Thin composite anodic layers formed on cadmium in sodium hydroxide-sodium sulphide solutions

Thin Solid Films, 182 (1989) 185 195

185

PREPARATION AND CHARACTERIZATION

THIN COMPOSITE ANODIC LAYERS FORMED ON CADMIUM IN SODIUM HYDROXIDE-SODIUM SULPHIDE SOLUTIONS S. B. SAIDMAN, J. R. VILCHE AND A. J. ARVIA

lnstituto de lnvestigaciones Fisicoquimieas Te&icas y Aplicadas ( INIFTA ), Facultad de Ciencias Exactas, Universidad Naeional de La Plata, Casilla de Correo 16, Sucursal 4, 1900 La Plata (Argentina) (Received June 3, 1988: revised April 26, 1989; accepted June 13, 1989)

The electroformation and electroreduction of passlvating layers on cadmium in alkaline solution containing sodium sulphide is followed through cyclic voltammetry and complex potential controlled perturbations. By changing the Na2S concentration from 0.0001 up to 0.03 M the anodic layer, as followed through voltammetry, varies from a cadmium sulphide-cadmium oxide-hydroxide layer to a thin cadmium sulphide layer depending on where the anodic limit of the voltammetric scan is set. The formation of the cadmium sulphide at different pH values can be interpreted through adsorption and charge transfer processes with a rate-determining step which depends on the Na2S concentration. The corresponding electroreduction appears to be a predominantly ohmic polarization-controlled reaction.

| . INTRODUCTION

The electrochemistry of cadmium in alkaline solution has been extensively studied in relation to the application of this metal in batteries 1-5. Furthermore, the formation and electrochemical properties of cadmium sulphide layers are of interest for comparison with cadmium chalcogenides used as promising semiconductors in photoelectrochemistry6 9. The electrochemical anodization of cadmium in plain alkaline solution involves, at low potentials, the formation of a complex hydrous cadmium hydroxide-cadmium oxide layer, at intermediate potentials, the simultaneous formation of soluble cadmium species and layer growth and, finally, at high potentials, the change of the anodic layer into another layer which exhibits passive film characteristics 1°. The different stages of the entire reaction can be followed through the corresponding electroreduction cycle. The relative contribution of each stage in the overall process depends on both the perturbing potential program applied to the electrode and the hydrodynamic conditions. The appearance of soluble cadmium species together with the hydrous oxide layer in the anodization of cadmium in alkaline solutions offers the possibility of interfering with the overall process through the addition of anions yielding insoluble 0040-6090/89/$3.50

~') ElsevierSequoia/Printedin The Netherlands

186

S. B. S A I D M A N , J. P. V I L C H E , A. J. ARVIA

cadmium salts to produce either a different or a composite thin anodic layer. This layer can be formed through competitive adsorption processes between O H and foreign anions. The anodic formation of cadmium sulphide layers has been studied at pH 14 by using both a rotating ring disc electrode technique ~~ and still electrode cyclic voltammetry ~2, and also at pH 9 by employing potentiostatic, potentiodynamic and galvanostatic measurements 13 15. Nevertheless, the postulated mechanisms for the initial stages of the reaction are neither coincident nor fully substantiated because both the number of voltammetric peaks and the charge involved in the different contributions strongly depend on the experimental conditions including sodium sulphide concentration 16. In addition, one should consider the uncertainty about the type of sulphide ion prevailing in solution as the second dissociation constant of aqueous hydrogen sulphide expressed as pKa. 2 varies within the range 11.96-17.3 at 25 C ~7 2o The present paper is devoted to a study of the voltammetric electroformation and electroreduction processes of thin anodic layers on cadmium resulting in alkaline electrolytes with different concentrations of sodium sulphide and perturbing potential conditions. 2. E X P E R I M E N T A L DETAILS

