Materials Sciences and Applicatio ns, 2011, 2, 237-243
doi:10.4236/msa.2011.24030 Published Online April 2011 (http://www.scirp.org/journal/msa)
Copyright © 2011 SciRes. MSA
An Investigation on the Electrochemical
Characteristics of Ta2O5-IrO2 Anodes for the
Application of Electrolysis Process
Joo-Yul Lee1,*, Dae-Keun Kang1, KyuHwan Lee1, DoYon Chang1
1Electrochemistry Research Group, Korea Institute of Materials Science 797 Changwondaero, Changwon, Korea.
E-mail: leeact@kims.re.kr
Received February 8th, 2011; revised February 14th, 2011; accepted February 28th, 2011.
ABSTRACT
The elec trochemical cha racteristics o f Ta2O5-IrO2 electrodes prepared from different chemical compositions and coat-
ing methods were observed by using cyclic voltammetry, potentiostatic polarization, galvanostatic polarization and
scanning electron microscopy. The efficiency fo r chloride oxidation and oxygen evolutio n processes was not only influ-
enced by the chemical composition but also by the surface morphology of the oxide electrode which was susceptible to
the ratio of the two components and the coating method. Ta2O5(50) -IrO2(5 0) electrodes revealed the highest catalytic
activity for the chloride ion oxidation and oxygen evo lu tion reaction because they had th e la rgest effective su rface area .
The durability of the oxide electrodes in the accelerated life tests was improved as the thickness of the oxide layer in-
creased and the ratio of [IrO2] to [Ta2O5] approac hed 80/20.
Keywords: Ta2O5-IrO2 Anode, Chloride Oxidation, Oxygen Evolution, Accelerated Life Test, Corrosion Resistance
1. Introduction
Mixed metal oxides have been developed as insoluble
anodes to protect steel-concrete structures in corrosive
atmospheres induced by chloride ions [1,2]. Oxide elec-
trodes used in severe environments require a high
chemical stability as well as electrocatalytic activity,
whose properties are closely related with their surface
exfoliation, electrochemical dissolution, and an increase
of electrochemical impedance at the substrate/metal ox-
ide interface [3,4]. For the fabrication of highly stable
metal oxide electrodes, interfacial properties at the metal
oxide/solution interface should be considered in addition
to the chemical composition of the metal oxide itself.
Various types of metal oxides such as PtOx, IrO2,
Ta2O5, SnO2, Co3O4 have been tested to improve the
corrosion resistance of mixed metal oxide anodes in re-
cent times [3-14]. According to this previous research, it
is generally accepted that a noble metal tends to increase
the catalytic activity of the anode while a general metal is
added to strengthen the electrochemical stability of the
composite catalytic layer. IrO2 was used as an oxygen
evolution catalyst due to its low overpotential for the
oxygen evolution reaction, high electrical conductivity
and corrosion resistance. Furthermore, an IrO2 catalyst
layer admixed with SnO2 or Ta2O5 component was very
effective to extend the service life in a corrosive envi-
ronment [3]. On the other hand, Ta2O5 is chemically/elec-
trochemically stable and is known to prohibit separation
between the substrate and upper catalytic layer by hin-
dering passivation of the Ti substrate metal [5,6]. For this
reason, the importance of the mixed metal oxide prepared
from IrO2 and Ta2O5 was stressed for the industrial ap-
plications including electroplating, electrowinning, elec-
trosynthesis, salt-splitting, descaling, cathodic protection
and so on [15-28].
In this research, we investigated the relationship be-
tween the electrochemical properties of Ta2O5-IrO2 mix-
ed metal oxides and their surface morphology both in
anodic and cathodic process. And, electrochemical char-
acteristics of Ta2O5-IrO2 electrodes prepared from dif-
ferent chemical compositions and coating methods were
observed by using cyclic voltammetry, potentiostatic po-
larization, galvanostatic polarization and surface mor-
phologies from scanning electron microscopy.
