Surface and CoatingsTechnology,28 (1986) 93 - 111 93

THE HYDROGEN EVOLUTION REACTION ON Ni-Sn ALLOYSAND INTERMETALLICS

ANDRÉ BELANGER andASHOK K. VIJH

Institut de Recherched ‘Hydro-Quebec, Varennes,QuébecJUL 2P0 (Canada)

(ReceivedNovember18, 1985)

Summary

Ni—Sn alloyscontaining0%, 1.1%, 11.6%,25.5%, 40.3%, 63.5%, 77.9%,83.5%, 98% and 100% tin were preparedand examined regarding theiractivity towardsthe hydrogenevolutionreactionin sulphuricacid solutions.The composition of these alloys and intermetallics was determinedfromthe initial weight of the constituentsused in the alloy preparation.Theidentification of various phaseswascarriedout by X-ray diffraction analysisusing ASTM standardmicrofiles. The surfacecomposition of thesealloyswas determinedby Auger electronspectroscopyfor several typical cases,bothbefore andafter the cathodicpolarization.

The electrochemicalmeasurementsconsistedof steadystatepotentio-static polarization curves at various temperaturesand potentiodynamicprofiles. The electrochemicaldata deduced include Tafel slopes,exchangecurrent densities, apparent heatsof activation and potentiodynamic be-haviour.

On nickel and nickel-rich intermetallics, electrochemicaldesorptionis indicated as the rate-determiningstep whereasthe hydrogenevolutionreaction appearsto proceedby the initial dischargemechanismon tin andtin-rich alloys. Also the activity of nickel-rich intermetallics approachesthat of nickel whereasthe tin-rich alloys tend to exhibit activity similar tothat of tin.

1. Introduction

In a previouspaper [1], we reportedwork on the hydrogenevolutionreaction (HER) on an alloy systemthat forms a solid solution in its entirecomposition range, namely Ag—Pd. In the presentinvestigation, the HERwas studiedon avery different alloy systemthat doesnot form solid solu-tions: the Ni—Sn couple. The Ni—Sn phasediagram is quite complex [2]and revealsat least three known intermetallic compounds,namely Ni3Sn,Ni3Sn2andNi3Sn4(Fig. 1).

0257-8972/86/$3.50 © Elsevier Sequoia/Printedin The Netherlands

94

I~~ I I ~1~23l0C T

2320C

100 —~ - ___________ _______0 25 50 75 100Ni Composition ot the alloy (at 0/0 Sn) Sn

Fig. 1. Phasediagram of the Ni—Sn system.

The HER has beenexaminedon eight Ni—Sn alloysandon purenickel

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 compositionaxis ofthe phasediagramshownin Fig. 1.

2. Experimentaldetails

2.1. Preparationand analysisof the alloysThe alloys were preparedstarting from nickel (purity, 99.97%) ob-

tained from the International Nickel Corporationand tin (purity, 99.999%)purchasedfrom A. D. Mackay, New York. The main impurities containedin the nickel were carbon at 200 ppm or less, iron at 50 ppm or less andsulphur, silicon, copper, chromium, titanium, cobalt and magnesium,all atless 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 annealedin two groups. Alloys containing less than 40.3 at.% Sn were annealedfor36 h at 900 °Cwhile those having a tin content higher than 40 at.% wereannealedat 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 roomtemperature.Micrographic examinationwas usedto verify the homogeneityof the sample.For all the alloys, we usedthe etchingsolution suggestedfortin by the Material ResearchCorporation [31.This solution containsa 1:1mixture of concentratedHF and HNO

3 solutions. A dip for a few secondsprovedto be sufficient to reveal different phasesin the varioussamples.

Figure 2 shows four micrographs of Ni—Sn alloys containing 1.1,78, 84 and 98 at.% Sn in which the different phasesare revealed.The

- .,‘ .- .. ‘,~- ____

V ~

-

~ ~i- 2 -

‘-~

(c)__________ ____ (d)[ig. 2. ~1icrugraphs oF sonw Ni Sn alloys etcht~~lin a solution of HF HNOt in a 1 :1

ratn : I 1 .1 at . Sn: I hi 77.9 ai . : Sn: IC) ~ at .‘ Sn; id I ~M.() at. Sn.

coinposition Of th~alloys was determimd by weighing the initial constitu-ents. ld.ontifieation of various l~hases~~‘ascarried out by X-ray diffractionanalysisusing .AS’lM standard microfil~s( I’ig. :31.

