Hydrated lead dioxide

Another model assumes that gel zones are formed by hydrated lead dioxide (PbO(OH)2) and act as bridging elements between the crystallite particles. Electrons can move along the polymer chains of this gel and so cause electronic conductivity between the crystalline zones 137],... 

The images in Fig. 2.33a show that large zones in some of the Pb02 particles are nontransparent for the electron beams (dark zones), while others let the electron beams through. The latter zones are heterogeneous in structure comprising fully transparent and less transparent zones. When the sample is heated in the microscope, it boils and water is released, as a result of which the Pb02 phase concentrates into a compact mass. This indicates that the electron transparent zones in the particles are composed of hydrated lead dioxide (Pb(OH)4 and PbO(OH)2). 

When PAM is washed with water, the reverse process takes place and it almost restores its initial crystallinity. It can be concluded that there is an equilibrium between the crystal and gel zones and the solution. When Li cations from the solution enter the gel zones (Fig. 2.34c), an equivalent amount of H" " ions should leave the particles in order to preserve their electroneutrality. CP anions from the solution have to be exchanged with OH ions from the gel (Fig. 2.34b). This means that PAM has an amphoteric behaviour. Hydrated lead dioxide is known to have amphoteric properties [95,96]. 

The white precipitate of lead hydroxide (or hydrated lead(ll) oxide) is then oxidised by the chloraie(I) lo the brown dioxide ... 

Hydrolysis of solutions of Ti(IV) salts leads to precipitation of a hydrated titanium dioxide. The composition and properties of this product depend critically on the precipitation conditions, including the reactant concentration, temperature, pH, and choice of the salt (46—49). At room temperature, a voluminous and gelatinous precipitate forms. This has been referred to as orthotitanic acid [20338-08-3] and has been represented by the nominal formula Ti02 2H20 (Ti(OH). The gelatinous precipitate either redissolves or peptizes to a colloidal suspension ia dilute hydrochloric or nitric acids. If the suspension is boiled, or if precipitation is from hot solutions, a less-hydrated oxide forms. This has been referred to as metatitanic acid [12026-28-7] nominal formula Ti02 H2O (TiO(OH)2). The latter precipitate is more difficult to dissolve ia acid and is only soluble ia concentrated sulfuric acid or hydrofluoric acid. 

Beside the crystalline material, a certain portion of amorphous lead dioxide is always observed. In the working electrode such amorphous material is apparently hydrated and forms a gel structure at the phase boundary between the solid material and the electrolyte (cf. Ref. [6]). 

Hydrated zones exchange ions with the solution. Hence, the lead dioxide particle is an open system [39,42]. PbO(OH)2 may also be represented by the formula H2Pb03. Thus, both OH and H" " ions in the gel zones are in equilibrium with the anions and cations of the solution, which facilitates the discharge reaction [44]. It has been established that the crystallites which build up the crystal zones of the formed PAM particles are sized between 25 and 40 nm, whereas the size of the particles themselves varies from 50 nm to about 150nm, and a single particle may contain several crystallites [38]. These particle sizes give a specific surface-area of 5-7m g PAM. 

As evident from the SEM and TEM pictures, the electrochemically obtained lead dioxide particles comprise crystal (Ph02) zones and hydrated (amorphous) zones [PbO(OH)2]. The two types of zones are in equilibrium [56—58]. 

When this lead dioxide was heated and dehydrated, its capacity declined abruptly. These results indicate that the electrochemical reaction of Pb02 reduction proceeds in the hydrated (gel) zones of the particles and it is these gel zones that determine the electrochemical activity of the lead dioxide [59]. The gel zones of Pb02 particles have both electron and proton conductivity due to the polymer chains of the lead dioxide structure [59]. 

As to the electrochemical activity of chemically obtained lead dioxide, either it is composed of crystal zones only or, if hydrated, the gel zones in its structure are very small, which is the reason for the low specific capacity of such an electrode. 

The TEM pictures in Fig. 2.33 evidence that the lead dioxide active mass comprises particles with crystal (a- or p-Pb02) or amorphous zones and hydrated (gel) zones. Figure 2.33c features Pb02 particles with predominantly crystal structure. Only single surface zones are hydrated. 

This highly heterogeneous structure of the Pb02 particles obtained during the electrochemical formation of the lead dioxide active mass evidences that the processes of Pb02 formation proceed through hydration and dehydration stages ... 

The hydrated corrosion layer is readily oxidized to lead dioxide. The latter is ion conductive and is in contact with the PbS04 and basic lead sulfate particles of the paste. These particles have to be oxidized to Pb02- Oxidation of PbS04 proceeds by two mechanisms without... 

The higher the antimony content in the alloy, the higher the degree of hydration of the corrosion layer obtained, i.e. the volume of the gel zones in the lead dioxide layer increases due to incorporation of antimony ions in its structure. These results indicate that antimony increases the degree of amorphization of the PbOi layer, which is also confirmed by the XRD data for die anodic layer [34]. 

Reaction 3.3 is an equilibrium between lead hydroxide and lead dioxide in the presence of water. Then this water can transform the Pb02 particle into an open system. This open Pb02 particle allows for the transfer of ions between the solution outside the particles and the hydrated zones of the particle. Actually, the hydrated zones occupy about 30% of the PAM. Besides the equilibrium between the hydrated and crystal zones, hydrated zones are also in equilibrium with ions in the solution. In this way, the hydrated linear polymer chains containing Pb, O, and OH can be formed, which plays a critical role in both electron and proton conductions. Both high electron and proton conductivities of these polymer-like chains are necessary conditions for the reaction of discharge of Pb02 to proceed. 

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