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The anodic behaviour of cobalt in alkaline solutions
Electrochimica
Acta.
1969.
Vol. 14. PP. 981 to 989.
Pergamon
Press.
Printed
in Northern
Ireland
THE ANODIC BEHAVIOUR OF COBALT IN ALKALINE SOLUTIONS* Department
R. D. COWLING-~ and A. C. RIDDIPORD of Chemistry, The University of Victoria, Victoria, British Columbia, Canada
Abstract-The anodic polarization of cobalt in sodium hydroxide solutions has been studied under potentiostatic and galvanostatic conditions. Only two stages in the oxidation were found. It is suggested that these correspond to the transitions Co/Co(OH), and cO(OH),/Co(OH),. The nucleation and growth of the passivating layer of cobaltous hydroxide is discussed by analysis of the current/time curves at various constant potentials. R&sum&-Etude, sous de-s conditions potentiostatique et galvanostatique, de la polarisation anodique du Co en solution sodique. Deux &apes seulement de l’oxydation ont Bte trouv&s. Ceci conduit a penser qu’elles correspondent aux transitions Co/Co(OH), et Co(OI&/Co(OH),. Discussion de la nucleation et de la croissance de la couche passive d’hydroxyde cobalteux par, analyse des courbes courant-temps 11dit%ents potentiels constants. Znaammenfamnng-Kobalt wurde in Natriumhydroxydlosungen anodisch unter galvanostatischen turd potentiostatischen Bedingungen polarisiert. Es wurden nur xwei Oxydationsschritte gefunden. Diese werden den Ubergangen Co/Co(OH), und Co(OI-&,/cO(OH), zugeschrieben. Durch Analyse der bei verschiedenen konstanten Potentialen aufgenommenen Strom/Zeitkurven werden die Keimbildung und das Wachstum der passivierenden Kobalt(II) hydroxydschicht diskutiert. INTRODUCTION
COMPARJZD to the large amount of work on nickel and iron electrodes, very few studies been made on cobalt. The anodic formation of particular oxides on the metal was first reported by Grube and Feucht,r and later by Bessor? and by El Wakkad and HicklingS The results of these workers, obtained with cobalt electrodes plated on to platinum supports, have been collated by Deltombe and Pourbaix,4 who established an equilibrium potential/pH diagram for the metal at 25°C. Later work includes that of Schwabe and G&r6 The structure, phase-transitions and electrochemistry of the Co(OH), electrode have been studied by Wynne-Jones and co-workers.7 A comprehensive survey on cobalt is availab1e.s Previous investigations of the cobalt electrode were mostly concerned with anodic charging curves at constant current densities, the existence of several oxides and hydroxides being deduced from inflexions in these curves. The following forms are generally considered possible :4 COO, CO(OH,), CosO,, Co(OH), or CoOOH and Coos. It appears to be difficult to distinguish between anodically formed Co0 and Co(OH),; thus, Bessons claimed to have found a potential corresponding to CO/COO, while El Wakkad and Hi&h@ reported a value close to the theoretical equilibrium potential for Co/Co(OH),. The highest oxide was observed in each case,1-3 but was generally found to be unstable. The aim of the present investigation was to study the anodic behaviour of spectroscopically pure cobalt electrodes in alkaline solutions, in the absence of oxygen. The current/potential curve has been examined potentiostatically, as this does not appear have
l Manuscript received 11 July 1968. t Present address: Department of Chemistry, Laurentian University, Sudbury, Ontario, Canada.