96

100% Ni SF 7p1514 4 7 21 42 100 NS 4-0850

C1%Sn 4F 6P SFl1.6%Sn SF SF~ 20 5 5 5

~ SF SF101° SFNi,Sncalculated 20 10 20 55 103 lCD 20

4F~4F~4F4F3F ~-:~:-- G~ ~7~° ~ 5F50~ ~~°°

l4i~&~ 4-0845 3SF ~ 03 ~ SFi°~50 3D 0353 ~ 5 80 ~l0 50 ~ 3)81)

7~ ¶~ 44 1003

Ii SF SF~ ? ~96%Sn SF~SFISF SF

505330 5353 30 0350 100503 5

ofl I II III I El* a-Sn ~ ~i III 17 SF

I I I I 111111111 I

0.5 1.0 .5 2.0 2.5 3.0d interplanar distance i

1ii

Fig. 3. X-ray diffraction patternsobtainedon Ni-Sn alloys using a Debye—Sherrercam-era. Relative intensities are given on the lines of each spectrum. Asterisks indicate anASTM standardspectrum.

2.2. Alloy compositionsIn view of the Ni—Sn phasediagram, tin is consideredto be insoluble

in nickel at temperatureslower than 400 °Cand the sameis true for nickelin tin [2].

The various intermetallic compoundswere identified andtheir latticeconstantsdeterminedas follows: Ni3Sn possessesa hexagonalstructurewitha = 5.286 A, c = 4.242 A andc/a = 0.803. Ni3Sn2also hasahexagonalstruc-ture with a = 4.145 A, c = 5.213 A and c/a = 1.258 (saturatedin nickel) ora = 4.048 A, c = 5.123 A and c/a = 1.266 A (saturatedin tin). Ni3Sn4hasa monoclinic structure with a 12.17 A, b 4.06 A, c = 5.16 A and i3 =

104.3°[2]. !3-Tin has a tetragonalstructurewith a 5.83 A and c = 3.182A; nickel solidifies in the f.c.c. structurewith a = 3.524A.

Since the ASTM X-ray file wasnot available,we havereconstitutedthepredictedspectra(Table 1) from the experimentalresults of Bandyopadhyayand Gupta [4] usingthe relation that expressesthe lattice spacing“d” fora hexagonalsystemknowing the lattice parametersa and c and the planeindices [5], i.e.

1 ir h2+hh+k2 12

+—

d2 3 a2 c2

97

TABLE 1

Plane Intensity d (A)

110 10 2.645200 20 2.291002 100 2.12201 100 2.015112 5 1.654211 5 1.603202 20 1.560220 10 1.323203 20 1.203

a = 5.29 A, c = 4.24 A.

Figure 3 presentsthe detailed X-ray spectraobtainedfrom the inter-metallic compoundsand metals togetherwith the ASTM SpectraStandardsfor Ni3Sn2andNi3Sn4andthe calculatedNi3Sn spectrum.

The samplecontaining 25.5 at.% Sn is almost pure Ni3Sn phase.Thealloys with 1.1 and11.6at.% SncontainNi(c~)andNi3Sn mixtures;the leverrule predicts that the former contains 1% Ni3Sn and 99% Ni(o1) while thelatter consists of 72% Ni(a) and 28% Ni3Sn. Some tracesof undissolvedtin are presentin thesetwo samplesas indicated by the weak diffractionlines at 2.7 ~ d ~ 2.9 A. The diffraction pattern of the alloy with 40.3at.% Sn is also consistentwith the ASTM Standardfile for Ni3Sn2 (ASTMcard 6-0414) with the exceptionof a line at 2.57 A having a relative inten-sity of 30. The samplescontaining 63.5%, 67.4%, 83.5% and 98% Sn aremixtures of Ni3Sn4(~i) andpureSn(�).A leverrule evaluationof therelativeabundanceof thesetwo phasesis given in Table2.

