Journal of The Electrochemical Society, 149 (9) 2002
An electrochemical and X-ray diffraction study has been conducted on the formation of lead dioxide deposits on platinum, from
nitric acid solutions, as a function of potential and temperature. It has been shown that these parameters strongly influence the
morphology and electrocatalytic activity of the PbO2 films.
The electrodeposition process is satisfactorily described by an electrochemical,
chemical, electrochemical mechanism:
[i] H2O --> OHads + H+ + e-
[ii] Pb2+ + OHads --> Pb(OH)2+
[iii] Pb(OH)2+ + H2O --> PbO2 + 3H+ + e-;
the second electron transfer stage and Pb2+ diffusion control the dioxide formation in
the lower and higher overpotential range, respectively. Temperature and potential (or current) are important parameters in the
electrodeposition process. Depending on the potential region, the process can be kinetically or diffusion controlled. In an acid
electrolyte, where mainly the b-PbO2 modification is electrodeposited, the amount of a-phase impurity increases with increasing
potential in the kinetically controlled region and decreases in the diffusion controlled domain. In addition, relatively low electrodeposition
potentials and high temperatures favor an increase of the crystallite size with preferred crystallographic orientation
for both a- and b-PbO2 modifications.
The temperature of the growth solution affects the crystallinity of the resulting oxide
deposits and has a marked effect on their performance as anodes in processes at high positive potentials such as ozone generation.
Despite the fact that lead dioxide is not the latter day anode
material,1 its study is now of high interest, and it extends beyond the
wide field of battery research. Recent studies have been focused, in
particular, on the improvement of lead dioxide as an anode material
for use in anodic oxygen transfer reactions,2-5 including ozone
evolution6,7 and waste treatment processes.8,9 In this connection the
elucidation of PbO2 electrodeposition process is very important, and
although there are numerous recent studies on the mechanism of
lead dioxide electrodeposition,10-17 several aspects remain open to
investigation, especially regarding the influence of the electrodeposition
conditions on the morphology and electrochemical activity of
the resulting PbO2 deposit. In effect, one should not overlook the
fact that the apparent ~observed! electrocatalytic activity of an electrode
surface is dependent on surface roughness even if the inherent
activity remains constant. This is true when the dimension of roughness
is less than that of the diffusion layer, because increased surface
roughness increases the geometrically projected density of active
sites. For a polycrystalline surface, such as PbO2 , the surface roughness
is a function of the dimensions of crystallites, which, in turn,
are dependent on the current density for electrodeposition.
It is now accepted11-16 that the electrodeposition process involves
soluble species as reaction intermediates; these are likely to be
Pb~III! and/or a Pb~IV! oxygen complex. According to our recent
data,19 in nitric acid the rate of PbO2 electrodeposition process can
be limited by an electron transfer or a diffusion stage and the reaction
mechanism is described by the follow scheme
[i] H2O --> OHads + H+ + e-
[ii] Pb2+ + OHads --> Pb(OH)2+
[iii] Pb(OH)2+ + H2O --> PbO2 + 3H+ + e-;
first stage is the formation of oxygen-containing species such as
chemisorbed OH, followed by a chemical stage in which these parparticles
interact with the lead species forming a soluble intermediate
species, likely to contain Pb~III!, which is then oxidized electrochemically
forming PbO2 .
It has been previously reported20 that variations of the conditions
of PbO2 electroplating cause changes in the oxide properties, such
as the crystallographic nature, which result in a different electrocatalytic
behavior. Lead dioxide is electrodeposited from acid solutions
as the tetragonal b-form, although a small amount of the orthorhombic
a-form is also present depending on experimental conditions.21
In this respect, it is important to note that the literature data on the
influence of electrodeposition conditions on the crystallographic nature
of the electrodeposited PbO2 are rather contradictory. Thus, for
example, the formation of a-PbO2 was reported to be favored by
relatively high deposition current densities,22 while mainly pure
b-PbO2 was deposited at low current densities.23 However, other
authors21 report that in an acidic growth solution, at low current
densities, the oxide is always a mixture of the b- and a-phases with
the content of the latter decreasing on increasing the current density;
at high current density pure b-PbO2 was the only observed form of
the electrodeposited oxide.
In the present work we report on the electrodeposition of PbO2
from nitric acid solutions and the physicochemical properties of the
resulting oxide. It is intended to be a step toward a more systematic
study on the influence of the electrodeposition mechanism on the
physicochemical properties of the dioxide obtained.
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