981
982
R. D.
COWLING
and A. C. RIDDIFORD
to have been reported previously for alkaline solutions. Galvanostatic charging curves have been obtained at fairly low current densities, and current/time curves have been studied at several constant potentials in O-1 N and 1-ON NaOH. EXPERIMENTAL
TECHNIQUE
Cobalt electrodes These were prepared from spectroscopically pure wire, supplied by Johnson, Matthey and Co., Ltd. Electrodes 0.5 mm diameter and 2 cm long were mounted in polystyrene tubing. Each electrode was cleaned anodically in a mixture of 1 part of ethyl alcohol to 1 part of concentrated hydrochloric acid, washed with doubly distilled, de-ionized water and transferred quickly to the cell. This gave very reproducible results. Solutions Solutions were prepared from AnalaR reagents, using doubly distilled water, and were de-aerated with “white-spot” nitrogen. It was found that the results were not affected when the nitrogen was scrubbed by passage through a heated column of copper on kieselguhr, then through a solution of vanadous sulphate in sulphuric acid and finally through a potassium hydroxide solution. Measurements A Wenking potentiostat was used for determining current/potential curves. For the galvanostatic experiments, voltages were measured with a Solar&on 1420 or 1440 digital voltmeter, connected to a print-out unit. A quinhydrone reference electrode was used with suitable precautions to avoid contamination of, and by, the alkaline electrolyte. To assist uniform current distribution at the working electrode, a large cylindrical platinum counter-electrode was used. All potentials in this paper are referred to the normal hydrogen electrode. RESULTS
Current/potential curves The steady open-circuit potential of cobalt in 1 N NaOH was about -0.63 V(she). A typical anodic polarization curve from this rest potential, at room temperature, is shown in Fig. 1. In calculating cd, the apparent surface area of the electrode was used. The current/potential curve was obtained by increasing the potential in increments of 5 mV in the active regions and 25 mV in the passive region, and recording the steady current at each potential. The current increased from the rest potential up to -0595 V(she), at which point it fell to zero (strictly, to less than lo-’ A/cm2) over a period of 3-4 h. Further increase of overpotential caused a small rise in the current which, again, subsequently fell to zero. Between -0.595 and $0.200 V(she), the electrode was clearly passivated. A second active/passive region occurred between +0*200 and +0*265 V. At more positive potentials, oxygen evolution began. In O-1N NaOH, the electrode became passivated at -0.535 V(she), the transpassive region beginning at +0*250 V. Potential/time curves Constant-current charging curves were obtained on electrodes in NaOH solutions ranging from O-1 to 2 N. The cd varied between 50 and 500 pA/cm2. The curves
983
Anodic behaviour of cobalt in alkaline solutions
I
Lo-_b
-650
-600
-550
mV (she)
E, FIG. 1. Potentiostatic
anodic polarization curve for cobalt in 1 N NaOH, with nitrogen.
saturated
-600 r
I
9
30
I
4L
1
I
50
60
r, min
a
FIG. 2. Galvanostatic
anodic polarization curves for cobalt in 1 N NaOH, saturated with nitrogen. 1,400; 2, 310; 3,250; 4,200; 5, 150,uA/cma
obtained for a 1 N solution are shown in Fig. 2. Except at the higher cds, the plateaux which preceded passivation occurred at -0.590 to -0.600 V(she). A second process, marked by a considerable overshoot of potential, followed by a slight pause, was found in the region where the potential changed rapidly; this appears as a change of slope between about +a300 and +0*450 V, depending on the cd.
R. D. C~WLINOand A. C.
984
RIDDIFORD
In O-1 N NaOH, at cds less than 80 PA/ cm2, the initial plateau occurred at about -0.530 V(she), the second process being shown by a marked inflexion in each curve at +0.300 to +OMO V. Current/time curves The current/time relationship during the initial Glm formation was studied at various constant potentials in solutions of 1.0 N and 0.1 N NaOH. The curves obtained for a 1 N solution are shown in Fig. 3. As the overpotential was increased,
3oc
Q
2oc
i
< IOC
C
1
I
I
2
4
6 t,
0
IO
12
min
Fro. 3. Potentiostatic i/t curves for cobalt in 1 N NaOH, saturated with nitrogen. 1, -535; 2, -545; 3, -555; 4, -565 mV (she).
the maximum current also increased, the peak shifted to shorter times and both the rise and fall of the current were more rapid. Attempts to obtain potentiostatic current/ time curves for the second oxidation process were not successful. DISCUSSION
The potential at which passivation first becomes possible in both 1 N and O-1 N NaOH was found to be very reproducible. The Flade potential for cobalt in these solutions is therefore taken as -0.595 and -0.535 V(she), respectively. In neither solution was a reactivation potential observed. The equilibrium potential/pH diagram4 shows that the only oxides (or hydroxides) which can be formed from the metal at these potentials are Co0 and Co(OH),, the corresponding equilibrium potentials at pH 14 being -0.662 and -0.733 V(she), respectively. Thus, while many workers report passivation potentials close to reversible values for particular metal/metal-oxide equilibria,s*10 in the present case there is a large difference between the passivation potential and the nearest equilibrium potential. The change of 60 mV for a pH change of 1 suggests that passivation is due to a simple oxidation reaction in which the stoichiometric coefficients of hydroxyl ions and electrons are equal.