TABLE 2

Composition Relative abundance(at.%) Ni3Sn4phase Snphase

63.5 47% 53%77.9 14% 86%83.5 10% 90%98.0 1% 99%

In thesefour high tin content alloys the lines correspondingto planes200 and 101 of tin (ASTM card 4-0673) are slightly shifted to lower inter-planardistances.This could be duepartly to lattice distortionin the prepara-tion of the powderfor the X-ray analysis.Table3 summarizesX-ray diffrac-tion dataandcompositionanalysisof the preparedalloys.

98

TABLE 3

Alloy characterizationby X-ray diffraction

Alloys a Ni3Sn Ni3Sn2 Ni3Sn4 Remarks

(at.% Sn)

0 — f.c.c.;a = 3.524 A(Ni)

1.1 1% Containstracesof undissolved99% Ni(a) Sn

11.6 28% -— — Containstracesof undissolved72%Ni(81) Sn

25.5 100% — — Hexagonal Ni3Sn; a = 5.29 A,c = 4.24 A, c/a = 0.803

40.3 — 100% — HexagonalNi3Sn2a = 4.15 A,c = 5.21 A, c/a = 1.258

63.5 — -— 47% Ni3Sn4monoclinic;a = 12.1753% Sn(e) A, b = 4.06 A, c 5.16 A,

= 104.3 A

77.9 — — 14% Lines (200) and (101) slightly86% Sn(e) displaced

83.5 — — 10% Lines (200) and (101) slightly90% Sn(e) displaced

98.0 — — 1% Lines (200) and (101) slightly99% Sn(e) displaced

100% — — — Tetragonal;a = 5.83 A, c =

(Sn) 3.18 A

aThecomposition is determinedby weight andweight-lossmethods.

2.3. AugerelectronspectroscopyThe Auger electron spectroscopy(AES) techniquewas usedto deter-

mine the surfacecomposition of the Ni—Sn alloys in order to reveal anysegregationthat could have beenbrought about by samplepolishing or bysome electrochemicalphenomenasuch as preferential dissolution of oneof the constituents.

Three sampleswere used.They were 1 at.% Sn, 25.5 at.% Sn and 100at.% Sn. A thin slice of eachfreshlypolishedalloy wascut off andanalysed.The electrodeswere repolishedand submittedto cathodicpolarizationfor1 h at 20 mA cm ~2; three further sampleswere cut from thesepolarizedelectrodesand analysedby AES. Thesesampleswereonly rinsedwith triplydistilled water to remove any tracesof electrolyte on the surface.Concen-tration profiles areshownin Fig. 4.

A first observation is that the automatic profile tracing techniqueused in the present study at Ecole Polytechniquede Montréal indicatesthat the pure tin samplecontainsnickel concentrationsup to 1%. However,

99

100 tOO80 ~ 80 Sn

60 l.lat. % Sn before polarization i~60 tOO at % Sn after polarization

° 40 CL (Auger: 80t.%Sn( 40 (Auger. 99 at. /~Sn)C —020 ~1~5fl 20 ~0 Ni

0 ~ —i~--~~r~ TL.,,.. c ~ I0 00 200 300 400 500 0 100 200 300 0400 500

(a) Depth (A) Depth (A>

too too 255 at. ~ Sn betoce polarization~ •—.— Ni 80 (Auger: 5601 0/ Sn)

..~ 60 1.lat % Sn after polarization ~560 ~(Auger: 701 %Sn)

040 ~ 40,—

20 ~CL ,.,—Sn 20 0 and S

0 tOO =0 DCC 100 5CC 0 tOO 200 300 400 500

(b) Depth (A) Depth(~(

100 —•--_•—____ Sn 100

80 25.5 at. 0/ Sn after polarization00at. % Sn before polarization (Auger 58 at. ~ Sn)60 (Auger: 98 at. % Sn) 60 ~ •

~40 °40p~~

20 70 ~Jl 20 ~0andS

0 ~—--—~~ I i~ ~. I

0 tOO 200 300 400 500 0 tOO 200 300 400 500(c) Depth (A) Depth(A(

Fig. 4. Concentration profiles obtained by AES on alloys containing (a) 1.1 at.%, (b)25.5 at.% and (c) 100 at.% Sn. The spectrawere takenbefore and after cathodic polariza-tion on eachsample.

an examinationof the correspondingAuger spectrarevealsthat the nickelpeak is indistinguishablefrom the backgroundnoise of the spectrum.The1% is thus due to an error in the profiling techniqueand, in reality, the tinsampleis absolutelyfree of any nickel.

