Co(II) 1 M NaOH electrode

J. Electroanal Chem, 145 (1983) 379-387 Elsevier Sequom S.A., Lausanne - Prmted m The Netherlands

EFFECT O F T E M P E R A T U R E O N T H E P O T E N T I O D Y N A M I C OF Co/Co(II) 1 M NaOH ELECTRODE

379

RESPONSE

H. GOMEZ MEIER, R. SCHREBLER GUZMi~N and R CORDOVA ORELLANA Instttuto de Qulmtca, Umverstdad Catthca de Valparaiso, Casdla 4059, Valparaiso (Chde)

(Received 6th April 1982; in rewsed form 26th August 1982)

ABSTRACT The potenUodynamac response of polycrystalhne cobalt electrode m 1 M NaOH solution has been investigated at different temperatures m the potential range related to electroformatlon of CoO and Co(OH) 2. The results show that the E l i displays are gradually modified as temperature is increased. Ttus behav~our can be reasonably interpreted through the reaction model recently proposed which assumes a sandwich-type structure for the interface Ageing effects that were dlstmgmshed through the potentiodynamlc E / I records are also reported.

INTRODUCTION The electrochemical characteristics of cobalt in alkaline solutions have been relatively less investigated than those of nickel and iron, partly because it has not been found useful as an electrode material in alkaline storage devices, except when it is added to positive plates of nickel alkaline batteries [1]. A review of the relevant literature shows that, within the potential range of water t h e r m o d y n a m i c stability, the nature of electroformed species and the m e c h a m s m of electro-oxidation process are not well established [2-6]. The electrochromic properties of cobalt oxide films electroformed on platinum or cobalt base have been investigated b y Burke et al. [7,8]. These authors also reported the cyclic voltammetric behaviour of thick h y d r o u s oxide films on cobalt base and its electrocatalytic activity for the oxygen evolution reaction [9]. Recently, it has been reported that the c o b a l t / a l k a l i n e aqueous solutions interface exhibits a complex potentiodynamic E / I display with a relatively large n u m b e r of anodic and cathodic current peaks and shoulders [10]. However, when the p o t e n t i o d y n a m i c response only covers the potential range where the first stages of electrodissolution and onset of passivity take place, it is possible to obtain a rather less involved E l i record which can be interpreted through a reasonable reaction mechanism [10]. This mechanism assumes that when a stabilized potentiodynamic E l i response is attained, the interface acquires the sandwich-type structure C o / C o O . n H E O / C o ( O H ) 2 • m H 2 0 . This type of multiple interface has been assigned to several electrochemical systems [11-16]. 0022-0728/83/0000-0000/$03.00

© 1983 Elsevier Sequoia S.A.

380 In order to gain further information about this type of structure at the cobalt/1 M N a O H interface, its potentiodynamic behaviour has been investigated at different temperature values. The results show that the changes in the E/I profiles can be explained with the same reaction mechanism, including some minor changes. On the other hand, ageing effects were also detected. The phenomenological aspects of these effects are similar in principle to those described for nickel [17-20] and iron [21-22] in alkaline solutions. They indicate that the reaction products at the interface are generally a mixture of species with different chemical reactlvities that undergo chemical reactions to attain the more stable configuration. EXPERIMENTAL A conventional three-compartment glass cell was used. The working electrodes were made from Johnson Matthey "Specpure" cobalt wires (0.5 mm diam. and 0.2 cm 2 apparent electrode area). The counterelectrode was a large area Pt sheet and a SCE was used as reference. The electrode potentials, however, are referred to the N H E scale. The 1 M N a O H solution was prepared from triple-distilled water and Merck Analytical grade reagent. Experiments were made under N 2 gas saturation at 0. 25, 50 and 75°C. After an initial mechanical polishing the electrode was cathodized for 10 min at - 1.1 V; then each experiment was initiated with a sequence of repetitive triangular potential sweeps (RTPS) until a stabilized E l i display was recorded. In some cases, systematic decrease of the cathodic (E~.c) and anodic (E~,a) switching potential respectively, was made once the stabilized contour was attained. RESULTS When the RTPS are recorded at different temperatures in the potential range associated with the first steps of dissolution and passivity of cobalt (Es, c = - 1.1 V , E~., = - 0 . 0 5 ) the resulting E/I profiles exhibit generally similar behaviour, but the relative contributions of the different processes depend on the temperature. Figure 1 a - d shows the potentiodynamic E/I displays recorded at 0, 25, 50 and 75°C respectively, taken from the first cycle until the stabilized contour is reached. The analysis of the first anodic potential excursions shows two current contributions: one as a shoulder (peak I) located at a potential value that remains practically temperature independent; and the other (peak II) whose potential peak moves to more negative values as the temperature is increased. In the subsequent scans, peak I remains unchanged and any slight variation should be atributed to a baseline modification. On the other hand, the current contribution from peak II diminishes and shifts to more negative potentials. When the stabilized E l i contour is attained, the main anodic contribution at 0 and 25°C corresponds to peak I, while at 50 and 75°C it is due principally to peak II. The change in the cathodic current contributions depends on the temperature and the number of cycles. At 0°C (Fig. la) the first cathodic potential excursion shows a