Anodic behaviour of cobalt in alkaline solutions
985
The second stage of the anodic oxidation began at +0*200 V(she) in 1 N NaOH and at +0*250 V in O-1 N NaOH. In view of the fact that only two steps were found in the oxidation of the metal, it seems unlikely that the unstable oxide COO, was formed under the conditions of these experiments. This, at first, seems surprising, as other authors1-3 have reported the existence of this oxide. The probable explanation is that while Coos is stable on a substrate of inert platinum (or platinum oxide), it is not likely to be formed in contact with cobalt, which would immediately reduce it to a lower oxidation state. It can be assumed, then, that the second stage in the potentiostatic oxidation of cobalt is due to the transition Co0 or Co(OH), to Co(OH), or CoOOH. The galvanostatic potential/time curves, such as those in Fig. 2, also indicate the occurrence of only two stages in the oxidation. The potential of the first plateau in 1 N and O-1N NaOH corresponds closely to the potential of the first process found in the potentiostatic examination. For the second stage, however, the inflexions in the charging curves occur at somewhat more positive potentials than the corresponding process in the current/potential curves. This is due, in part, to the overshoot of the potential beyond the equilibrium value as the thin film on the electrode surface becomes oxidized,ll and in part to the effect of the cd through the film. The higher the cd, the further will the potential be displaced from the reversible value (see Fig. 2). In the charging of the cobalt electrodes, the overpotential associated with the second state was undoubtedly greater in 1 N NaOH than in the 0.1 N solution, since in the latter case the cds were considerably lower. Wynne-Jones et ~1’ examined plated Co(OH), electrodes, and found a potential of 0.25 V(she) for the transition Co(OH),+ CoOOH, in 1 N NaOH. In O-1N NaOH, one would therefore expect a plateau at about $0.31 V, in reasonable agreement with the present observations. It is suggested that the second stage in the oxidation therefore corresponds to Co(OH),/CoOOH, the initial passivation reaction being due to the formation of Co(OH), rather than COO. This implies an overpotential of about 130 mV for each stage in the potentiostatic oxidation of cobalt, a magnitude which is often associated with nucleation overpotentials. When the electrode is forced to sustain a constant current, the overpotentials may increase. The general form of the current/time curves at constant potentials, Fig. 3, indicates that the rate of formation of the passivating film is controlled by the rate of growth of the nuclei, rather than by the rate of dissolution of the metal.12 Figure 4 shows that during the initial stages of growth in 1 N NaOH, the current at lower overpotentials (with respect to the equilibrium potential for Co/Co(OH)d is proportional to P. Using a simple model for the geometry of growing nuclei,ll this implies that threedimensional (hemispherical) growth occurs, with progressive nucleation. The particular case of the initiation, and growth, of three-dimensional nuclei upon a bare solid metal surface has not been fully treated in the literature with respect to the calculation of the exact amount of overlap of the growing centres. The ensuing discussion, therefore, follows the lines of Fleischmann and Thirsk’s13 treatment of the growth of lead dioxide nuclei in a lead sulphate layer. As such, it must be regarded as a first approximation. It is assumed that the growing nuclei effectively reach a limiting size at a fixed time after formation, this time being equal to the maximum in the i/t curve. The physical limit to the growth of nuclei would, in the present case, be the grain boundaries of the
R. D.
CCWLING
and A. C.
RIDDIFORD
120-
IOO-
60a 3. 60.<
L
I
I I.0
0
2.0
min3
131
FIG. 4. Dependence
of current on (time)” during the initial stages of film formation in 1 N NaOH. 1, -555; 2, -575 mV (she).
metal substrate. If the nucleation law can be represented as diV/dt = AN, exp (--At), and the growth law by i = Bu2, where N is the number of nuclei at time t, N,, is the maximum possible number of sites, A is the nucleation rate constant, B is the growth rate constant and u is the age of a centre, the@ rate =
AN$u2 exp [-A(t
where uz is the time of the rate maximum. rate -h
- u)] . du,
For u, > t,
BN, . At3
3
’
and for uz < t, rate = BN, exp [-A(t
- uJ] ula - 2
+ $ - $
exp (-AU,) 1.
Thus, during the initial stages of growth, the current should be proportional to t3, as observed. For uz < t, a plot of log (rate) us t should be linear, the gradient giving the nucleation rate constant A. Figure 5 shows the experimental log i us t plots for cobalt at various potentials in 1 N NaOH. The variation of A with potential is shown in Fig. 6(A). As A is very small, the exponential in the expression Ul2 -
2 +5 - f
exp (-Au,)]
was expanded to obtain log (rate) = log BN, + log F
+ Au, - At,
Anodic behaviour of cobalt in alkaline solutions
987
P20 -
* 2.00 x .= I.80 B I.60 -
I.401 0 FIG.
5.