Furthermore,the atomic percentagesshown on the profiles are onlyapproximateand this is due to the lack of precisionof the AES technique,the precisionbeingof the order of a few percent [6]. Preferentialsputteringcould also be responsiblein part for the errors in the concentrations.Theimmediately sampledsurface is far from being representativeof the alloycomposition. Important concentrations(5 - 30 at.%) of elementssuch ascarbon, sulphur, oxygen and chlorine are found on the surface(Table 4).Carbon is particularly abundanton the surfaceof the 1.1 at.% Sn sampleand may be due to the rather high carbon content of the nickel used.Itcould also arise from the adsorptionof oxides of carbonon a surfacecon-taining a catalytic metal suchasnickel. The surfaceconcentrationof oxygenis nearly the samefor all thealloysandthis oxygencontentnormally showsadecreaseat depthsgreaterthan5 A, exceptfor the unpolarized1.1 at.% Snwhere the oxygen remainsat 7 at.% at depthsup to 1000 A. The sulphurcontent is always higher in the polarized samplesand is indicative of a

100

(0 I ~ I I I

(0 (0 01 C~1~~0 I I

IS ~

10 co (0C0(0 I c’~

c~ -~ I I I

(0 (0 01 — ,-H 01‘-C —

a .~

(0 (0 C~1 aj (0(0 01 01

(0I co I I I I

(0 c~ I ,- I —

CS

(0 01 -t cC Cl V(0 ,- —

(0 t— -1’ 01 Cl C’110 ,H I

(0 (0 C~i -~ t- 100~C

(0 (0 (0 ~.- .~5l Cl0 (0 Cl ,-~ Cl ,-~ ,.H

00 0 0 0

.2 o 0 ~0 ~ ~ a15 .-. .~ 15 N .I~C

CS 1- N •~ CSC.. N ~ .~ 15-~ •~ 0 15 — =

~~ 0 a 00 0a 00 10 10 C-

10 1 0 0 ~

C-.. 5) Ci) ,~ ~a ~ a

a a~ 0 CS CS

~ ‘. (0 10 (0 (0H ~ — — Cl Cl ,-~

101

slight cathodic reduction of the S042_ anion onto the electrodesurface.The fact that the 1.1 at.% Sn, again, containsmore sulphur and at greaterdepthcould be due to the facility of nickel to form stablecompoundswithsulphur, as wasshown for the Pd—S interactionsby Haque [7]. Apart fromthe sulphur contamination,we can assumethat mostof the other impuritieswill be removed from the surfaceduring the cathodic electrolysis at highcurrent density underhydrogen bubbling. Thus the surfaceconcentrationof the alloy is only slightly different from the bulk composition of thealloy.

It is difficult to quantify the effect of theseimpurities on the electro-catalytic propertiesof the alloys. It is well known, for examplein gas-phasecatalysis,that platinum can be totally poisoned by sulphur contamination.In electrochemistrylittle hasbeendone so far to correlatethe lossin electro-activity with the natureandamountof impuritieson the catalyticsurface.

2.4. Electrochemicaltechniquesand apparatusAll the electrochemicaltests were performedin a three-compartment

Pyrex cell which has been describedrecently togetherwith details on theelectrolytesand electrodemounting techniques[1]. Classicalpotentiostaticand voltammetric procedureswere used [8, 9J. Correctionsfor ohmic dropsweremadethroughoscillographicinterruptiontechniques.

Freshly pre-electrolysedsolutions (24 h, on a nickel foil at 20 mAcm2) consisting of mixtures of 1 N H

2S04and 1 M Na2SO4in triply dis-tilled water wereused.

3. Results

3.1. PotentiostaticmeasurementsPotentiostaticTafel curvesobtainedin the mixture of 1 N H2S04and

1 M Na2SO4(pH 0.9) arepresentedin Fig. 5. To obtainreproducibleresults,it is important to takea few precautionsas follows. The electrodemust beunder applied potential before it entersthe electrolyte; this impedesanypreferential dissolution of the less noble constituentwhich would modifythe surfacecomposition of the alloy. For similar reasons,only the experi-mental points obtained by going from the higher to the lower cathodicpotentialswere retained;in fact, curves recordedin the oppositedirectiongive much higherTafel slopes(140 - 170 mV) that can be attributedto thepresenceof an oxide film on the electrodesurface [10]. This oxide filmcould have formed at the lower starting potentials where the corrosionreaction of tin is likely to occur. A freshly pre-electrolysedsolution on asacrificialnickel electrodewasusedfor every run.

