Mechanism of passivation processes of the lead sulphate electrode

EbctrochfmlcaAcm, 1970,Vol. 15,pp. 1483 to 1491. Pormmoa Press. Printed in Northern Ireland

MECHANISM OF PASSIVATION PROCESSES LEAD SULPHATE ELECTRODE*

OF THE

D. PAVLOV and R. POPOVA Institute of Physical Chemistry, Division of Electrochemical Power Sources, Bulgarian Academy of Sciences, Sofia 13, Bulgaria Abstract-The passivation pro&sses of the Pb/PbSO, electrode were investigated by measuring the potential, capacitance and resistance during galvanostatic anodic polarization and a suwuent open-circuit period. Electron micrographs show that the anodic deposit is a polycrystalline PbSO, layer. The passivation and depassivation phenomena take place in the spaces left among the PbSOI Two kinds of passivation of the Pb/PbSO, electrode were recognized: stable and unstable. The stability of the passive state as well as the way in which it is achieved are determined by the distances between the PbSO, crystals. When these distances are of the order of magnitude of the ionic diameters the layer acts as a lective membrane. Basic sulphates of lead and lead monoxide precipitate in the intercrysta#?== me spaces and the electrode is stably passivated. When the width of the intercrystalline spaces exceeds the membrane size, passivation takes place only upon anodic polarization. Basic sulphates of lead and lead monoxide form in the intercrystalline spaces. Upon interrupting the polarization these compounds dissolve and the electrode is self-depassivated. R&m&-Lets processus de passivation de 1’Blectrode Pb/PbSO, ont t% 6tudi6s en mesurant la capacit6, le potentiel et la r&stance au cuurs de Ia polarisation anodique galvanostatique et en circuit ouvert. Les micrographics &ctroniques montrent que le dbp&tanodique est constitu6 par une couche polycristalline de PbSO,. Les ph&om&nes de passivation et d’autod&passivationont lieu dans les espaces entre les cristaux de PbSO,. Deux esp&es de passivation dans 1ulectrode Pb/PbSO, ant &t6 mises en 6vidence: stable et instable. La stabilit6 de l%tat passif et le mode de passivation sent d&ermin& par les distances entre les cristaux de PbSO,. Lorsque ces distances sont de l’ordre des diam&es ioniques la couche agit comme une membrane permselective. Des sulfates basiques de plomb ainsi que du monoxyde de plomb sent p&ipit&s dans Ies espaces intercristallins. L%lectrode est pas&& de maniere stable. Dans le cas oh la largeur des interstices d+asse la dimension de membrane permselective la passivation a lieu uniquement au tours de la polarisation anodique. Les sulfates basiques et le monoxyde se forment entre Ies cristaux. En interrompant la polarisation ces composds se dissolvent et l’blectrode est autod&passivke.

Zusamme&hssung-Durch Messung des Potentials, der KapazitLit und des Widerstandes wahrend galvanostatischer anodischer Polarisation und einer nachfolgenden ‘open circuit’-Periode wurde die Passivierung der Pb/PbSO,-Elektrode untersucht. Elektronenmikroskopische Aufnahmen zeigten, dass der anodische Niederschlag aus einer Schicht von polykristallinem PbSOl besteht. Die Passivierungs- und Depassivierungserscheinungen spielen sich in den Zwischenr&unen zwischen den PbSO,-Kristallen ab. Es wurden zwei Arten von Passivierung gefunden: Eine stabile und eine instabile. Die Stabilitgt des passiven Zustandes wie such der Weg, auf welchem er erreicht wird, werden durch die Absttide zwischen den PbSO,-Kristallen bestimmt. Wenn die Abstlnde in der Grbssenordnung von Ionendurchmessem liegen, dann wirkt die Schicht als permselektive Membran. In den interkristallinen Zwischenr2iumen fallen basischcs Bleisulfat und Bleimonoxyd aus und die Elektrode ist stabil passiviert. Wenn die Griisse der interkristallinen R&une die Dimension der Membran iibert.riiTt,so findet Passivierung nur bei anodischer Polarisation statt. Zwischen den Kristallen bilden sich basisches Bleisulfat und Monoxyd. Bei Unterbruch der Polarisation l&en sich diese Verbindungen auf und die Elektrode ist selbst-depassiviert. UPON anodic polarization of a metal electrode, an insoluble anodic layer is frequently formed at the surface of the electrode. According to its structure the anoclic deposit

may be either a homogeneks non-porous film or a polycrystalline layer. The processes of formation and growth of the anodic films as well as their properties are * Manuscript received 19 May 1969. 1483

D.