I
I
I
2
4
6 1,
Dependence of log i on t dutiNt;k 1, -535;
2, -545;
3, -555;
I
I
8
IO
I
12
J
14
min later stages of film formation in 1 N 4; -565;
5, -575
mV (she).
and the plots of log
i us t shown in Fig. 5 were extrapolated back to t = 0, log BN, being calculated from the intercept. Figure 6(B) shows the dependence of log BN,, upon potential. For the assumed model of growing hemispheres, it can be shownu that BN,, is proportional to k3, where k is the rate constant for the growth process in mole/s/cma. Hence, if 7 is the overpotential for the growth process, 1 . dq dq d log BN, =5%$ Since the slope of Fig. 6(B) is dg/d log BN, = 28.2 mV, then dq/d log k = 84.6 mV. If ,5 is the transfer coefficient for the anodic reaction, and z the number of electrons involved in the rate-determining step, then dg/d log k =
2.303 RT O-059 /3zF = 7’
In the present case, pz = O-69. Therefore, if z = 1, /? = O-69, or if z = 2, p = O-35. If the rate-determining step involves a two-electron transfer, a possible mechanism would be
Co + 2 OH- = Co(OH), + 2 e-, although
an effectively termolecular reaction such as this seems unlikely. Since the basis of the foregoing discussion is that Glm growth, rather than metal dissolution, is rate-determining, the reaction Co + Co s+ + 2 e- is not considered as the slow step in the process. A possible mechanism for the growth of the passivating Urn on cobalt in 1 N NaOH, is Co + OH- = Co(OH),, + e-,
Co(OH),,,
+ OH- = Co(OH), + e-,
R. D. Coma
988
and A. C.
RDDIFORD
o.kl-1--10 E,
mV (she)
-7.oor N E -670Y e 3! ii -640_ -0 5
-6.10-
4 -560-
E, Fro.
6. Dependence
mV (she)
of (A) nucleation rate constant, and (B) log of composite growth rate constant, on potential, in 1 N NaOH.
where the first step, the formation of adsorbed OH at cobalt sites, is rate-determining, with /I = O-69. A similar analysis of the current/time curves in O-1 N NaOH was not successful. Further studies are being made on this. SUMMARY
The results of the present studies have shown that the oxidation of a clean cobalt surface takes place in two stages. This is supported by both potentiostatic and galvanostatic experiments, and in no case was a significant third process found between the rest potential and the point of oxygen evolution. The two oxidation stages are attributed to the transitions Co + Co(OH), and Co(OH), + CoOOH. The current/time curves show that in 1 N NaOH the passivating layer grows in three dimensions (on the simple model, hemispherical growth being assumed), and that progressivenucleation occurs. From the analysis of these curves, a simple mechanism for growth is proposed.
Anodic behaviour of cobalt in alkaline solutions
989
Acknowledgment-R. D. C. thanks Energy Conversion Ltd. for the provision of a Research Grant, during the tenure of which the present work was undertaken. REFERENCES 1. 2. 3. 4. 5. 6. I: 9. 10. 11. 12. 13.
G. GRUBEand 0. FEUCHT, Z. Elektrochem. 28,568 (1922). J. BESSON,C.r. hebd. S&w. Acad. Sci., Paris 223, 28 (1946). S. E. S. EL WAKKADand A. HICKLING, Trans. Furuduy Sot. 46,820 (1950). E. DELT~MBBand M. POURB~, Proc. 6th Meeting CITCE, p. 153. Butterworths, London (1955). K. Scnw~se, Proc. 9th Meeting CITCE, p. 339. Butterworths, London (1957). H. G&m, Electrochim. Acta 11,827 (1966). P. BENSON,G. W. D. Baras and W. F. K. WYNNE-JONES,Efectrochim. Acta 9,275 (1964). Battelle Memorial Institute (ed. Centre dInformation du Cobalt), Cobalt Monograph. Brussels (1960). T. S. DE GROMOBOY and L. L. SHREIR,Electrochim. Actu 11,895 (1966). T. P. HOAR, in Modern Aspects of Electrochemistry No. 2, ed. J. O’M. B~CKRIS. Butterworths, London (1959). M. FLEISCHMANNand H. R. THIRSK, in Advances in Electrochemistry and Electrochemical Engineering, ed. P. DELAHAYand C. W. TOBIAS,Vol. 3. Interscience, New York (1963). D. A. VER~~L~EA, in Advances in Electrochemistry and Electrochemical Engineering, ed. P. DELAHAYand C. W. TOBIAS,Vol. 3. Interscience, New York (1963). M. FLEISCHMANN and H. R. Tr-nns~, Trans. Furuday Sot. 51, 71 (1955).