A rigorous analysisof the results is difficult to achievesinceeachofthe testedalloyscontainsat leasttwo constituentsthat possessvery differentelectrocatalytic properties.As a first hypothesiswe shall assumea simpleadditivity rule for eachphasewithin the alloy electrode.

102

0%Sn-0.2 -

-0.1 -

C IIII~1Il 1~111111

io~ io-~ to~ io° 1o2 to_iCurrent density (A cm~)

Fig. 5. Plots of the electrodepotential i~against Iog(current density) for the various Ni--Snalloys (solution, 1 N H

2S04 with 1 M Na2SO4pH 0.7; T = 25°C).

The results obtained on pure nickel give an exchangecurrent densityof 2.5 X 10-6 A cm

2 and a Tafel slope of 110 mV. Levin and Rotinyan[11] found a value of i

0 = 2.7 x 10-6 A cm2 in a sulphuric acid medium

while Conway et al. [121 obtained i0 = 6.3 X 106 A cm

2 in 0.1 N HC1solutions at 38 °C.ShamsulHuq and Rosenberg[131 obtainedi

0 = 2 X 10-6A cm

2 with a slope of 125 mV in 1 M HC1O4 (pH 0.04). Thus our results

comparerather well with the publishedresults on nickel. It is thuspossibleto assumethat the nickel usedwas of a similar purity and that the tech-niques used to prepare the electrode (mounting, polishing and cleaning)were adequate.In the case of tin, we obtained,at 25 °C, a value for i0 of5.6 X 1O~A cm

2 with a slopeof 120 mV. On tin, the i~versuslog i relationexhibits an inflection of some 250 mV in the current density range from5 X 10~to 5 X iO~~A cm2. This behaviouris common to metals such astin, bismuth, gallium andantimony and correspondsto the crossingof thepotential of zero charge; the lower potential region (small values of i) isrelated to a positively chargedsurfacewhile in the upper potential region(high values of i) the HER would proceedon a negativelychargedsurface.At 25°C,Kiimnik andRotinyan [14] report a jump of 230 mV betweenthetwo types of surfacewhile we obtain a value of 160 mV. However, as wenote,the maximum jump only occurs when the transitionregion is scannedvery slowly (a fewhours);in our case,this periodnever exceeded30 mm andthis can explain the smaller jump. However,it was also shown that thejumpheight dependedstrongly on temperature,beinghighestat 5 °Candreachingacompleteextinction at 65 °C(Fig. 6).

103

—0.9

5~a~’/-0.8- -

2

‘1 3—0.7 - -

4Lul 55I -0.6 - -

z

- -

-0.4- -

Pure tin6555\___

—0.3 - 45-=

5—0.2 11111111 III..,,) ,IL~ 1111111 111111,

to~ t~-~ t02 to1Current density (A cm~2)

Fig. 6. Plots of the electrodepotential i~againstlog(current density) for a pure tin elec-trode as a function of temperature(solution, 1 N H

2SO4 with 1 M Na2SO4pH 0.7).

Trasatti [15] has interpreted this jump as the result of the changein water molecule orientation at the electrodesurface. From this model,Trasatti has shown that the jump could attain 400 mV from a water dipoleparallel to the surfaceto a dipole perpendicularto the surface. For puretin, Kilimnik and Rotinyan [14] have reported i0 1.8 X 10_b A cm

2(negativelychargedsurface) and i

0 = 1.8 x 108 A cm2 (positively charged

surface) for 2 N H2S04 solutions. We havedeterminedi0 = 5.6X io~A

cm2 for a negatively chargedsurfacebut we havenot observedany linear

Tafel region at low overpotentials.Qualitatively, if we assumea Tafel slopeof 120 mV at 10~A cm2, the extrapolationat zero overpotentialwouldgive i

