Rare Metal Materials and Engineering
Volume 42, Issue 5, May 2013
Online English edition of the Chinese language journal
Cite this article as: Rare Metal Materials and Engineering, 2013, 42(5): 0885-0890.
ARTICLE
Preparation and Characterization of Titanium Based
PbO2 Electrodes Doped with Some Common Elements
Xu Hao1,2,
1
Li Jingjing1,
Yan Wei 1, 3,
Chu W2
2
3
Xi’an Jiaotong University, Xi’an 710049, China; The Hong Kong Polytechnic University, Hong Kong, China; Suzhou Academy of Xi’an
Jiaotong University, Suzhou 215012, China
Abstract: Four kinds of PbO2 electrodes were prepared by electrochemical deposition and three of them were doped by Fe, Ni
and Ag, respectively. The characters of the electrodes were analyzed by SEM, XRD, accelerated life tests, and linear scanning
voltammetry. The results show that the electrode surfaces are covered by quadrangular β-PbO2 crystals. The average grain
sizes of the undoped PbO2 electrode and Ni-doped PbO2 electrode are similar and smaller than that of the Fe-doped PbO2 and
Ag-doped PbO2 electrodes. The Fe-doped PbO2 electrode has the longest accelerated test life due to its dense surface. The LSV
tests show that the Ni-doped PbO2 electrode possesses the highest oxygen evolution potential. From the phenol electrolysis,
the Ni-doped PbO2 electrode demonstrated the best performance in COD removal efficiency, average current efficiency, and
energy consumption.
Key words: PbO2 electrode; doping; accelerated life; oxygen evolution potential; phenol wastewater degradation
In recent years, the electrochemical oxidation treatment
has gained more and more attention for its potential application in the degradation of refractory pollutants[1-9]. In the
electrochemical oxidation method, one of the most crucial
components is the anode where the decomposition reactions
of pollutions take place. Currently, the most typical electrodes are dimensionally stable anodes (DSA) and boron
doped diamond (BDD) [4,10-12]. The DSA electrode is used
widely due to the easy preparation and cost-efficiency, e.g.
PbO2, SnO2 [13-16], IrO2 [17] and RuO2 [18].
Among these DSA electrodes, PbO2 is considered as an
excellent metal oxide electrode because of its lower price
compared to noble metals, chemical stability in corrosive
media, and relatively high over-potential for the oxygen
evolution reaction. Many attempts have been made on the
enhancement of PbO2 in order to further improve the performance of the PbO2 electrode in the application. A frequently used method is to dope the PbO2 layer with some
materials [19]. For instance, Amadelli et al. [20] prepared the
F-PbO2 electrodes and found that the oxygen evolution
process of the F-PbO2 electrodes shifted to higher potentials
and the current efficiency of the formation of O3 rose at a
given current density, compared to unmodified PbO2. Song
et al. [21] used cerium nitrate as the dopant and obtained the
PbO2-CeO2 electrode, which has excellent performance in
the accelerated life test and the antibiotic wastewater treatment. Zhou et al. [22] used the fluorine resin as the dopant to
prepare the PbO2 electrode and similar results like Song’s
work were obtained.
Phenols are aromatic compounds with one or more hydroxyl groups attached to the aromatic ring and are found as
wastes from a variety of industries. The oxidation removal
of phenol has been of particular interest because of their
high yield in industrial effluents [23]; therefore, phenol was
chosen as the probe in the electrochemical oxidation process.
In this work, three familiar elements, including Ni, Fe,
and Ag were applied as dopants to prepare PbO2 electrodes
with Ti/Sb-SnO2 by electrochemical deposition. X-ray
fluorescence (XRF), scanning electron microscopy (SEM)
Received date: May 25, 2012
Foundation item: Ph. D. Programs Foundation of Ministry of Education of China (20090201110005); Natural Science Foundation of Jiangsu Province;
(SBK201022919) and the Joint supervision scheme of Hong Kong Polytechnic University (GU-784)
Corresponding author: Yan Wei, Ph. D., Professor, Department of Environmental Science and Technology, Xi’an Jiaotong University, Xi’an 710049, P. R. China, Tel:
0086-29-82664731, E-mail: [email protected]
Copyright © 2013, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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Xu Hao et al. / Rare Metal Materials and Engineering, 2013, 42(5): 0885-0890
and X-ray diffraction (XRD) were used to examine the
changes of the coating of these electrodes. The electrochemical analysis (including linear scanning voltammetry,
accelerated life test, and electrochemical degradation) of
the electrochemical performance was used to determine
whether these changes result in improvement of the doped
PbO2 electrodes.