Acta.
1969.
Vol. 14. PP. 981 to 989.
Pergamon
Press.
Printed
in Northern
Ireland
THE ANODIC BEHAVIOUR OF COBALT IN ALKALINE SOLUTIONS* Department
R. D. COWLING-~ and A. C. RIDDIPORD of Chemistry, The University of Victoria, Victoria, British Columbia, Canada
Abstract-The anodic polarization of cobalt in sodium hydroxide solutions has been studied under potentiostatic and galvanostatic conditions. Only two stages in the oxidation were found. It is suggested that these correspond to the transitions Co/Co(OH), and cO(OH),/Co(OH),. The nucleation and growth of the passivating layer of cobaltous hydroxide is discussed by analysis of the current/time curves at various constant potentials. R&sum&-Etude, sous de-s conditions potentiostatique et galvanostatique, de la polarisation anodique du Co en solution sodique. Deux &apes seulement de l’oxydation ont Bte trouv&s. Ceci conduit a penser qu’elles correspondent aux transitions Co/Co(OH), et Co(OI&/Co(OH),. Discussion de la nucleation et de la croissance de la couche passive d’hydroxyde cobalteux par, analyse des courbes courant-temps 11dit%ents potentiels constants. Znaammenfamnng-Kobalt wurde in Natriumhydroxydlosungen anodisch unter galvanostatischen turd potentiostatischen Bedingungen polarisiert. Es wurden nur xwei Oxydationsschritte gefunden. Diese werden den Ubergangen Co/Co(OH), und Co(OI-&,/cO(OH), zugeschrieben. Durch Analyse der bei verschiedenen konstanten Potentialen aufgenommenen Strom/Zeitkurven werden die Keimbildung und das Wachstum der passivierenden Kobalt(II) hydroxydschicht diskutiert. INTRODUCTION
COMPARJZD to the large amount of work on nickel and iron electrodes, very few studies been made on cobalt. The anodic formation of particular oxides on the metal was first reported by Grube and Feucht,r and later by Bessor? and by El Wakkad and HicklingS The results of these workers, obtained with cobalt electrodes plated on to platinum supports, have been collated by Deltombe and Pourbaix,4 who established an equilibrium potential/pH diagram for the metal at 25°C. Later work includes that of Schwabe and G&r6 The structure, phase-transitions and electrochemistry of the Co(OH), electrode have been studied by Wynne-Jones and co-workers.7 A comprehensive survey on cobalt is availab1e.s Previous investigations of the cobalt electrode were mostly concerned with anodic charging curves at constant current densities, the existence of several oxides and hydroxides being deduced from inflexions in these curves. The following forms are generally considered possible :4 COO, CO(OH,), CosO,, Co(OH), or CoOOH and Coos. It appears to be difficult to distinguish between anodically formed Co0 and Co(OH),; thus, Bessons claimed to have found a potential corresponding to CO/COO, while El Wakkad and Hi&h@ reported a value close to the theoretical equilibrium potential for Co/Co(OH),. The highest oxide was observed in each case,1-3 but was generally found to be unstable. The aim of the present investigation was to study the anodic behaviour of spectroscopically pure cobalt electrodes in alkaline solutions, in the absence of oxygen. The current/potential curve has been examined potentiostatically, as this does not appear have
l Manuscript received 11 July 1968. t Present address: Department of Chemistry, Laurentian University, Sudbury, Ontario, Canada.