0 = 4 X 10~A cm2 which is comparablewith the results of Kilimnik

andRotinyan [14].All the Ni—Sn alloys give n—log i curveswhich are found to lie in be-

tween thoseof the parentmetals.Of these,threealloys exhibit a potentialjump, like tin, aroundtheir point of zerocharge(PZC); they are 1.1, 11.6and 40.3 at.% Sn. It thus seemsthat the Ni(cx) phaseand the Ni

3Sn2phase,even though the former contains little tin, behavelike tin with respecttothe potentialjump at the PZC. It could also meanthat at low overpotentialsome tin could dissolve and redepositon the surface,maskingmost of theinitial catalytic activity. In fact, it was often observedthat an electrodebehaviour similar to that of tin could be artificially provoked only byleaving some electrodes at low overpotentials, thus producing dissolvedtin that was redepositedin the following cathodicsweep.From this it canbe seen that the following precautions becomevery important: (i) pre-polarizationof the electrodebefore immersion in the solution; (ii) polishingthe electrode beforeeachrun; (iii) use of freshly pre-electrolysedsolutions

104

TABLE 5

Experimentalparametersderivedfrom the potentiostatic curvesin Fig. 5

Alloy Alloy (—log io)2y°~ (—log i)~=—0.5 v b

composition composition (mV) (kca)(at.% Sn) (wt.% Sn) moY’)

0 0 5.6 1.0 110 4.21.1 2.2 5.9 2.4 145 11.2

11.6 21.0 6.3 2.14 120 9.2

25.5 40.9 5.4 14a 125 4.640.3 57.7 6.4 - 07a 90 5.763.5 77.9 6.5 1.24 118 5.077.9 87.7 6.5 2.37 120 12.283.5 91.1 6.2 2.78 135 13.098.0 99.0 6.3 2.73 140 8.7

100 100 8.3 3.70 120 11.9

i0 and i are in amperesper centimetresquared.aExtrapolated.

to eliminate dissolvedtin. Table 5 presentsthe log i0 and Tafel slopevaluesderivedfrom the curvesof Fig. 5.

To obtain the apparentheatof activation ~H* of the HER on Ni—Snalloys, we obtained for eachalloy a seriesof potentiostaticcurvesat vari-ous temperaturesbetween 5 and 65 °C.Figure 6 shows the temperaturedependencefor tin while Fig. 7 depicts that observedfor the Ni3Sn2phase(40.3at.% Sn).

-0.5

15°C30°C

I

to1 to~ to_s ~ t0~° t02

Current density (A cm2(

Fig. 7. Plots of the electrode potential i~against log(current density) for an alloy con-taming 40.3 at.% Sn at various temperatures(solution, 1 N H

2SO4 with 1 M Na2SO4pH 0.7).

105

3.2. Cyclic voltammetryVoltammetric studieson the Ni—Sn alloys were carriedout in order to

establish the electrochemicalstability of the alloy in the H2S04—Na2SO4medium. The method can also indicatewhether the alloy tendsto passivateor to dissolveactively. It may also reveal qualitatively the type of modifica-tions undergone by the electrode surface during consecutive potentialsweeps.Figures 8(a) - 8(g) depict the observedbehaviour for the differentalloys in the order of increasingtin content.Analysis of the voltammogramsrevealsthat all the alloy compositionsbetweenpure tin and the 63.5 at.%Sn have a similar behaviour and their electrochemicalstability domaincompareswith that of pure tin. Thus we haveto concludethat the phaseNi3Sn4, which constitutes47% of the 63.5 at.% Sn alloys, possesseselectro-chemical characteristicsthat approachthose of tin. Thesealloys are alsosimilar with respectto their open-circuitpotential which standsin an activecorrosion region —0.22V> 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 passivefar into the anodic region:+0.6 V measuredagainsta normal hydrogenelectrode(NHE). The cathodic

side of the voltammogramrevealsa shoulderlocatedat about—0.4 V(NHE)and, since it shows up only on returning from the anodic sweep,it is asso-ciated with the depositionof a speciesthat has beendissolvedduring thepreviousanodiccycle, i.e. the small inflexion at +O~3V.