2. Experimental
2.1. Electrode Preparation
Commercial 99.5 wt% titanium plate (10 × 40 × 1.5 mm)
An Investigation on the Electrochemical Characteristics of TaO-IrO Anodes for the
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Application of Electrolysis Process
was used as a substrate for the electrode and its surface
was pretreated by sand blasting and degreasing in a
mixed solution of trichloroethylene and acetone prior to
the formation of the oxide layer. Each layer of mixed
metal oxide was prepared by successive oxide precursor
coating, drying, and thermal decomposition. The precur-
sor solution for mixed metal oxide coating consisted of
0.1 M H2IrCl6·4H2O (Strem Chemicals, 99.9%), 0.1 M
Ta(OC2H5)5 (Aldrich, 99.98%) and 8.3 M isopropyl al-
cohol (Duksan Pure Chemicals, 99.8%) as a solvent
which was stirred vigorously at room temperature before
use. The application of the oxide precursor to the sub-
strate was done by brushing or dipping method for com-
parison. The metal Ta:Ir mole ratio in the precursor solu-
tion varied from 20:80 to 80:20.
The coating layer was dried at 100˚C in air for 10
minutes and then thermally decomposed at 450˚C for 20
minutes. Also the outermost layer was annealed at the
same temperature for 60 minutes for the complete oxida-
tion of the coating. The thickness of oxide was controlled
by the repetition of the above successive processes and
the loading amount of the oxides weighed 1.07 ~ 1.35
mg/cm2 after l0 layers were completed.
2.2. Electrochemical Measurements
Mixed metal oxide (area: 2 cm2), platinum plate(area: 4
cm2), and SCE (saturated caromel electrode) were used
as working, counter, and reference electrodes respec-
tively for the electrochemical analysis. Also the electro-
chemical characteristics of the mixed metal oxides were
measured by using a Gamry PC4/FAS1 potentiostat
(Gamry Instruments). Cyclic voltammograms of each
mixed metal oxide anode were obtained in 5 M NaCl
(Duksan Pure Chemicals, 99%) aqueous solution in the
potential range of –0.15 ~ 1.00 V at 50 mV/s. Potentio-
static polarization was performed in 1 M HClO4 (Sam-
chun, 60% ~ 62%) aqueous solution from 1.20 V to 1.42
V with a potential step of 20 mV in the positive direction,
where the potential was maintained for 30 seconds at
each potential step and thereby steady state currents were
recorded as a function of applied potential.
For the evaluation of durability of the mixed metal
oxide anodes in real plant environments, galvanostatic
polarization experiments were executed with a 2-electro-
des configuration – a mixed metal oxide and a platinum
plate as an anode and a cathode respectively. The corro-
sion resistance of each anode was assessed by the poten-
tial transition curve in a mixed solution composed of 1M
NaCl and 2 M HClO4 at 50˚C at a constant current den-
sity, 2 A/cm2, under the conditions that the concentration
of chloride ions and overall conductivity of the solution
were kept in the range of 32 ~ 38 g/l and 900 ~ 1000 mS
by periodic addition of the concentrated mixed solution.
2.3. Microscopic Observations
The surface morphology and surface composition of the
mixed metal oxides were analyzed by scanning electron
microscopy (SEM, JSM-5800, JEOL)) and energy dis-
persive x-ray spectroscopy (EDX).
3. Results and Discussion
3.1. Redox of Chloride Ions
Figure 1 shows cyclic voltammograms of chloride ions
in a brine solution at the mixed metal oxide anodes pre-
pared by (a) a dipping and (b) a brushing method. Redox
peaks appearing at 0.3 ~ 0.6 V represent the oxidation/
reduction by Ir3+/4+ species on the anode surface, indicat-
ing that the IrO2 component is the electrochemically ac-
(a)
(b)
Figure. 1. Cyclic voltammograms of Ta2O5-IrO2 electrodes
in a brine solution (5M NaCl, pH 2.3, temperature = 80˚C)
at 50 mV/s. (a) Curves: (solid) Ta2O5(80)-IrO2(20), (dash)
Ta2O5(50)-IrO2(50), (dot) Ta2O5(20)-IrO2(80) prepared by a
di pp.ing method; (b) Curves: (solid) Ta2O5(80)-IrO2(20),
(dash) Ta2O5(50)-IrO2(50), (dot) Ta2O5(20)-IrO2(80) pre-
pared by a brushing method. Number of oxide layers was 4.
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tive species participating in the redox process of chloride
ions [29-31]. When the composition of a mixed metal
oxide layer changes in the order of Ta2O5(50)-IrO2(50),
Ta2O5(80)-IrO2(20) and Ta2O5(20)-IrO2(80), their peak
potentials shifted in a positive direction and also their
current peaks decreased. This means that the overpoten-
tial for chloride ion oxidation increases and the current
efficiency decreases in the order of the above composi-
tion.