The experimental set-up was described in previous publications ~°'1~'~2 "Specpure" cadmium discs (Johnson Matthey Chemicals Ltd.; 0.30cm 2 apparent area) supported in Teflon (polytetrafluoroethylene) holders were used as working electrodes in 0.01 M N a O H + y M Na2S(0 ~< 3' <~ 0,03)andin x M N a O H + 0 . 0 1 M NazS (0.01 ~< x <~ 1) solutions at 25 ' C under purified N2 gas saturation conditions. Electrolyte solutions were prepared from analytical grade reagents (Merck) and triply distilled water, and they were carefully deaerated before the addition of NazS. Cadmium electrodes were mechanically polished with 400 and 600 grade emery papers and l Iam and 0.3 gm grit size alumina-acetone suspensions, and then thoroughly rinsed in triply distilled water. Potentials were measured against a properly shielded saturated calomel electrode, but in the text they are referred to the normal hydrogen electrode scale. After a 5 rain cathodization in the potential region of the hydrogen evolution reaction, the following perturbing potentials were applied: (i) single triangular potential sweeps (STPSs) and (ii) repetitive triangular potential sweeps (RTPSs) between present cathodic (E~,c) and anodic (Es.,) switching potentials at a scan rate v. 3. RESULTS The v o l t a m m o g r a m of cadmium in 0.01M N a O H at 0.1Vs ~ (Fig. 1) (reference system), run from E~,c = - 1.76 V to E~,, = 0.24 V, shows two main anodic peaks (I and II), a cathodic peak (III), a nearly limiting current in the - 1.0 V to - 1.5 V range, and a net cathodic current extending from - 1.5 V towards more negative potentials. According to results reported in a previous paper ~°, peaks I and II are attributed to the formation of the anodic layer and soluble cadmium species, peak Ill is principally related to the electroreduction of the anodic layer, the

ANODICLAYERSONCd I N NaOH-Na2S SOLUTION

(II

1:

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187

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-2

V=O.1V/S

/

001 I'4 NaOH

('in)

-4

-1.'6

-1'.2

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Fig. 1. Voltammogram(RTPS) run at v = 0.! V s- xfor Cd/0.01 M NaOH (referencesystem). cathodic limiting current is associated with the electrodeposition of soluble cadmium species, and the cathodic current at potentials more negative than - 1.5 V is related to the hydrogen evolution reaction (HER) on a presumably completely electroreduced cadmium surface. Practically no anodic current is recorded up to c a . - 0 . 7 5 V. The voltammogram at 0.1Vs 1 for 0.01 M N a O H containing NazS run between E~.~ = - 1.76 V and E~.a = - 0.76 V exhibits a new pair of conjugated peaks (A and B) (Fig. 2). The start of peak A coincides with the thermodynamic threshold potential of the Cd/Cds/S 2- electrode 23. The profiles of peaks A and B, as well as their charges, depend strongly on the NazS concentration. On increasing the latter the potential of peak A moves in the negative direction and, for Na2S concentrations greater than 0.003 M, peak A tends to split into a doublet, although the charge of the

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Na2S(0.0005~
+ y M

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latter (ca. 1.0 mC c m 2 ) becomes practically independent of Na2S concentration. This charge is associated with the formation of a very thin solid c a d m i u m sulphide layer, with a thickness in the range of few monolayers, on the electrode 1('. The first positive potential scan shows that the characteristics of peak A depend on the previous cathodization potential, as the H E R changes the local pH and, correspondingly, influences the H 2 S - H S S z equilibria. For constant cathodization conditions tiffs can be seen by running v o l t a m m o g r a m s for E~,~ = - 1.36 V up to E .... = - 0 . 7 6 V at different z and constant Na2S concentration (Fig. 3). Under these conditions the contribution of the H E R can be minimized, and then peak A appears as a single peak in the entire Na2 S concentration range. F o r NazS concentrations greater than 0.001 M the anodic charge density Qp,A recorded up to the potential of peak A is practically constant (Qp,A z 0.5 mC cm - 2) and under this condition the peak parameters fit the following relationships:

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(la) 1

(lb) (lc) (ld)

where Ep. A, J~,.A and CN,,S denote the potential and height of peak A and the concentration of NazS respectively. At lower NazS concentrations ./p,A tends to increase linearly with v 12. The processes taking place in the 1.5 to - 0 . 9 V range, that is the potential range where the electroformation and electroreduction of a thin anodic layer take place, have a considerable influence on the distribution of peaks 1, II and III (see Fig. 1). Furthermore, the potentials of peaks A and B shift more negatively according to

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Fig. 3. Influence of c on the voltammogram (STPS) obtained between k.',.~.: -- 1.36V and E,., : -0.76Vin 0.01 M NaOH +0.001 M Na2S.

ANODIC LAYERS ON C d IN

NaOH-Na2S SOLUTION

189

the Na2S concentration. During the potential cycling the contributions of peaks A and B increase and the overall charge related to peaks I, II and III diminishes. This gradual change in the voltammograms turns out to be clearer at relatively low NazS concentrations (Fig. 4). The condition E,,~ ~< 0.24 V was set to avoid the formation of a thicker less compact layer, as this process is accompanied by an abrupt change in the kinetics of the reaction ~-~5. In contrast, at high positive potentials the formation of either sulphur or polysulphides ~'13 and pits ~3 at the metal surface interferes with the overall process. E/V 1

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The stabilized voltammograms between E~.c = - 1.76V and E s , a = 0.24V exhibit a symmetric peak B at c a . - 1.6 V, which is related to the electroreduction of cadmium sulphide as the main component of the anodic layer (Fig. 5). In addition, the HER is appreciably enhanced as a result of the presence of Na2S in solution. As the concentration of the latter increases, the charges of peaks A and B increase and their contributions become greater than those related to the formation of hydrous cadmium oxide as the main component of the anodic layer, but the anodic current associated with the formation of soluble cadmium species increases. These results suggest a probable dissolution of cadmium assisted by the presence of Na2S. Therefore the chemical precipitation of CdS at the interface means an increase in the electroreduction charge of peak B. At a fixed Es.a the relative yields of Cds and hydrous cadmium oxide depend on both the NazS:NaOH concentration ratio at the interface and the potential sweep rate. For low E,.a values, peak B is preceded by a small limiting current, which can presumably be assigned to the electroreduction process at the anodic layer. Similarly, for constant switching potentials, i.e. E , . c =

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Fig. 5. Influence of v on the stabilized (RIPS) v o l t a m m o g r a m s run between E~,~ = - 1 . 7 6 V and E~., = 0.24 V in 0.01 M N a O H + v M Na2S: (a) y = 0.001; (b) y = 0.01. -1.76V and E~, a = 0.24V, the stabilized voltammetric charge Qp.~ of peak B decreases slightly with increasing v (Fig. 6). At constant pH and Na2S concentration, linear log.jpm vs. log v relationships are obtained with slopes of 0.60 +_0.08. The voltammograms resulting from the stepwise decrease of E~,a (Fig. 7) provide further information. Thus for E~., < - 0 . 8 V the charges of peaks A and B are practically the same and the corresponding peak potentials are nearly equally shifted at both sides from the equilibrium potential of the C d / C d S / S 2 (0.001 M) electrode. However, for Es,, > - 0 . 7 V, the electroreduction charge comprises the sum of that of peak A and those associated with CdS resulting from two reactions, namely that between sulphide ions and cadmium oxide-hydroxide and that between sulphide ion and soluble cadmium species. Similarly, even in the absence of peak Ill,

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ANODIC LAYERS ON Cd

NaOH-NazS

IN

SOLUTION

191

that is when practically no cadmium oxide-hydroxide layer has been formed, soluble cadmium species are already present when Es, a is set to c a . - 0 . 8 V. This suggests that soluble cadmium species can already be formed at the early stages of cadmium anodization in base solution. Otherwise, for Es.a > - 0 . 7 V the ascending branch of peak B becomes practically independent of the electroreduction charge. In general, from these runs one can conclude that the kinetics of the electroreduction process probably depends on the apparent resistivity of the anodic layer, which in turn may involve a heterogeneous structure as already concluded from previously reported scanning electron micrographs 16 ]