1484

and R. POPOVA

PAWV

known from investigations on valve metals. The elementary processes taking place upon anodic polarization of electrodes having a polycrystalline layer have been less

investigated and differ from those involved in the formation of homogeneous anodic films. Electron microscopic investigations of a lead sulphate electrode immersed in a sulphuric acid solution showed that the lead sulphate layer is constituted by well formed crystals. The formation of a crystaIIine layer was investigated by determining the changes in potential and resistance upon galvanostatic oxidation of lead in sulphuric acid. f Lead sulphate nuclei form and subsequently grow upon anodic polarization of the lead electrode. Concomitantly the resistance increases. When the entire surface is covered by lead sulphate crystals the potential of the electrode rapidly increases, while the resistance remains constant. The electrode is passivated. This increase in potential does not affect the capacitance and resistance values.2 In these investigations the polarization is maintained up to the transition of the electrode into a lead dioxide one. This considerably increases the variety of the processes and hampers the study of the phenomena taking place during passivation. The lead sulphate layer was considered also as a film,* the passivation of the electrode occurring when a definite film thickness is reached. The present paper is concerned with the investigation of processes involved in the passivation and depassivation of the lead sulphate electrode when the formation of lead dioxide is avoided. EXPERIMENTAL

TECHNIQUE

MtdlOd The lead sulphate electrode was oxidized under galvanostatic conditions. In order to avoid the formation of lead dioxide, current was passed through the electrode until the electrode potential reached 0 mV with respect to the Hg/Hg,SO, electrode. The measurement of the electrode potential continued during the following periods of several hours at open circuit. Besides the potential the values of two other paramthe capacitance and resistance of the electrode. To this eters were measured: effect a galvanostatic pulse method was used. During polarization and at open circuit galvanostatic pulses with duration 6-8 ps and period 90 ms were applied to the electrode. Figure 1 represents the potential change during the pulse. The capacitance

I 1

I

I

I

3

1, g/c

transientat potential -860 mV.

5

4 L

FIG.

I

I

I 2

6

7

r

e

I

9

p

is determined by the slope of the tangent to the origin of the transient, C = i,(dt/d$),

where iP is the pulse current. R = (A&i,).

The resistance is determined

by the intercept

A+,

Passivation processes of the lead sulphate electrode

1485

Figure 2 illustrates the block diagram of the electric circuit. The current upon polarization of the electrode was maintained constant by means of a galvanostat. The electrode potential was measured by means of a high-impedance vacuum-tube CP-2 voltmeter, and determined with respect to the Hg/Hg,SO, electrode; all values of the potential are given with respect to this electrode. Rectangular galvanostatic pulses were applied with a Solartron GO-1377 pulse generator. The dc component of the pulse generator was filtered with a 1 ,uF capacitor. The change in the potential during the pulse was determined with respect to a low-resistance, non-passivated Pb/PbSO, electrode (second reference electrode in the cell). The pulse transient 4/i was recorded by means of a differential oscillograph Cl-15.

FIG. 2. Block diagram of the electrical circuit. K, cell (1, working electrode; 2, counter-electrode; 3, Hg/HgSOa electrode; 4, Pb/PbSO, electrode) Gs, galvanostat; Mv, millivoltmeter; I& pulse generator; Os, oscillogaph.

Electrodes and soktions The working electrode was 99*999x purity lead machined as a cylinder with a diameter of 2 mm with a conical tail. The electrode was tightly pressed in a Teflon holder with a conical opening so that only the base of the cylinder remained exposed. The same Teflon holder held the lead sulphate reference electrode. The cell contained also the mercury sulphate reference electrode. A platinum counter-electrode was used. The 3 N H$04 solution was additionally purified by previous electrolysis. Before the experiment the working electrode was scraped with a knife and immediately immersed in the electrolyte. After completion of the experiment the ?nodic layer obtained was observed under an EF-4 Zeiss electron microscope. To this effect the electrode was washed and dried and an electron micrograph of its surface was made, using the platinum-carbon replica technique. RESULTS