Table 1 summarizes the anodic charge densities at the
mixed metal oxide anode prepared by a dipping method
and a brushing method. From the change of the anodic
charges at the electrodes prepared by different coating
methods for the same chemical compositions, mixed
metal oxide anodes coated by dipping showed a higher
catalytic activity than those by brushing for all the
chemical compositions. Comparing the variation of an-
odic charges among different chemical compositions for
each preparation method, it was revealed that the Ta2O5-
(50)-IrO2(50) electrode had the highest anodic charge
density and the Ta2O5(80)-IrO2(20) electrode was com-
parable to or a little higher than the Ta2O5(20)-IrO2-(80)
electrode.
Since electric charge is proportional to the electrode
surface area, we can say that the dipping method effec-
tively increases the active area of the catalyst taking part
in the electrochemical oxidation reaction of chloride ions
more than by brushing. Therefore, it was estimated that
the Ta2O5(50)-IrO2(50) electrode had the largest catalytic
area and the Ta2O5(80)-IrO2(20) electrode and the
Ta2O5(20)-IrO2(80) electrode had similar effective sur-
face areas.
3.2. Oxygen Evolution Reaction
Figure 2 represents the steady state current densities at
each applied step potential in 2 M HClO4 to compare the
oxygen evolution reactions of anodes as a function of the
chemical composition. Because the oxygen evolution
reaction competes with the oxidation of chloride ions in
the saline solution due to their similar ovepotentials, it is
necessary to differentiate the former from the latter for
practical use.
The Ta2O5(20)-IrO2(80) electrode was expected to
show the highest steady state current density due to the
inherently low overpotential characteristics of IrO2 for
the oxygen evolution process. However, for the mixed
metal oxide anodes prepared by dipping, the efficiencies
of oxygen evolution reaction were Ta2O5(50)-IrO2(50)
Ta2O5(20)-IrO2(80) > Ta2O5(80)-IrO2(20) in Figure 2(a).
The fact that the Ta2O5(50)-IrO2(50) electrode had a
comparable efficiency to the Ta2O5(20)-IrO2(80) elec-
trode was assumed to be due to the large active surface
area of the former as shown in Figure 3. On the other hand,
Table 1. Superficial anodic charge densities of chloride oxi-
dation reactions with the variation of [Ta2O5]/[IrO2] of
Ta2O5-IrO2 electrodes prepared by a di pp.ing method and
brushing method.
Composition Di pp.ing method Brushing method
Ta2O5(80)-IrO2(20)7.07 mC/cm2 6.49 mC/cm2
Ta2O5(50)-IrO2(50)9.44 mC/cm2 8.51 mC/cm2
Ta2O5(20)-IrO2(80)6.87 mC/cm2 4.84 mC/cm2
the Ta2O5(20)-IrO2(80) electrode was more active than
the Ta2O5(80)-IrO2(20) electrode owing to its higher
(a)
(b)
Figure 2. Steady state current densities of Ta2O5-IrO2 elec-
trodes in 1 M HClO4 aqueous solution (20˚C) from –1.20 V
to –1.42 V with a potential step of +20 mV for 30 s. (a)
Curves: (solid) Ta2O5(80)-IrO2(20), (dash) Ta2O5(50)-IrO2
(50), (dot) Ta2O5(20)-IrO2(80) prepared by a di pp.ing
method; (b) Curves: (solid) Ta2O5(80)-IrO2(20), (dash)
Ta2O5(50)-IrO2(50), (dot) Ta2O5(20)-IrO2(80) prepared by a
brushing method. Number of oxide laye r s was 4.
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Application of Electrolysis Process
content of the IrO2 component. Anodes formed by
brushing also had a similar tendency to those formed by
dipping in Figure 2(b).
3.3. Microstructure Analysis
Figure 3 displays the surface morphologies of Ta2O5-
IrO2 mixed metal oxide layers prepared by (a-c) dipping
and (d-f) brushing. Oxide layers formed by dipping
showed smoother surfaces than those by brushing and the
transition of surface microstructures with a molar ration
of Ta: Ir was similar for both methods.
Oxide layers containing high Ta2O5 contents (Figures 3
(a) and (d)) formed a type of agglomerates of IrO2 on an
amorphous Ta2O5 background, while oxides with high
IrO2 contents showed a high crack density on the surface.
Interestingly, the Ta2O5(50)-IrO2(50) electrode had both
agglomerates and many cracks on the surface, which may
lead to a large active surface area and result in the highest
steady state current densities in Figure 1 and Figure 2.