981
982
R. D.
COWLING
and A. C. RIDDIFORD
to have been reported previously for alkaline solutions. Galvanostatic charging curves have been obtained at fairly low current densities, and current/time curves have been studied at several constant potentials in O-1 N and 1-ON NaOH. EXPERIMENTAL
TECHNIQUE
Cobalt electrodes These were prepared from spectroscopically pure wire, supplied by Johnson, Matthey and Co., Ltd. Electrodes 0.5 mm diameter and 2 cm long were mounted in polystyrene tubing. Each electrode was cleaned anodically in a mixture of 1 part of ethyl alcohol to 1 part of concentrated hydrochloric acid, washed with doubly distilled, de-ionized water and transferred quickly to the cell. This gave very reproducible results. Solutions Solutions were prepared from AnalaR reagents, using doubly distilled water, and were de-aerated with “white-spot” nitrogen. It was found that the results were not affected when the nitrogen was scrubbed by passage through a heated column of copper on kieselguhr, then through a solution of vanadous sulphate in sulphuric acid and finally through a potassium hydroxide solution. Measurements A Wenking potentiostat was used for determining current/potential curves. For the galvanostatic experiments, voltages were measured with a Solar&on 1420 or 1440 digital voltmeter, connected to a print-out unit. A quinhydrone reference electrode was used with suitable precautions to avoid contamination of, and by, the alkaline electrolyte. To assist uniform current distribution at the working electrode, a large cylindrical platinum counter-electrode was used. All potentials in this paper are referred to the normal hydrogen electrode. RESULTS
Current/potential curves The steady open-circuit potential of cobalt in 1 N NaOH was about -0.63 V(she). A typical anodic polarization curve from this rest potential, at room temperature, is shown in Fig. 1. In calculating cd, the apparent surface area of the electrode was used. The current/potential curve was obtained by increasing the potential in increments of 5 mV in the active regions and 25 mV in the passive region, and recording the steady current at each potential. The current increased from the rest potential up to -0595 V(she), at which point it fell to zero (strictly, to less than lo-’ A/cm2) over a period of 3-4 h. Further increase of overpotential caused a small rise in the current which, again, subsequently fell to zero. Between -0.595 and $0.200 V(she), the electrode was clearly passivated. A second active/passive region occurred between +0*200 and +0*265 V. At more positive potentials, oxygen evolution began. In O-1N NaOH, the electrode became passivated at -0.535 V(she), the transpassive region beginning at +0*250 V. Potential/time curves Constant-current charging curves were obtained on electrodes in NaOH solutions ranging from O-1 to 2 N. The cd varied between 50 and 500 pA/cm2. The curves
983
Anodic behaviour of cobalt in alkaline solutions
I
Lo-_b
-650
-600
-550
mV (she)
E, FIG. 1. Potentiostatic
anodic polarization curve for cobalt in 1 N NaOH, with nitrogen.
saturated
-600 r
I
9
30
I
4L
1
I
50
60
r, min
a
FIG. 2. Galvanostatic
anodic polarization curves for cobalt in 1 N NaOH, saturated with nitrogen. 1,400; 2, 310; 3,250; 4,200; 5, 150,uA/cma
obtained for a 1 N solution are shown in Fig. 2. Except at the higher cds, the plateaux which preceded passivation occurred at -0.590 to -0.600 V(she). A second process, marked by a considerable overshoot of potential, followed by a slight pause, was found in the region where the potential changed rapidly; this appears as a change of slope between about +a300 and +0*450 V, depending on the cd.
R. D. C~WLINOand A. C.
984
RIDDIFORD
In O-1 N NaOH, at cds less than 80 PA/ cm2, the initial plateau occurred at about -0.530 V(she), the second process being shown by a marked inflexion in each curve at +0.300 to +OMO V. Current/time curves The current/time relationship during the initial Glm formation was studied at various constant potentials in solutions of 1.0 N and 0.1 N NaOH. The curves obtained for a 1 N solution are shown in Fig. 3. As the overpotential was increased,
3oc
Q
2oc
i
< IOC
C
1
I
I
2
4
6 t,
0
IO
12
min
Fro. 3. Potentiostatic i/t curves for cobalt in 1 N NaOH, saturated with nitrogen. 1, -535; 2, -545; 3, -555; 4, -565 mV (she).
the maximum current also increased, the peak shifted to shorter times and both the rise and fall of the current were more rapid. Attempts to obtain potentiostatic current/ time curves for the second oxidation process were not successful. DISCUSSION
The potential at which passivation first becomes possible in both 1 N and O-1 N NaOH was found to be very reproducible. The Flade potential for cobalt in these solutions is therefore taken as -0.595 and -0.535 V(she), respectively. In neither solution was a reactivation potential observed. The equilibrium potential/pH diagram4 shows that the only oxides (or hydroxides) which can be formed from the metal at these potentials are Co0 and Co(OH),, the corresponding equilibrium potentials at pH 14 being -0.662 and -0.733 V(she), respectively. Thus, while many workers report passivation potentials close to reversible values for particular metal/metal-oxide equilibria,s*10 in the present case there is a large difference between the passivation potential and the nearest equilibrium potential. The change of 60 mV for a pH change of 1 suggests that passivation is due to a simple oxidation reaction in which the stoichiometric coefficients of hydroxyl ions and electrons are equal.