AND

DISCUSSION

polarization of lead in sulphuric acid, first a polycrystalline layer of lead sulphate is built up, after which passivation of the electrode takes place. The lead sulphate electrode is passivated when a very weak current passes through it under potentiostatic conditions. Under galvanostatic oxidation conditions a rapid polarization of the passive electrode is observed, whereas at open circuit the electrode potential takes values lying between the equilibrium potentials of the lead sulphate and the lead-dioxide/lead-suiphate electrodes. In the present experiments it was found that in a sulphuric acid solution the lead Upon

the

anodic

D. PAVLUV and R. POF'OVA

1486

by anodic polarization of the electrode

sulphate electrode is passivated in two ways: and by self-passivation at open circuit. A. Anodic

passivation and depassivationof the lead electrode in sulphuricacid

A series of successive anodic oxidations were performed on each electrode in order to clarify the passivation phenomena. Figure 3 represents the changes in potential, capacitance and resistance of on electrode in three successive oxidation runs. The values of the capacitance (1 to 3 pF/cm2) and resistance (O-l-l Qcm2) of the passivated electrode coincide with those measured by the impedance methoda On some of the passivated electrodes, however, capacitances of 1 to O-2 pF/cm* were found.

437mV,

PbO -. Fm

562mV. 635mV.

ml

I I

)

15-<

P

745mV,

I

20

25

30

35

4PbO. PbSOq/ 3PbO. PbSO,. PM.

Pb H@/Pb

PbSO4/Pb

1

I

I

I

I

40

45

50

55

T

_1.,,

I

60

65%+-

min

4 C, resistance,R, and potential, E, of the lead sulphate electrode upon three successive polarizations. Qi, quantity of electricity passed upon galvanostatic oxidation performed at current i.

FIG. 3. Change in the capacitance,

According to the changes in potential, capacitance and resistance during the polarization and during the subsequent open circuit time interval, several periods can be recognized. They are best seen in the third oxidation run on Fig. 3. First, a slow polarization of the electrode takes place upon galvanostatic oxidation, in the interval a-b. At this the capacitance of the electrode decreases, while the resistance rises. At point b the electrode is passivated. It remains passive throughout the interval b-m. The capacitance of the electrode retains an almost constant value. In the interval b-f the passive electrode is under current and the electrode potential rapidly increases. A series of oxidation runs showed that throughout the period b-e the slope of the straight line dE/ds depends not only on the current but on the previous history of the electrode as well. At point f the polarization is interrupted and in the interval f-k processes proceed during which self-depassivation of the electrode takes

Passivation processes of the lead sulphate electrode

1487

place. A potential arrest g-h is observed in this interval. From point m the capacitance rapidly increases, m-k, the resistance decreases and the potential reaches the equilibrium potential of the lead sulphate electrode. The electrode is activated. In the interval k-l the electrode remains active. The values of the capacitance as a function of the electrode potential upon polarization and at open circuit are given in Fig. 4. It is seen that under our experimental a* 30KC 25-

I -6aO

I -900

x.+~~~~x--“--y-x-x-x-Y I I I -700 -600 -500. E,

1 -400

-300

I -200

-100

0

mV

FIG. 4. Dependence of capacitance on the x, upon polarization 0, at open circuit

electrode potential.

conditions the passivated electrode shows the same capacitance values upon polarization and at open circuit. Only between -956 and -800 mV is hysteresis observed. Figure 5 shows electron micrographs of the anodic layer on the electrode. Wellformed crystals with different size are clearly seen. The crystalline character of the anodic deposit suggests that the passivation and depassivation processes take place in the spaces left between the lead sulphate crystals. Thus the changes in potential, capacitance and resistance of the electrode upon polarization and at open circuit may be explained in terms of the following model.’ The lead sulphate electrode is constituted by lead sulphate crystals precipitated on the lead surface. Consequently, the capacitance of an electrode with an area of 1 cm2 consists of two terms: the first is the capacitance of the portion 8 of the lead surface which is covered with lead sulphate crystals, and the second takes into account the capacitance of the portion left between the crystals and which is in contact with the solution, (1 - 6), and

c-

(1 -

e)G + 0 0 .lT3 d

, 01Flcm2) 1

where C, is the electrical double layer capacitance of lead, e1 the dielectric constant of PbSOl (for a polycrystalline layer el = 28), 4 the effective thickness of PbSO, layer in A, which was approximately evaluated from the electron micrographs. For the different electrodes, values ranging between 800 and 5000 A were obtained for dl. Consequently, the capacitance of the active lead sulphate electrode is determined by the first term of the above equation.