The durability of an anode is influenced by its surface
state, which also may be controlled by the molar ratio
between constitutional chemical compositions. Therefore,
the adjustment of the compositional ratio of the IrO2-
Ta2O5 system with a suitable preparation method should
make it possible to enhance the corrosion resistance of an
anode by inducing compact surface structures and so can
be effectively used for the fabrication of the mixed metal
oxide layer durable in severe environments.
3.4. Accelerated Life Tests
For the evaluation of the durability of individual mixed
metal oxide anodes, accelerated life tests were executed
in a mixed solution of 1 M NaCl and 2 M HClO4 (50˚C)
at a constant current density of 2 A/cm2. Figure 4 com-
pares potential transition curves among the mixed metal
oxide anodes prepared from different molar ratios of
[Ta2O5]/[IrO2] and with different coating methods. For
each anode, an abrupt potential increase appears before
its breakdown, which means complete deactivation of the
electrode surface. The mixed metal oxide anodes pre-
pared by the brushing method showed relatively better
corrosion resistance than those coated by the dipping
method. Correlating the effect of constituent chemical
compositions of anodes with a coating method, the coat-
ing method made little differences for the oxide layers of
low IrO2 contents, but the brushing method proved its
superiority to the dipping method as the IrO2 content
became higher.
Figure 5 shows the effect of a coating repetition num-
ber-that is, oxide thickness - of the Ta2O5(20)-IrO2- (80)
layer on the electrode’s serviceable life. The continuous
process of coating, drying, and thermal decomposition was
Figure 3. SEM photographs of Ta2O5-IrO2 electrodes pre-
pared by a di pp.ing method (a-c) and by a brushing method
(d-f). (a,d) Ta2O5(80)-IrO2(20); (b,e) Ta2O5(50)-IrO2(50); (c, f)
Ta2O5(20)-IrO2(80), whose coating number was 4.
repeated three times, six times, and ten times respectively,
which were confirmed as 0.3 μm, 0.7 μm, 1.2 μm thick,
to achieve the comparative thickness of oxides. We could
get the linear relationship between the oxide thickness
and anode span in curve in a very aggressive solution (1
M NaCl and 2 M HClO4, 50˚C, at a constant current den-
sity, 2 A/cm2) as below.
Service life (hr) = –12.4 + 28 X (coating number) (1)
Since the oxide layer was formed in proportional to the
coating number, around 0.1 ~ 0.15 μm/1 coat, this fact
facilitates us to calculate the necessary oxide thickness
for a certain service time.
Figure 6 shows typical potential–time curves repre-
senting the change of durability of anodes with different
chemical compositions, whose layers were prepared by a
brushing method with ten repetitions. Judging from the
fact that the serviceable time of the electrodes has been
extended with an increase of IrO2 content in the
IrO2-Ta2O5 system, it is concluded that the IrO2 content
is one of the critical factors dominating the corrosion
resistance of this type of mixed metal oxide anode.
However, it is noticeable that a mixed metal oxide anode,
Ta2O5(20)-IrO2(80), showed a superior durability to the
single component pure IrO2 electrode.
4. Conclusions
1) The efficiency for the chloride oxidation and the oxy-
gen evolution process of the Ta2O5-IrO2 mixed metal
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An Investigation on the Electrochemical Characteristics of Ta2O5-IrO2 Anodes for the
Application of Electrolysis Process
Copyright © 2011 SciRes. MSA
241
(a) (b)
(c)
Figure 4. Typical potential-time curves of Ta2O5-IrO2 electrodes prepared by (solid) di pp.ing method and (dot) brushing
method with coating number of 10. (a) [Ta2O5] : [IrO2] = 80:20; (b) [Ta2O5] : [IrO2] = 50:50; (c) [Ta2O5] : [IrO2] = 20:80. Ac-
celerated life tests were executed at constant curren t densities of 2 A/cm2 in a (1 M NaCl + 2 M HClO4) mixed solution (50˚C).
Figure 5. Typical potential-time curves of Ta2O5(20)-IrO2(80) electrodes with different coating numbers, which were pre-
pared by the brushing method and recorded under the same acceleration life test conditions of Figure 3.
An Investigation on the Electrochemical Characteristics of TaO-IrO Anodes for the
242 252
Application of Electrolysis Process
Figure 6. Typical potential-time curves for Ta2O5-IrO2 elec-
trodes with a variation of [Ta2O5]/[IrO2], which were p-
Next Generation Core En-
elopment Project (No.021-
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