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t in 0.01 M N a O H + 0 . 0 0 1 M

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When E~,a is fixed at potentials more positive than that of peak B (Fig. 8) the voltammetric response of cadmium in 0.01 M N a O H + 0 . 0 0 1 M Na2S changes substantially during cycling. Thus, the first positive potential scan exhibits an increase in the height of peak A at c a . - 0.9 V, followed by peaks I and II, and the reverse scan presents peak III and a rather constant cathodic current for E < 1.0 V. The subsequent voltammograms exhibit a gradual increase in the electrode polarization for the reactions related to the cadmium oxide-hydroxide layer, even though the cathodic current in the -1.0 to - 1 . 3 V range remains practically constant. The presence of Na2S produces a remarkable decrease in the current related to the cadmium oxide-hydroxide layer formation compared with the -

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Fig. 8. Voltammograms ( R I P S ) r u n = 0.1Ws-1 between E~.c = - 1.76V and E~, a = 0.24V in 0,01 M NaOH +0.001 M NazS. The first and second scans and those obtained with different potential cycling times are shown.

192

S. B. S A I D M A N ,

J. P. V I L C H E ,

A. J. A R V I A

reference system. There voltammograms confirm that the cadmium sulphide layer initially formed cannot be completely electroreduced under the above-mentioned conditions. The heterogeneous structure of the initial cadmium sulphide layer probably favours the formation of the cadmium oxide-hydroxide layer and soluble cadmium species at more positive potentials. During cycling the precipitation of soluble cadmium species as CdS contributes to the CdS layer growth, but simultaneously the rate of the anodic layer growth decreases. Accordingly, the increasing ohmic polarization at the anodic layer begins to distort the symmetry of the electroreduction peak. It is interesting to note that no pitting of the base metal can be detected and the passive layer can be removed only through a prolonged cathodization at E < - 1.8 V. The relative change in the magnitudes of the voltammetric peaks A and B and | II and Ill can also be observed by working at a fixed Na2S concentration and different pH values (Fig. 9). These results give further support to the conclusions derived from Fig. 5 in terms of a competition between both cadmium sulphide and cadmium hydroxide formation. 3

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F i g . 9. S t a b i l i z e d , , o l t a m m o g r a m s inxM

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obtained

. ,x=0.01;

1

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4. DISCUSSION Voltammetric data show that the formation of cadmium sulphide in alkaline solutions appears to be a complex reaction as deduced from the fact that, when the potential scan starts from a region where a net evolution of hydrogen takes place, peak A then consists of two components An and A2. Peak A1 can be clearly observed even for Na2S concentrations lower than 0.001 M, whereas peak A2 increases according to Na2S concentration (Fig. 2). Peak A 2 involves a charge considerably

ANODIC LAYERS ON C d IN

NaOH-Na2S

SOLUTION

193

lower than that of peak A1, the overall charge being of the order of only one or two monolayers. Similarly, the increase in pH enhances the contribution of peak A 2. From the thermodynamic standpoint the formation of the first surface layer assigned to CdS can be observed when the applied potential scan started from a high negative potential reaches a value close to the calculated standard potential for the reaction CdS + H 2 0 + 2e- ~ Cd + H S - + OH -

E2 ° = - 1.22 + 0.08 V

(2)

The value of E2 ° is estimated from the standard equilibrium potential of the Cd/Cd 2+ redox couple (E ° = -0.402 V) 23, the equilibrium constant for the CdS dissociation reaction (Ko = 3.6 x 10-z9 M)2O, the dissociation constants of HzS in water ( K a , 2 ~ 10 1 2 - 1 0 - ~ M) 17-2° and K,o. According to reaction (2) the formation of CdS occurs at a potential lower than that related to the formation of the cadmium hydroxide-cadmium oxide layer 16'23. The standard potentials of the corresponding reactions are z4 Cd(OH)z+2e