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single-current peak (peak IV) located at - 0 . 9 9 V which in the following cycles diminishes and disappears completely when the stabilized El1 profile is reached. From the second cycle on, a new peak located at more anodlc potentials is observed. Its height increases gradually on continuing the potential cycling, and when the stabilized profile is attained the peak current maximum is located at - 0 . 8 8 V.

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E/V Fig. 2 Influence of the anodic switching potential on the potentiodynanuc E a c h r u n w a s m a d e b y c h a n g i n g Es, a d o w n w a r d s a f t e r a p r e w o u s R T P S Es, a = - 0 . 0 5 V t o a t t a i n a s t a b l e p r o f i l e . (a) 0 ° C ; (b) 5 0 ° C , (c) 7 5 ° C .

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383

A similar behaviour is observed at 25 and 50°C (Fig. lb, c), but in these cases the peak located at more negative potentials is better defined in the first cycles. Once the stabilized profile is reached, the E/I display at 50°C differs from those at 0 and 25°C as there are two well-defined current contributions, peak IV at - 1.0 V and peak III at - 0 . 8 4 V . At 75°C the change in the cathodic profile is somewhat different to those just described because during the first cycle the current peak contribution located at more negative potentials keeps increasing with cychng. In the stabilized E l i profile it defines a broad current peak with a maximum at - 0 . 9 3 V that encloses peak IH which now appears poorly resolved. The anodic and cathodic charges are modified with both RTPS and temperature. Whatever the temperature investigated, the anodlc charge during the first cycles is greater than the corresponding cathodic charge, and once the stabilized profile is attained the anodic to cathodic charge ratio (Qa/Qc) is--within experimental error--equal to unity. When a perturbation program, in which the anodic switching potential is systematically reduced, is applied to the C o / 1 M N a O H interface, as depicted in Fig. 2, it is possible to correlate the different anodic and cathodic conjugated processes. Figure 2a0 recorded at 0°C, shows that as the positive potential limit becomes more negative, the E l i response appears more reversible. This can be deduced from the fast response of the positive potential switching and from the shift of cathodic peak current to more positive potentials. A similar behaviour can be observed at 25°C. At 50°C (Fig. 2b) where the two cathodic contributions are clearly observed, as Es,a decreases the current contribution located at more negative potentials becomes less

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i m p o r t a n t . W h e n Es, a is - 0.64 V o n l y the c o n t r i b u t i o n at the m o r e positive p o t e n t i a l c a n b e observed. I n this case, the reversibility o f the process is similar at 0 a n d 25°C. A t 7 5 ° C also, a single c a t h o d i c c o n t r i b u t i o n a p p e a r s a n d the effect o f progressive decrease in Es,a is similar to those f o u n d at 0 a n d 25°C. However, the less reversible

385 response which is observed would indicate the participation of a different redox couple. Similar correlations can be found when over the stabdized E/I display Es,c is systematically decreased. Figure 3 shows the correlation between the different anodic and cathodic processes at 50°C. The influence of temperature on the different processes taking place in the C o / 1 M N a O H interface and the subsequent changes of the E l i display can be also followed, starting from a potentiodynamic stabilized profile at 0°C and then raising the temperature according to a linear program with a slope of 0.4°C rain-1. The results (Fig. 4) indicate that the anodic contributions associated with peak II undergo a remarkable increase. In the same way, the cathodic sweep shows a current increase of peak IV from 35°C on, and at 55°C the latter sphts into two current contributions. At higher temperatures current peak IV overlaps peak III, the latter appearing as a shoulder in the E l i display in accordance with the behaviour of the stabilized profile at 25°C (Fig. ld). DISCUSSION The analysis of the potentiodynamic E/I response of a cobalt/1 M N a O H electrode, recorded at different temperatures, presents two well-defined aspects to consider. One is the increase of anodic and cathodic charges with temperature, both in the first potential scan as well as in the stabilized E l i displays. The other is related to the net splitting of the cathodic current peaks when the stabilized E l i profile is reached at a temperature between 50 and 75°C. Both facts indicate that the metal electrodissolution process and the composition of the electrode interface are strongly dependent on temperature. A reasonable explanation of this potentiodynamlc behaviour in the whole range of temperature can be made in terms of the following reaction model previously proposed at 25°C for this interface [10]: Co + OH- = Co(OH) ~ds Co(OH)~-ds= Co(OH)ads+ e /~ Co(OH)ads ~ Co(OH)2 -F ( ? / - 2) Co(OH)ads -}-Co