Anodic behaviour of cobalt in alkaline solutions
985
The second stage of the anodic oxidation began at +0*200 V(she) in 1 N NaOH and at +0*250 V in O-1 N NaOH. In view of the fact that only two steps were found in the oxidation of the metal, it seems unlikely that the unstable oxide COO, was formed under the conditions of these experiments. This, at first, seems surprising, as other authors1-3 have reported the existence of this oxide. The probable explanation is that while Coos is stable on a substrate of inert platinum (or platinum oxide), it is not likely to be formed in contact with cobalt, which would immediately reduce it to a lower oxidation state. It can be assumed, then, that the second stage in the potentiostatic oxidation of cobalt is due to the transition Co0 or Co(OH), to Co(OH), or CoOOH. The galvanostatic potential/time curves, such as those in Fig. 2, also indicate the occurrence of only two stages in the oxidation. The potential of the first plateau in 1 N and O-1N NaOH corresponds closely to the potential of the first process found in the potentiostatic examination. For the second stage, however, the inflexions in the charging curves occur at somewhat more positive potentials than the corresponding process in the current/potential curves. This is due, in part, to the overshoot of the potential beyond the equilibrium value as the thin film on the electrode surface becomes oxidized,ll and in part to the effect of the cd through the film. The higher the cd, the further will the potential be displaced from the reversible value (see Fig. 2). In the charging of the cobalt electrodes, the overpotential associated with the second state was undoubtedly greater in 1 N NaOH than in the 0.1 N solution, since in the latter case the cds were considerably lower. Wynne-Jones et ~1’ examined plated Co(OH), electrodes, and found a potential of 0.25 V(she) for the transition Co(OH),+ CoOOH, in 1 N NaOH. In O-1N NaOH, one would therefore expect a plateau at about $0.31 V, in reasonable agreement with the present observations. It is suggested that the second stage in the oxidation therefore corresponds to Co(OH),/CoOOH, the initial passivation reaction being due to the formation of Co(OH), rather than COO. This implies an overpotential of about 130 mV for each stage in the potentiostatic oxidation of cobalt, a magnitude which is often associated with nucleation overpotentials. When the electrode is forced to sustain a constant current, the overpotentials may increase. The general form of the current/time curves at constant potentials, Fig. 3, indicates that the rate of formation of the passivating film is controlled by the rate of growth of the nuclei, rather than by the rate of dissolution of the metal.12 Figure 4 shows that during the initial stages of growth in 1 N NaOH, the current at lower overpotentials (with respect to the equilibrium potential for Co/Co(OH)d is proportional to P. Using a simple model for the geometry of growing nuclei,ll this implies that threedimensional (hemispherical) growth occurs, with progressive nucleation. The particular case of the initiation, and growth, of three-dimensional nuclei upon a bare solid metal surface has not been fully treated in the literature with respect to the calculation of the exact amount of overlap of the growing centres. The ensuing discussion, therefore, follows the lines of Fleischmann and Thirsk’s13 treatment of the growth of lead dioxide nuclei in a lead sulphate layer. As such, it must be regarded as a first approximation. It is assumed that the growing nuclei effectively reach a limiting size at a fixed time after formation, this time being equal to the maximum in the i/t curve. The physical limit to the growth of nuclei would, in the present case, be the grain boundaries of the
R. D.
CCWLING
and A. C.
RIDDIFORD
120-
IOO-
60a 3. 60.<
L
I
I I.0
0
2.0
min3
131
FIG. 4. Dependence
of current on (time)” during the initial stages of film formation in 1 N NaOH. 1, -555; 2, -575 mV (she).
metal substrate. If the nucleation law can be represented as diV/dt = AN, exp (--At), and the growth law by i = Bu2, where N is the number of nuclei at time t, N,, is the maximum possible number of sites, A is the nucleation rate constant, B is the growth rate constant and u is the age of a centre, the@ rate =
AN$u2 exp [-A(t
where uz is the time of the rate maximum. rate -h
- u)] . du,
For u, > t,
BN, . At3
3
’
and for uz < t, rate = BN, exp [-A(t
- uJ] ula - 2
+ $ - $
exp (-AU,) 1.
Thus, during the initial stages of growth, the current should be proportional to t3, as observed. For uz < t, a plot of log (rate) us t should be linear, the gradient giving the nucleation rate constant A. Figure 5 shows the experimental log i us t plots for cobalt at various potentials in 1 N NaOH. The variation of A with potential is shown in Fig. 6(A). As A is very small, the exponential in the expression Ul2 -
2 +5 - f
exp (-Au,)]
was expanded to obtain log (rate) = log BN, + log F
+ Au, - At,
Anodic behaviour of cobalt in alkaline solutions
987
P20 -
* 2.00 x .= I.80 B I.60 -
I.401 0 FIG.
5.
