The low i0 (5 X iO~ A cm common with many “sp” metals such as lead, indium, mercury and cadmium. The proton discharge is the most probable rate-determining step for the HER on tin. Owing to the applications of nickel in electrolysers and in alkaline fuel cells, the interest in seeking nickel-based materials of high electrochemical activity and stability is obvious. Many metals, when alloyed with

110

nickel form intermetallics, e.g. titanium, zirconium and tin. In the work of Miles [19], intermetallics of titanium and zirconium were found to exhibit high hydrogen overpotentials, i.e. between —1.0 and —1.50 V at a current density of 2 mA cm2 in potentiodynamic measurements, at a sweep rate of 2 V min~. It is most surprising, therefore, to see a report by Yeager and coworkers [201 in which intermetallics of titanium and zirconium with nickel are shown to give values of overpotential which are only one tenth of those given by Miles, i.e. the graphs of these workers [20] indicate that, at overpotentials of around 100 150 mV, a current density of 2 mA cm2 can be obtained on these intermetallics. In our own work on Ni—Sn intermetallics the compounds do not manifest an especially high activity nor do they appear to involve the very high overpotential values obtained by Miles for intermetallics of titanium and zirconium. In general (Table 6), intermetallics high in nickel content approach nickel in electroactivity whereas those high in tin have activities that tend to lie close to that of tin. The detailed theoretical aspects of the composition—activity relationships for the HER on these alloys will be examined elsewhere [21]; in particular, we shall seek to explore the question as to whether the atomic composition of the surface or the bulk electronic configuration of these alloys and intermetallics determines their electrode activity [21]. -

Acknowledgments This paper is based on the Ph.D. thesis of Dr. A. Bélanger, under the direction of Dr. Ashok K. Vijh, submitted to the Institut National de la Recherche Scientifique Energie, Université du Québec. The financial support of this work by the National Research Council of Canada and the Institut de Recherche d’Hydro-Québec is gratefully acknowledged. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14

A. Bélanger and A. K. Vijh, Surf. Technol., 15 (1982) 59. M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958. Metal Catalog, The Material Research Corporation, Orangeburg, NY, 1972. J. Bandyopadhyay and K. P. Gupta, Metal!. Trans., 1 (1970) 327. B. D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, London, 1956. R. E. Weber, Research and Development, 23 (1972) 22. C. A. Haque, Proc. 6th mt. Conf. on Electrical Contact Phenomena, Illinois Institute of Technology, Chicago, June 5 - 9, 1972. A. K. Vijh and B. E. Conway, Chem. Rev., 67 (1967) 623. G. Belanger,J. Electrochem. Soc., 118 (1971) 583. B. E. Conway, Theory and Principles of Electrode Processes, Ronald, New York, 1965. E. D. Levin and A. L. Rotinyan,Russ. J. Phys. Chem., 43(1971)2546. B. E. Conway, E. M. Beatty and P. A. D. Demaine, Electrochim. Acta, 7 (1962) 39. A. K. M. Shamsul Huq and A. J. Rosenberg,J. Electrochem. Soc., 111 (1964) 270. A. B. Kilimnik and A. L. Rotinyan, Soc. Electrochem., 5 (1969) 1175.

111 15 S. Trasatti,J. Electroanal. Chem., 39 (1972) 163. 16 M. Pourbaix, Atlas d’Equilibres Elect rochimiques, Gauthier-Villars, Paris, 1963. 17 G. Ehrlich, in W. M. H. Sachtler, C. G. A. Schuit and P. Zwietering (eds.), Third Congress on Catalysis, Vol. I, North Holland, Amsterdam, 1967. 18 R. J. Mannan, Ph.D. Thesis, University of Pennsylvania, Philadelphia, PA, 1967, p. 263. 19 M. H. Miles, J. Electroanal. Chem., 60 (1975) 89. 20 Z. Y. Jiang, D. Trysk and E. Yeager, Extended Abstracts, Vol. 82-1, The Electrochemical Society, Pennington, NJ, 1982, p. 600. 21 A. Belanger and A. K. Vijh, to be published.