~-Cd+2OH-

CdO+H20+2e-.-~-~Cd+2OH-

E3°=-0.824V

(3)

E4 ° = - 0 . 7 8 3 V

(4)

The complexity of peak A suggests that already at potentials lower than - 0.8 V cadmium sulphide can be produced either through reaction (2) (peak A1) or through electrochemical dissolution and chemical precipitation (peak A2). The charge and location of peak A 2 depend on the value of the potential holding at the HER potential range (local pH) and the NazS concentration. However, the formation of CdS can also occur through the following reaction: Cd(OH)2(s ) + S 2 ~ CdS(s) + 2OH -

(5)

In this case the solution pH can determine the amount of Cd(OH) 2 which can be formed under preset voltammetric conditions, whereas the NazS concentration can determine the extent of reaction (S). Probably, the remaining Cd(OH)2 and soluble cadmium species are responsible for the cathodic current plateau and hump of peak B which becomes more marked as the Na2S concentration decreases. From the kinetic standpoint the initial stages of the anodic reaction should involve two competitive adsorption reactions for the O H - and sulphide ions in solution on cadmium yielding ( C d O H ) a a and (CdS 2-)ad species respectively 11 15. Subsequently, (CdS 2 -)ad yields (CdS)ad at the monolayer level through an electron transfer step at the electrode surface. Thus, at low Na2S concentrations one can explain in this way the low voltammetric charge of peak A (Fig. 3). However, at low Na2S concentrations the kinetics of the reaction becomes diffusion controlled since then Jp.A VS. V 1/2 plots are approached. Unfortunately, these aspects of the reaction can only be discussed qualitatively because the actual distribution of sulphurcontaining ionic species in solution cannot be established as the pH value of sulphide-containing aqueous solution is not known with sufficient accuracy 1~. Some unusually large values for pKd. a have been reported from UV absorption spectroscopy results is and from glass pH electrode measurements in concentrated alkaline solutions ~9. In contrast, for Na2S concentrations greater than 0.001 M, the

194

S. B. SAIDMAN, J. P. VILCHE, A. J. ARVIA

cadmium sulphide electroformation charge abruptly reaches a constant coverage of the order of a few monolayers. In this case the initiation of cadmium sulphide formation occurs just at the potential of the reversible CdS electrode (reaction (2)) 12. Otherwise, the electroreduction of thick CdS layers formed by anodization at relatively large positive potentials implies either a transport rate control or an ohmic control as the slope of the logjp.R v s . log l~curve is close to 0.5 (ref. 25). Otherwise, in the presence of a high sulphide ion concentration the classical formation of cadmium sulphide also takes place. This can explain the comparatively large value of Qp,~3with respect to the anodic charge (Fig. 5) and its decrease as v increases (Fig. 6), as well as the change of the voltammetric profiles during potential cycling (Fig. 4). There effects become clearer on decreasing the NazS concentration and on increasing the pH. The latter favours the formation of CdOH-- species at the initial stage of the anodic reaction (Fig. 9). Consequently, the voltammetric charges related to cadmium oxide-cadmium hydroxide in the first potential cycle become considerably greater than those of peaks A and B. According to results shown in Fig. 9, the change in Qp,r3with pH suggests that the formation of cadmium sulphide takes place mainly through reaction (5), and to a lesser extent through the precipitation of soluble cadmium species. A similar explanation can be advanced for the growth of the cadmium oxide-cadmium hydroxide layer at high positive potentials. 5. CONCLUSIONS

The voltammetric anodization of cadmium in alkaline solutions containing sulphide ions produces a thin complex passivating layer, the structure of the latter varying from that of CdS to that of CdS + Cd(OH)2 depending on the potential limits and solution composition. At low potentials, the prepassivating layer consists of CdS (peak A in the voltammogram). The formation of this layer is a typical complex electroadsorption process. At high potentials, the growth of a thin cadmium sulphide--cadmium oxide-hydroxide layer takes place. The growth process can be interpreted through a solid state reaction and to a lesser extent through a dissolution-precipitation process, the latter being assisted by OH ions. The kinetics of the passive layer electroreduction appears to be predominantly ohmic polarization controlled, ACKNOWLEDGMENTS

