PHYSICAL CHEMISTRY

I. G. KISELEVA and B. N. KABANOV

ON THE FORMATION AND ELECTROCHEMICAL PROPERTIES OF CRYSTALLINE MODIFICATIONS OF LEAD DIOXIDE

(Presented by Academician A. N. Frumkin, 10 VI 1958)

The phenomenon of polymorphism in the case of lead dioxide has been described in a number of works \((^{1-5})\). It has been established that \(\mathrm{PbO_2}\) exists in two modifications: tetragonal (\(\beta\)-form) and rhombic (\(\alpha\)-form). The occurrence of one or the other modification of lead dioxide depends on the conditions under which it is obtained. Thus, the \(\alpha\)-form is obtained by anodic deposition from neutral or alkaline solutions of lead salts, by oxidation of \(\mathrm{PbSO_4}\) in dilute \(\mathrm{H_2SO_4}\) solutions, and by transformation from the \(\beta\)-form under pressure; the \(\beta\)-form—by oxidation of \(\mathrm{PbSO_4}\) in concentrated \(\mathrm{H_2SO_4}\) solutions.

Analysis of the available data shows that the conditions for obtaining the \(\alpha\)- or \(\beta\)-form at normal pressure differ, in essence, in whether the formation of \(\mathrm{PbO_2}\) proceeds in the presence or in the absence of \(\mathrm{H_2SO_4}\). It could therefore be assumed that the production of different crystalline forms is connected with the adsorption of sulfuric acid, which, as was shown earlier, is retained on \(\mathrm{PbO_2}\) very strongly and in large amounts \((^6)\).

To confirm this assumption, we carried out comparative measurements of the adsorption capacity and an investigation of the structure of electrodes made of \(\mathrm{PbO_2}\) obtained under various conditions. The results are given in Table 1. The adsorption measurements were performed by a radiochemical method, by measuring the activity of the electrodes; the structure was investigated by the X-ray diffraction method*. The initial \(\mathrm{PbO_2}\) electrodes were obtained in the form of a smooth deposit on gold from a 15% solution of \(\mathrm{Pb(NO_3)_2}\) at a current density of \(2 \cdot 10^{-4}\ \mathrm{A/cm^2}\).

In accordance with the literature data, the deposits obtained by us from a neutral solution or by oxidation of \(\mathrm{PbSO_4}\) in \(0.01\,N\ \mathrm{H_2SO_4}\) consisted mainly of \(\alpha\)-\(\mathrm{PbO_2}\). As a result of electrochemical recrystallization of the electrode in \(8\,N\ \mathrm{H_2SO_4}\)—namely, cathodic reduction to \(\mathrm{PbSO_4}\) followed by anodic oxidation to \(\mathrm{PbO_2}\)—irreversible adsorption of \(\mathrm{H_2SO_4}\) on \(\mathrm{PbO_2}\) and transformation of \(\alpha\)-\(\mathrm{PbO_2}\) into \(\beta\)-\(\mathrm{PbO_2}\) occur**. The magnitude of adsorption in \(8\,N\ \mathrm{H_2SO_4}\) is from \(6 \cdot 10^{-8}\) to \(40 \cdot 10^{-8}\ \mathrm{M/cm^2}\), depending on the thickness of the \(\mathrm{PbO_2}\) layer. Adsorbed \(\mathrm{H_2SO_4}\) can be removed from the \(\mathrm{PbO_2}\) deposit by displacing it from the electrode with adsorbing cobalt \((^6)\). An X-ray structural analysis was carried out on lead dioxide which, after its deposition from \(8\,N\ \mathrm{H_2SO_4}\), was anodically polarized for 10–15 hours in a solution of \(8\,N\ \mathrm{H_2SO_4} + 5\%\,\mathrm{CoSO_4}\). It turned out that desorption of \(\mathrm{H_2SO_4}\) is accompanied by the transformation of \(\beta\)-\(\mathrm{PbO_2}\) into \(\alpha\)-\(\mathrm{PbO_2}\).

With regard to the nature of the adsorption of \(\mathrm{H_2SO_4}\) on \(\mathrm{PbO_2}\), one may make the assump—

* We express our deep gratitude to Z. V. Semenova for carrying out the X-ray structural analysis of the lead dioxide samples.