This combined dissolution—depositionprocessimproves the catalyticproperties of this alloy as can be observedon the following cycles. Partof this improvement could be due,however,to an increasein the realsur-face area of the electrode. The voltammogram of the compound Ni3Sn(Fig. 8(g)) is quite comparablewith that of Ni3Sn2in regardto its electro-activity domain. No visible redepositionwave can be seenon the cathodicsweepeven though it could be masked by the concurrentHER. The factthat the catalytic activity is improved when the sweepextendsto moreanodicpotentialssupportsthis interpretation.

Figure 8(h) shows the voltammogramof the 11.6 at.% Sn which con-tains 72% Ni(a) and 28% Ni3Sn. This alloy shows an appreciableanodicactivity centred around 0.4 V with a small shoulderat 0.5 V, followed bya passivationregion at moreanodic potentials.The 1.1 at.% Sn alloy (99%Ni(c~)and 1% Ni3Sn) has a behavioursimilar to the previousalloy, partic-ularly on the anodic side (Fig. 8(i)). It shows a first maximum at about0.5 V that moveswith cycling to 0.4 V, i.e. at the sameposition as theprevious alloy in Fig. 8(h). In the caseof nickel, evenif its immunity po-tential is around —0.4 V [16], we found experimentally that it remainsnoble as far as +100 mV where a generalizedactive corrosion begins.Likeearlier researchers[16], we found that it was almost impossibleto inducea passivationon nickel evenat potentialsexceeding1 V anodic,in sulphuricacidsolutions.

106

1O

o—_-~

-2 -~ -\~ \~~j

OT

/ Z C I I I.L....... I I/ I rD In ~ a Cu ‘- ‘- Cu a ~t

- c- (VW)iueIJnD -

- Cu

:1

(0 (0 ,O Ca — a ‘t a CD C-

(VW) Iuai~ro - ~-

___ ___ CCI ________ ______ ____

0

~ ~

2

Cu~ __

_J~Ji_ 1_LJJ~ L I I J~i~JL_JJ

L~5°3~ ~ lOW) IuSiJn3 ..- Cu 1°)

~ JI.~ CS zn

‘III ~

CS CD a CCI CCI a CD 0 (0 CS CD a C’I ‘ Cu 1) CD ICC CD cC

(0°-)~ . I ‘-C (OW) ius~~~3- I .—.

~ _J~ -_--Ca ______0 0

107

~C

a—.a

______________ r~ ~C1D C_______.io~ ° Ci)

0 +~5.. C

-d

~ ~

I I I _______ 2 -

ID 10 ~ II) Cu C’) II) CD In

(~w)iuGno . -

CI) - Z~o

I I I ~II) ~e,~C’J In CD H) C’) CI) H) CD ina (VW(IUS))flD -

~..~

a-~

______ ____ ______ _____ iii

I I I I I I I — —ID In CD II) CII CI) H) ~

wuejin I _________WIn CD 10 Cu — C’) II) CD In

CI) (vw)lueiino - ~

0 ow-~

i~~_w I -

Cu ~

III

0

108

-3OC

S

-200 - Active dissolution

Active dissolution

200 -

300 I I......L~L...._ I

0 10 20 30 40 50 60 70 80 90 100

Alloy composition (at °/ Sri)

Fig. 9. Plot of open-circuit potentialsof Ni--Sn alloys in solutionsof 1 N H2S04 with 1 M

Na2SO4 (pH 0.7; T 25 °C).Active dissolution and passivationregionsare shown.

4. Discussion

4.1. Mechanismsof the hydrogenevolution reactionThe experimentalresults indicate that the electroactivityof the various

Ni—Sn alloys does not follow a smoothrelationship with composition,asis the case with metals forming completely miscible solid solutions such asthe Cu—Ni [121 and Ag—Pd [11 systems.This observationseemsquite nor-mal, however,when we considerthat the intermetallic compoundsformedbetweennickel and tin possesschemical and physical propertiesvery differ-ent from those of their parent metals. Thus, it would be logical to considerthese compoundsas individual constituents,which would have their ownelectrochemicalbehaviour; the following table contains the experimentalresults on nickel, tin, Ni3Sn and Ni3Sn2 relevant to the discussionof themechanismsof HER on thesematerials(Table 6).