I
I
I
2
4
6 1,
Dependence of log i on t dutiNt;k 1, -535;
2, -545;
3, -555;
I
I
8
IO
I
12
J
14
min later stages of film formation in 1 N 4; -565;
5, -575
mV (she).
and the plots of log
i us t shown in Fig. 5 were extrapolated back to t = 0, log BN, being calculated from the intercept. Figure 6(B) shows the dependence of log BN,, upon potential. For the assumed model of growing hemispheres, it can be shownu that BN,, is proportional to k3, where k is the rate constant for the growth process in mole/s/cma. Hence, if 7 is the overpotential for the growth process, 1 . dq dq d log BN, =5%$ Since the slope of Fig. 6(B) is dg/d log BN, = 28.2 mV, then dq/d log k = 84.6 mV. If ,5 is the transfer coefficient for the anodic reaction, and z the number of electrons involved in the rate-determining step, then dg/d log k =
2.303 RT O-059 /3zF = 7’
In the present case, pz = O-69. Therefore, if z = 1, /? = O-69, or if z = 2, p = O-35. If the rate-determining step involves a two-electron transfer, a possible mechanism would be
Co + 2 OH- = Co(OH), + 2 e-, although
an effectively termolecular reaction such as this seems unlikely. Since the basis of the foregoing discussion is that Glm growth, rather than metal dissolution, is rate-determining, the reaction Co + Co s+ + 2 e- is not considered as the slow step in the process. A possible mechanism for the growth of the passivating Urn on cobalt in 1 N NaOH, is Co + OH- = Co(OH),, + e-,
Co(OH),,,
+ OH- = Co(OH), + e-,
R. D. Coma
988
and A. C.
RDDIFORD
o.kl-1--10 E,
mV (she)
-7.oor N E -670Y e 3! ii -640_ -0 5
-6.10-
4 -560-
E, Fro.
6. Dependence
mV (she)
of (A) nucleation rate constant, and (B) log of composite growth rate constant, on potential, in 1 N NaOH.
where the first step, the formation of adsorbed OH at cobalt sites, is rate-determining, with /I = O-69. A similar analysis of the current/time curves in O-1 N NaOH was not successful. Further studies are being made on this. SUMMARY
The results of the present studies have shown that the oxidation of a clean cobalt surface takes place in two stages. This is supported by both potentiostatic and galvanostatic experiments, and in no case was a significant third process found between the rest potential and the point of oxygen evolution. The two oxidation stages are attributed to the transitions Co + Co(OH), and Co(OH), + CoOOH. The current/time curves show that in 1 N NaOH the passivating layer grows in three dimensions (on the simple model, hemispherical growth being assumed), and that progressivenucleation occurs. From the analysis of these curves, a simple mechanism for growth is proposed.
Anodic behaviour of cobalt in alkaline solutions
989
Acknowledgment-R. D. C. thanks Energy Conversion Ltd. for the provision of a Research Grant, during the tenure of which the present work was undertaken. REFERENCES 1. 2. 3. 4. 5. 6. I: 9. 10. 11. 12. 13.
G. GRUBEand 0. FEUCHT, Z. Elektrochem. 28,568 (1922). J. BESSON,C.r. hebd. S&w. Acad. Sci., Paris 223, 28 (1946). S. E. S. EL WAKKADand A. HICKLING, Trans. Furuduy Sot. 46,820 (1950). E. DELT~MBBand M. POURB~, Proc. 6th Meeting CITCE, p. 153. Butterworths, London (1955). K. Scnw~se, Proc. 9th Meeting CITCE, p. 339. Butterworths, London (1957). H. G&m, Electrochim. Acta 11,827 (1966). P. BENSON,G. W. D. Baras and W. F. K. WYNNE-JONES,Efectrochim. Acta 9,275 (1964). Battelle Memorial Institute (ed. Centre dInformation du Cobalt), Cobalt Monograph. Brussels (1960). T. S. DE GROMOBOY and L. L. SHREIR,Electrochim. Actu 11,895 (1966). T. P. HOAR, in Modern Aspects of Electrochemistry No. 2, ed. J. O’M. B~CKRIS. Butterworths, London (1959). M. FLEISCHMANNand H. R. THIRSK, in Advances in Electrochemistry and Electrochemical Engineering, ed. P. DELAHAYand C. W. TOBIAS,Vol. 3. Interscience, New York (1963). D. A. VER~~L~EA, in Advances in Electrochemistry and Electrochemical Engineering, ed. P. DELAHAYand C. W. TOBIAS,Vol. 3. Interscience, New York (1963). M. FLEISCHMANN and H. R. Tr-nns~, Trans. Furuday Sot. 51, 71 (1955).