** It should be noted that, in pure form, neither of the modifications is obtained under these conditions. In all cases we had a mixture of both crystalline forms, but from a neutral solution a deposit was formed consisting mainly of the \(\alpha\)-form, and from an acid solution—of the \(\beta\)-form. These deposits we conventionally call, respectively, \(\alpha\)-\(\mathrm{PbO_2}\) and \(\beta\)-\(\mathrm{PbO_2}\).

position, that the absorption of H₂SO₄ occurs in the bulk of the electrode at the intercrystalline surface. This is evidenced by the large magnitude of adsorption and by its dependence on the thickness of the PbO₂ layer (Table 1). In addition,

Table 1

it has been established that appreciable irreversible adsorption occurs only in the process of formation of PbO₂ from PbSO₄, and is not observed during prolonged anodic polarization of PbO₂ in H₂SO₄. This apparently means that sulfuric acid is adsorbed not on the finished precipitate of lead dioxide, but in the course of its formation*. It is interesting to note that in 8 N H₂SO₄ the PbO₂ crystals obtained are 100 times smaller (in linear dimensions) than in Pb(NO₃)₂ solutions or 0.01 N H₂SO₄. This can be explained by the known phenomenon of hindered crystal growth due to chemisorption.

It is known that, with a change in crystalline modification, a number of properties of a substance change. In this connection it was of interest to compare the electrochemical behavior of α- and β-PbO₂. For this purpose, on smooth electrodes made of PbO₂ of the tetragonal and rhombic modifications, the rates of anodic formation of PbO₂ from PbSO₄ and of cathodic reduction of PbO₂ were measured. The rate of these reactions was measured by recording overvoltage curves in 8 N H₂SO₄. To measure the overvoltage, the electrode was partially discharged, i.e., part of the PbO₂ was converted into PbSO₄. The curves representing the dependence \(\varphi\)—\(\lg i\) for two different modifications of PbO₂ are shown in Fig. 1***. The anodic curves, taken rapidly and at low current densities, as well as the cathodic curves, run parallel to one another with a displacement of 30—40 mV. The exchange current on both modifications is almost the same. The course of the curves is determined by deviations of the magnitude of the con—

Fig. 1. Overvoltage curves. 1 and 1a — α-PbO₂ from Pb(NO₃)₂ solution; 2 and 2a — β-PbO₂ from 8 N H₂SO₄; 3 — α-PbO₂ from 8 N H₂SO₄ + 5% CoSO₄

* In the presence of CoSO₄ in solution, a change in crystalline modification and adsorption of H₂SO₄ during continuous anodic polarization may be explained by recrystallization proceeding according to the scheme β-PbO₂ → PbSO₄ → α-PbO₂. This process is accelerated in the presence of CoSO₄ because of the lowering of the oxygen overvoltage and, consequently, the approach of the electrode potential to the equilibrium value. It is also possible that the principal role is played by the preferential reduction of β-PbO₂, occurring at the expense of oxygen evolution.

** The absorption of H₂SO₄ cannot be explained by the formation of a stoichiometric compound of H₂SO₄ with PbO₂ (7), since in that case the amount of H₂SO₄ absorbed should have been two orders of magnitude greater than in the experiment.

*** All curves were recorded on the same electrode in the order of their numbering.

concentration of PbSO₄ at the electrode surface on the value of the saturation concentration. However, on α-PbO₂, in the region of current densities \(10^{-6}\)—\(3\cdot 10^{-6}\) A/cm², the overvoltage of the anodic process at one and the same current density increases with time, rising by approximately 80 mV. Therefore the curve, taken comparatively slowly (over 3–4 hours), has an anomalous form (Fig. 1, 1), and when the measurements are carried out in the reverse order, from higher current densities to lower ones, hysteresis is observed (dotted line, curve 1b). After anodic polarization of PbO₂ for 10–15 hours in \(8\,N\) H₂SO₄ + CoSO₄, during which desorption of H₂SO₄ occurs and β-PbO₂ is transformed into α-PbO₂, the oxidation of PbSO₄ proves to be retarded, and the overvoltage curve (Fig. 1, 3) runs almost the same as on α-PbO₂ obtained from a neutral solution.