1 Experiment
1.1 Electrode preparation
Titanium plate (99.6% purity) with thickness of 1.0 mm
was used in this study (BaoTi Co. Ltd China). All the
chemicals used were the analytical reagent. The deionized
water used was produced by water purifier (EPET-40TF,
EPET Co. Ltd, Nanjing, China).
Prior to the preparation process, Ti plates with a dimension of 5 cm×8 cm were mechanically polished by
1000-grid sand papers, and rinsed by deionized water. Then,
the foils were immersed into a mixture of acetone and 1
mol·L-1 NaOH (1:1 v/v) in an ultrasonic cleaner
(KQ2200DB, 80 W, 40 kHz, Kunshan Ultra Co. Ltd, Jiangsu, China) to remove organic compounds from the surface, etched in 10 wt% oxalic acid at 98 ℃ for 2 h, and then
washed by deionized water.
The Ti/Sb-SnO2/PbO2 electrode was prepared by
dip-coating plus thermal deposition for the inner coating
layer (Sb-SnO2) and electrochemical deposition for the
outer layer (PbO2). The precursor solution for the inter
metal oxide layer was prepared by dissolving 10%
SnCl4·6H2O and 1% SbCl3 in a solution consisting of 10 mL
n-butanol, 10 mL iso-propanol, 10 mL ethanol and 2 mL
hydrochloric acid (37%). The precursor solution must be
freshly prepared and used due to its poor stability. The titanium plate was brushed by the precursor solution, and then
was calcined in an oven at 450 °C for 15 min. This procedure was repeated for 10 times. Finally, the anode was annealed at 450 °C for 1 h. The average oxide loading for the
dip coating method was 1.10 mg·cm-2.
The electrochemical deposition of the PbO2 coating was
carried out on the as-prepared Ti/Sb-SnO2 electrode, which
can lessen the interface resistance between the substrate and
PbO2. The electrolyte for the undoped PbO2 electrode was
consisted of 0.5 mol·L-1 Pb(NO3)2, 0.1 mol·L-1 Cu(NO3)2,
0.01 mol·L-1 NaF, and 10 mg·dm-3 dodecyl sodium sulfonate, while the solution pH was adjusted to 2 using HNO3.
The power was supplied by a WYK-303B electrical source
and the electrochemical deposition was carried out in a two
electrode cell at 20 mA·cm−2 for 120 min in a 65 °C water
bath. Finally, the Ti/Sb-SnO2/PbO2 electrodes were rinsed
by doubly distilled water. The average PbO2 loading was
around 200 mg·cm-2. Either Ni(NO3)2, AgNO3, or Fe(NO3)3
was added into the electrolyte, when the correspondingly
doped electrode was prepared. The dopant concentration
was 10 mmol·L-1.
1.2 Electrode characterization
The structure of the as-prepared electrodes was characterized by a Rigaku D/MAX-2400X X-Ray Diffractometer
with Cu-Kα radiation. The morphology of the metal oxide
coating film was examined by SEM (JEOL, JSM-6390A).
The electrochemical character of the electrodes was performed in a three-electrode cell at room temperature in 1
mol·L-1 H2SO4. The as-prepared electrodes were served as
the working electrodes. The counter electrode was a copper
plate, and an Ag/AgCl/saturated KCl electrode was used as
the reference electrode. The electrochemical measurement
was made with an electrochemical system (LK3200A,
Lanlike, China). The linear scanning voltammetry was carried out on a scan at a speed of 50 mV·s-1. For the accelerated life test, the current density was increased to 1000
mA·cm-2.
1.3 Electrochemical degradation
The electrochemical degradation was performed in a two
electrode cell. The stainless steel plate with a dimension of
5 cm×8 cm was used as the cathode, and the as-prepared
electrodes were used as the anodes. The distance between
the two electrodes was 2.0 cm. The solution’s temperature
was controlled by the circular water bath and thermostat.