This work is financially supported by the Consejo Nacional de Investigaciones Cientificas y Tacnicas and the Comisi6n de Investigaciones Cientificas de la Provincia de Buenos Aires. REFERENCES P. C. Milner and V. B. Thomas, in C. W. Tobias (ed.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 5, lnterscience, New York, 1967, pp. 1 86. 2 S.U. Falk and A. J. Salkind, Alkaline Storage Batteries, Wiley, New York, 1969. 3 R.J. Latham and N. A. Hampson, in A. J. Bard (ed.), Eno'clopedia o[the Eh,ctrochemistry ql'the Elements, Vol. l, Dekker, New York, 1973, pp. 155- 233. I

ANODIC LAYERS ON C d IN N a O H - N a 2 S

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

SOLUTION

195

R. D. Armstrong, K. Edmondson and G. D. West, in Specialist Periodical Reports on Electrochemistry, Vol. 4, The Chemical Society, London, 1974, pp. 18-32. R. Barnard, J. Appl. Electrochem., 11 (1981) 217. G.P. Power, D. R. Peggs and A. J. Parker, Electrochim. Acta, 26 (1981) 68 I. M. Guti&rez and A. Henglein, Ber. Bunsenges, Phys. Chem., 87 (I 983) 474. P. Josseaux, A. Kirsch-De Mesmaeker, A. Roche, M. Romand and H. Montes, J. Electrochem. Sic., 132 (1985) 684. M. Evenor, D. Huppert and S. Gottesfeld, J. Electrochem. Soc., 133 (1986) 296. S.B. Saidman, J. R. Vilche and A. J. Arvia, Electrochim. Acta, 32 (1987) 395. B. Miller, S. Menczes and A. Heller, J. Electroanal. Chem., 94 (I 978) 85. V.I. Birss and L. E. Kee, J. Electrochem. Soc., 133 (1986) 2097. L.M. Peter, Electrochim. Acta, 23 (1978) 165. L.S.R. Yeh, P. G. Hudson and A. Damuanovic, J. Appl. Electrochem., 12 (1982) 153. A. Damjanovic, L. S. R. Yeh and P. G. Hudson, J. ,4ppl. Electrochem., 12 (1982) 343. S.B. Saidman, J. R. Vilche and A. J. Arvia, Electrochim. ,4cta, 32 (1987) 1153. W. Giggenbach, Inorg. Chem., 10 (1971) 1333. S. Ramachandra Rao and L. G. Hepler, Hydrometallurgy, 2 (1977) 293. S. Licht and J. Manassen, J. Electrochem. Soc., 134 (1987) 918. R.C. Weast (ed.), Handbook o f Chemistry and Physics, CRC Press, Boca Raton, FL, 60th edn., (1980) p. B-220. S.B. Saidman, M. Lopez Teijelo, J. R. Vilche and A. J. Arvia, Proc. 4th Simp. Bras. Electroquim. Electroanal., Instituto de Quimica-Universidade de Sao Paulo, Sao Paulo, 1984, pp. 87-98. M.L•pezTeije•••J.R.Vi•cheandA.J.Arvia•J.E•ectr•ana•.Chem.•131(•982)33•;•62(•984)2•7. Y. Okinaka, Cadmium. In A. J. Bard, R. Parsons and J. Jordan (eds.), Standard Potentials in Aqueous Solution, Dekker, New York, 1985, Chap. 10.II, pp. 257-265. S.I. Zhdanov, Sulfur. In A. J. Bard, R. Parsons and J. Jordan (eds.), Standard Potentials in Aqueous Solution, Dekker, New York, 1985, Chap. 6.I, pp. 93-110. A.J. Calandra, N. R. de Tacconi, R. Pereiro and A. J. Arvia, Electrochim. Acta, 19 (1974) 901.