1488

D.

PAVLOV

and R.

POPOVA

Upon anodic polarization lead ions are obtained. These are precipitated by the solute sulphate ions in the intercrystalline spaces and onto the crystals themselves. Thus the active lead surface decreases. This leads to a slow increase in potential, to a decrease in capacitance and to a rise in the resistance of the electrode throughout the period a-b (Fig. 3). The electrochemical process of formation of lead ions proceeds only at the free lead surface. The growth of the lead sulphate crystals leads to a decrease of this free surface, which brings about an increase of the cd there. When the amount of lead ions formed exceeds the flux of SOd2- moving towards the lead surface, the solution of the active metal surface is alkalinized in order to remain electro-neutral. This alkalinization is due to diffusion and migration causes.4 However, with the increase of pH, basic sulphates of lead and lead oxide are formed. These compounds precipitate filling the bases of the intercrystalline spaces and thus forming a continuous layer. The electrode is passivated (b, Fig. 3). This leads to an increase in the potential. The anodic deposit changes to a dense heterogeneous layer constituted by a mosaic of lead suIphate crystals held together by basic lead sulphates. The capacitance of the passivated electrode is now determined by

where e2 and d2 are the dielectric constant and the layer thickness of the basic sulphates of lead and lead oxide. In the period b-f (Fig. 3) the electrode potential increases at a rate depending on the current and the previous history of the electrode. The rate of increase of the potential is probably determined by the rate of increase of the basic sulphate layer thickness, by the increase of the electrical field in this layer, and lastly, by the change in the number and area of the intercrystalline spaces. The current passing through the layer among the lead sulphate crystals is ionic. The ionic conductivity is due to the motion of lead ions from the metal toward the basic sulphatejsolution interface and of 02- in the opposite direction. The sulphate ions are too large and probably do not participate in the current transfer through the basic sulphate layer. Generally speaking, lead is dissolved at the lead/basic-sulphate (or lead/lead-oxide) interface, while the layer is dissolved at the basic-sulphate/sulphuric acid interface. The lead ions thus obtained diffuse and precipitate on to the lead sulphate crystals. At these small supersaturations the nucleation rate of lead sulphate is very small and therefore the lead ions participate mainly in the growth of the existing lead sulphate crystals. The latter grow in the bulk of the solution (Fig. 5). The X-ray diffraction investigation of the composition of the anodic deposit obtained by potentiostatic oxidation in the range between -950 and 0 mV indicates that above -300 mV the deposit contains besides lead sulphate considerable amounts of tetragonal lead oxide, as well as minor amounts of PbO . PbSO, and 3PbO . PbSO, . H205. Figure 4 shows, however, that already at -800 mV, the lead surface is isolated from the solution by a continuous layer. Owing to its low sensitivity X-ray diffraction analysis is incapable of detecting very small amounts of basic sulphates. Upon interruption of the polarization, point f, Fig. 3, the open-circuit potential is arrested between g and h in the region of the equilibrium potentials of basic sulphates in I N H,SO,. The occurrence of this arrest is due to the low dissolution rate of the

(4

cc> FIG. 5. Electron

c

micrographs of the anodic layer a, c, front view b, side view. 1488

Passivationprocessesof the lead sulphateelectrode

1489

basic sulphates of lead. The lead ions obtained are also precipitated on the lead sulphate crystals. The anodic deposit at open circuit remains continuous down to about -900 mV. This is seen from Fig. 4. In the range -900 to -956 mV the layer breaks down at many intercrystalline spaces and the lead surface is exposed. The capacitance increases abruptly (m-k, Fig. 3) and the electrode is self-activated. The self-activation potential is more negative than the passivation one. At open circuit after depassivation a slow increase in capacitance and a decrease in resistance, k-l, were observed in many electrodes. This indicates that in the active state there are processes that continue in the crystalline deposit. As a result the Therefore the quantity of electricity required for the active surface is changed. subsequent anodic passivation depends on the previous history of the electrode. B. Self-passivation

of the lead mphate

electrode

After depassivation

some electrodes show an increase in potential and a decrease Figure 6 illustrates the behaviour of such an electrode. After the first oxidation run, the capacity of the electrode decreases in the period kl-a,. The potential changes by in capacitance and are passivated, although no current flows through them.