4.1.1. NickelThe Tafel slope for the HER in acid media is near 120 mV (2RT/F)

in agreementwith previously publishedresults [11, 12]. Thisvalue indicatesthat the rate-determiningstep will be the proton dischargemechanism(Volmer) at nil hydrogencoverage(0 —~- 0) or the electrochemicaldesorptionmechanism(Heyrovsky) at full coverage (0 —~ 1). The fact that in the gasphasehydrogen is adsorbed strongly by nickel would favour the secondmechanism;this was also the conclusionof Mannan [18] in a study of theisotopeseparationfactor on nickel.

109

TABLE 6

Parameter Ni Ni3Sn Ni3Sn2 Sn

Tafel slope 110 125 90 120(mV)

Exchange 2.5 x 10—6 4 x 10—6 4 x i0~ 5 x i0~current density(Acm

2)

Open-circuit 4 -7 —110 +120 —280potential (mV)

Apparentheat 4.2 4.6 5.7 11.9of activation(kcal molt)

Hydrogen In gasphase Probable Probable Niladsorption [17] heat of(kcal mol’) adsorption

is 64

Electroactivity From 0 to From —0.1 to From +0.15 to From —0.25domain (V) cathodic cathodic cathodic to cathodic

Proposed Electro- Electro- Electro- Protonmechanism chemical chemical chemical discharge

desorption desorption desorption (Volmer)

4.1.2. Intermetallic compounds:Ni3Sn,Ni3Sn2,Ni3Sn4

No results on either the HER or the hydrogenadsorption in the gasphaseare available for these compounds.For Ni3Sn, the Tafel slope of125 mV favours either the Volmer step at 0 —~ 0 or the Heyrovsky step at0 —~ 1 as the rate-determiningstep.The ratherhigh exchangecurrentdensity(i0 = 4 X 10-6 A cm

2) combinedwith an apparentheatof activation similarto that of nickel (i.~H*= 4.2 kcal mol’) favours the Heyrovsky step.Forthe samereasons,for Ni

3Sn2 andNi3Sn4the rate-determiningstepis also theelectrochemicaldesorptionof hydrogen.

4.1.3. TinThe Tafel slope on tin is also of the order of 2RT/F (120 mV). How-

ever, it is well known that tin doesnot adsorbhydrogenfrom the gasphase.The low i0 (5 X iO~ A cm

2) makes tin a rather poor HER catalyst, incommon with many “sp” metals such as lead, indium, mercury and cad-mium. The proton dischargeis the mostprobablerate-determiningstep forthe HER on tin.

Owing to the applications of nickel in electrolysersand in alkalinefuel cells, the interest in seeking nickel-basedmaterials of high electro-chemical activity and stability is obvious. Many metals,whenalloyed with

110

nickel form intermetallics, e.g. titanium, zirconium and tin. In the workof Miles [19], intermetallics of titanium and zirconium were found to ex-hibit high hydrogen overpotentials, i.e. between—1.0 and —1.50 V at acurrent densityof 2 mA cm2 in potentiodynamicmeasurements,at a sweeprate of 2 V min~.It is most surprising,therefore,to seea reportby Yeagerand coworkers[201 in which intermetallics of titanium andzirconiumwithnickel are shown to give valuesof overpotentialwhich areonly onetenth ofthose given by Miles, i.e. the graphs of theseworkers [20] indicate that,at overpotentialsof around 100 - 150 mV, a current densityof 2 mA cm2can be obtained on theseintermetallics. In our own work on Ni—Sn inter-metallics the compoundsdo not manifest an especiallyhigh activity nordo they appearto involve the very high overpotentialvalues obtained byMiles for intermetallics of titanium and zirconium. In general (Table 6),intermetallics high in nickel content approach nickel in electroactivitywhereasthosehigh in tin haveactivities that tend to lie closeto that of tin.

The detailed theoreticalaspectsof the composition—activity relation-ships for the HER on these alloys will be examined elsewhere [21]; inparticular, we shall seek to explore the questionas to whether the atomiccomposition of the surface or the bulk electronic configuration of thesealloys and intermetallicsdeterminestheir electrodeactivity [21].

Acknowledgments

This paper is basedon the Ph.D. thesisof Dr. A. Bélanger,under thedirection of Dr. Ashok K. Vijh, submitted to the Institut National de laRechercheScientifique Energie,Universitédu Québec.The financial supportof this work by the National ResearchCouncil of Canadaand the Institutde Recherched’Hydro-Québecis gratefully acknowledged.

References

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