The retardation of the oxidation process of PbSO₄ on α-PbO₂ in \(8\,N\) H₂SO₄ is probably connected with the hindrance of the electrocrystallization process (⁹), which

Fig. 2. Reduction of PbO₂ (discharge curves). 1 — 25% β-PbO₂, 2 — 50% β-PbO₂, 3 — 75% β-PbO₂

can be explained by adsorption of H₂SO₄. It is known that anodic oxidation of Pb²⁺ ions in \(8\,N\) H₂SO₄ leads to the formation of β-PbO₂. If the electrode is PbO₂ of the same modification, the electrocrystallization process proceeds easily, since growth of the existing crystal lattice continues (¹⁰). In the case of an electrode of α-PbO₂, however, on which H₂SO₄ has not yet had time to be adsorbed, a deposit of α-PbO₂ is readily formed at the active sites. Gradually the number of active sites decreases, probably because of the specific adsorption of H₂SO₄ (which occurs slowly), as a result of which the deposition of α-PbO₂ gradually ceases. After this, already at an increased overvoltage, only β-PbO₂ can form. In this case the increase in overvoltage is due not only to the formation on the surface of a deposit that is foreign with respect to the PbO₂ being formed, but also to the adsorption of sulfuric acid on this surface. Therefore, after the first portions of the deposit have been obtained, the rate of the process does not increase, as would be expected. Apparently, the increase in potential attained promotes very strong specific adsorption of H₂SO₄, which hinders the growth of the forming PbO₂ nuclei, and the process continues to proceed at a retarded rate.

Judging from curves 1a and 2a (Fig. 1), at a given potential the reduction of α-PbO₂ proceeds at an incomparably lower rate than that of β-PbO₂. Accordingly, during discharge of an electrode consisting of a mixture of both modifications of PbO₂, two plateaus appear on the discharge potential–time curve, differing in potential by approximately 30 mV (Fig. 2). The starting material for obtaining such electrodes was α-PbO₂ deposited from a neutral solution of Pb(NO₃)₂, or from \(0.01\,N\) H₂SO₄. By partial reduction of PbO₂ followed by oxidation of the PbSO₄ formed in \(8\,N\) H₂SO₄, the corresponding part of α-PbO₂ was transformed into β-PbO₂. The length of the first plateau is determined by the amount of β-PbO₂. In the case of curves 1, 2, and 3 in Fig. 2, the partial preliminary discharge of α-PbO₂ amounted, respectively, to 25, 50, and 75% of the capacity.

A similar phenomenon was observed in the experiments of Ruetschi and Cahan (4). However, the authors were inclined to explain it both by a difference in the discharge potentials of \(\alpha\)-PbO\(_2\) and \(\beta\)-PbO\(_2\), and by passivation phenomena. Burbank (5) concluded that the discharge of \(\alpha\)-PbO\(_2\) is slowed on the basis of the experimental fact that, in the corrosion products of lead covered with a mixture of \(\alpha\)-PbO\(_2\) and \(\beta\)-PbO\(_2\), \(\alpha\)-PbO\(_2\) is found along with PbSO\(_4\).

Thus, the investigation carried out permits one to consider that the cause of the slowing of the process PbO\(_2\) \(\to\) PbSO\(_4\), as well as of the formation of the \(\beta\)-form, is the chemical adsorption of sulfuric acid on the surface of PbO\(_2\). The influence of H\(_2\)SO\(_4\) adsorption on the rate of another anodic process on PbO\(_2\) (oxygen evolution) had already been established earlier (8).

In conclusion it should be noted that further study of the conditions for the appearance of various modifications of lead dioxide and of their properties is of practical interest. Thus, using deposits of lead dioxide on gold as an example, we observed a considerable difference in mechanical strength between deposits of \(\alpha\)-PbO\(_2\) and \(\beta\)-PbO\(_2\). In addition, it is known that the hardness of PbO\(_2\) from 8 \(N\) H\(_2\)SO\(_4\) is less than that of PbO\(_2\) from 0.1 \(N\) H\(_2\)SO\(_4\) (11). Consequently, it is possible that the so-called “creep” of the active mass of the positive electrode in a lead storage battery is connected with a decrease in strength due to the transformation of \(\alpha\)-PbO\(_2\) into \(\beta\)-PbO\(_2\) during cycling (3).

Institute of Electrochemistry
Academy of Sciences of the USSR

Received
10 VI 1958

CITED LITERATURE

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