-

1.5

“E Y k

Y-X-

1

E G

b-

1.0 Q.?

l -.-

L

min t, FIG. 6. Change in capacitance,C, resistance,R and potential, E, of the lead sulphate electrode during two successive polarizations. Q,, quantity of electricity passed upon galvanostatic oxidation performed at current i.

several millivolts. These changes in the electrode parameters were interrupted by a new galvanostatic polarization. After switching off the current upon this particular polarization the potential rapidly reaches -950 mV whereas the capacitance increases from 1 to 5 pF/cme. Subsequently a comparatively rapid potential increase and a

1490

D. PAVLOVand R. POPOVA

decrease in resistance are observed, h-n, and the electrode is passivated. In other electrodes this self-passivation proceeds very slowly-for hours and sometimes days. Thus passivated, the electrode can be activated only by applying an external current. It should be noted that not all electrodes self-passivate. Such a behaviour of the electrode can be explained in terms of the phenomena taking place in the intercrystalline spaces. When the lead surface is very densely covered with lead sulphate crystals the intercrystalline distances become commensurable with the diameters of the ions in solution. In this case there is no free diffusion of the solute ions through the intercrystalline spaces and the anodic deposit acts as a permselective membrane. The smaller ions (HsO+, OH- and Pb2+) arrive freely at the metal surface, while the access of the larger ones (SOpz-) is strongly hindered. We shall call membrane size the intercrystalline space size at which the lead sulphate layer begins to act as a permselective barrier. In this case a Donnan equilibrium is established and the electrolyte at the lead surface is alkalized. This brings about the precipitation of basic sulphates of lead, which passivate the electrode. If there are no macropores in the crystalline layer, a membrane equilibrium is established among all crystals. The electrode is stably passivated. Its capacitance is low (0.2-2 ~F[cm2), its potential is more positive than -950 mV (in our experiments this value ranged between -850 and about -680 mV). When the intercrystalline distances exceed the membrane size all solute ions have free access to the metal surface and the crystalline deposit is devoid of membrane properties. The electrode behaves as the classical Iead sulphate electrode ; it has the potential of the latter and a relatively high capacitance. Upon anodic polarization of such an electrode basic sulphates of lead may form, owing to kinetic reasons, in the intercrystalline channels according to the mechanism discussed in section A, already before they can reach membrane size. Upon interruption of the polarization these compounds dissolve and the electrode is self-depassivated. In the crystalline layer there are, as a rule, intercrystalline spaces of both types. From Figs. 3 and 6 it is seen that crystallization processes continue in the lead sulphate layer after depassivation of the electrode, in the period after k. This is shown by the changes exerted by these processes on the capacitance and resistance of the electrode at open circuit. These processes are brought about by the recrystallization of the anodic deposit. Conjugated reactions can take place, too, such as reduction of 0, and H+ and oxidation of lead. As a result of these processes, removal or introduction of material from or in the intercrystalline channels can take place. If the number of intercrystalline spaces having a width exceeding the membrane size is small, the electrode will have a Iow capacitance after depassivation. As a result of recrystallization processes and conjugated reactions, material may be introduced in the channels at open circuit and the spaces among these crystals can reach membrane size. Thus the electrode self-passivates (Fig. 6). Conversely, if the number of channels having a width exceeding the membrane size is sufficiently large, the capacitance of the electrode at open circuit will be relatively high. If material is removed from the in&crystalline spaces in the period after k, such an electrode self-activates (Fig. 3). A comparison between these considerations and the regularities established in the formation and growth of homogeneous non-porous films reveals the differences which exist in the elementary processes taking place in these two types of anodic layer.

Passivation processes of the lead sulphate electrode

1491

authors wish to thank Prof. Dr. E. Budevski and Dr. R. Moshtev for their help in preparing the manuscript and to the laboratory of Dr. M. Marinov for the electron micrographs. REFERENCES 1. W. FEITKNXXT and A. GXUMANN, J. Chim. phys. 49C, 135 (1952). 2. B. KABANOV and D. LEIKIS,Z. Hektrochem. 62,660 (1958). 3. P. Rrmrscm and R. ANGSTADT, J. electrochem. Sot. 111,1323(1964). 4. D. PAVLOV, Electrochim. Acta 13, 2051 (1968). 5. D. PAVLOV, C POULIEIT, E. KLAJA and N. IORDANOV, J. efectruchem. Sot. 116,316 (1969). Acknowle&ements-The