Co[Co(OH)-H20] [Co(OH)2 H20 ] + OH- = Co[CoO-H20 ] [Co(OH)2- 2 H20 ] + e

(1) (2) (3)

(4)

Through reactions (1-4) a film of Co(OH)2is formed at the electrode surface, and at the same time the primary sandwich structure C o / C o O H / C o ( O H ) 2 is established, After RTPS, the thickness of the Co(OH)2 film becomes large enough to inhibit the cobalt electrodissolution. Then, the electrochemical reaction mainly occurs at the middle of the primary sandwich structure as indicated by eqn. (4). Under these circumstances, when at 25°C the stabilized E l i profile is attained, a definitive sandwich structure is established at the interface. Peaks I and III in the potentiodynamic E l i display are then associated with the electroformatlon and electroreduction of a oxygen monolayer according to the reaction Co[CoO. n 2 0 ] [ Co(On)2 • 2 H20] + 2 e ~ Co[ Co(OH)2 • 2 H 2 0 ] + 2 O H -

(5)

386 The preceding reaction scheme is consistent with the experimental findings at temperatures lower than ca. 45°C, but is not completely applicable for interpreting those obtained at higher temperature values. To account for the greater charge values found at temperature > 45°C where a greater electrodlssolution of cobalt should occur, the following two possible reactions associated to complex anodic peak III should be included in the above-mentioned scheme: Co[Co(OH). H20] [Co(OH)2- 2 H20 ] = Co[Co(OH)2" H20] + H C o O / + 2 H + + e (6a) Co[Co(OH) • H20] [Co(OH)2 • H20] = Co[2 Co(OH)2 • H20] + H + + e

(6b)

Reaction (6a) considers the active dlssoluUon of cobalt and reaction (6b) represents another way to form Co(OH)2 at the electrode surface, both being alternative paths of reactions that can compete with that expressed by eqn. (4). Therefore, the course that reaction should follow after the primary sandwich structure was formed will be dependent almost exclusively on the temperature. The experimental results at 0 and 25°C confirm that in these conditions reaction (4) is favoured in such a manner that in both cases the definitive sandwich structure C o / C o O HzO/Co(OH)2 • H 2 0 is accomplished. However, at temperatures > 45°C, the satuation is rather different because of the large charge values and their low rate of dirmnution w~th the successive potenual scans. These facts indicate that reactions (6a) and (6b) occur at the expense of reaction (4) whose contnbution is now negligible. Under these circumstances the formation of the sandwich structure becomes more difficult to attain. Therefore, at 75°C, the corresponding electroreduction process after prolongated RTPS is now associated with peak IV and can be better represented by the following formalism: Co[Co(OH)2. H20 ] + 2 e ~ 2Co. H20 + 2 O H -

(7)

As can be expected, the situation at 50°C exhibits an intermediate behaviour which represents a compromise between those found at lower and higher temperature. On fact the observed splitting of cathodic current peaks in the stabilized profile can be interpreted by assuming that the electroreduction process includes contributions of both reactions (5) and (7). which can be related to peaks III and IV respectively in the E l i display. Further evidence that the interface changes its compositions with temperature can be inferred from the potentiodynamic E l i profiles recorded with the linear temperature increase program starting from the stabilized condition at 0°C. The gradual increase of anodic and cathodic charge and the evolution of the current peaks prove that the interface ~s changing progressively from one configuration to another. At 50°C the splitting of cathodic peaks is produced as due to reactions (5) and (7), as previously stated. At 75°C only one cathodic current contribution is found which corresponds to electroreduction of Co(OH)2 according to reaction (7). Finally, the results obtained with the sequence of progressive shortening of the anodic switching potential over the stabilized E l i contour, show a shift of cathodic

387

current peaks to more negative value as Es, a is increased. This behaviour can be associated with the possibility that oxygen-containing species formed at the interface undergo ageing processes through which the formation of the more stable species is promoted. Therefore, it can be assumed that the anodlcally formed species in the potential range of current peaks I and II present a simultaneous chemical change during its electroformatlon which is associated with the structural stabilization of the product. ACKNOWLEDGEMENTS

We thank the Direccibn General de Investlgacibn from Universidad Catblica de Valparaiso for support of this research. We are also indebted to Prof. Dr. A.J. ArvJa for revision of the manuscript and his helpful comments. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

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