The power was supplied by a WYK-303B electrical source.
In the treatment process, after 450 mL phenol wastewater
(1000 mg·L-1 phenol and 1 mol·L-1 Na2SO4 as the supporting electrolyte) was added into the stainless steel reactor,
the stirring and controller systems were turned on. The
process was carried out under a constant current condition
and the duration of electrolysis was 2 h. For the UV-vis and
COD analyses, the wastewater was sampled for every half
hour during the treatment.
The COD removal efficiency was calculated by the following formula:
Removal efficiency=
A0 − At
× 100%
A0
(1)
where A0 is the COD value of initial wastewater and At is
the COD of the solution at the given time t.
The average current efficiency (ACE) was calculated
by the following equation:
ACE=
(COD 0 - COD t )FV
× 100%
8 It
(2)
where COD0 and CODt are the chemical oxygen demand at the
initial and the reaction time t (gO2·L-1), respectively. I is the
current (A), F is the Faraday constant (96,487 C·mol-1), t is the
treatment time (s), and V is the volume of the wastewater (L).
The energy consumption (EC), expressed in kWh·kg
COD−1, was calculated by the following equation:
UIt
(3)
EC=
3.6(COD 0 - CODt )V
where U is the voltage applied (V).
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Xu Hao et al. / Rare Metal Materials and Engineering, 2013, 42(5): 0885-0890
2
Results and Discussion
2.1 Surface morphology
Fig.1 shows the SEM images of various Ti/SnO2-Sb/PbO2
electrodes. All the 4 different electrodes of
Ti/SnO2-Sb/PbO2 have uneven deposits, which provide
more surface areas and potentially increase the electrode
efficiency, and thus the application potential in wastewater
treatment is improved[24]. The shape is quadrangular for the
undoped PbO2 electrode, as shown in Fig.1a. But there are
some areas (black spots) which are not fully covered with
the PbO2. The Ni-doped PbO2 electrode surface is denser
than the undoped PbO2 electrode surface. The quadrangular
grains of the former are clearer than the latter, and there are
some cavities in the quadrangular grains as shown in Fig.1b.
The Fe-doped PbO2 electrode has the densest surface
among the four electrodes and its average grain size is bigger than that of the undoped PbO2 and Ni-doped PbO2. The
Ag-doped PbO2 electrode has a similar grain size compared
with the Fe-doped PbO2. Its surface is less denser and had
many cracks.
2.2 XRD analysis
The XRD patterns of the 4 Ti/SnO2-Sb/PbO2 electrodes
are shown in Fig.2. There are no diffraction peaks of the
doping elements, because their dosages in the PbO2 film are
too low to be detected by XRD. This is further confirmed
by XRF analysis, in which no doping elements are identified.
Two types of crystallites can be distinguished on the
surface: small fraction of α-PbO2 crystals and the dominant
β-PbO2 crystals. The main crystal planes for the undoped PbO2
electrode are β (101) at 31.67º and β (301) at 62.20º, which is
consistent with the results reported by Kong et al. [25]. For the
Ni-doped PbO2 electrode, it has the similar spectra with the
undoped PbO2 electrode. But the angle positions of the two
a
b
crystal planes are slightly shifted to 31.77º and 62.27º, respectively. And the diffraction intensity of the Ni-doped
PbO2 electrode in these two positions is higher than the
undoped PbO2 electrode, indicating the former has a better
degree of crystallinity of PbO2. The main crystal planes for
the Fe-doped PbO2 electrode are β (110) at 25.34o and β
(211) at 49.04º. The main crystal planes for the Ag-doped
PbO2 electrode are β (301) at 62.40º and β (211) at 49.00º.
The 4 electrodes have their main crystal planes which are
different from each other. The reason for this phenomenon
is the doping in the electrochemical deposition. Thus, the
doping process affects the growth of PbO2 and changes its
growth orientation, resulting in a change of its crystal
plane.
The average crystal size of PbO2 particles can be calculated from the XRD data according to Scherrer formula.
The average crystal size is 27.78, 29.75, 33.11 and 34.84
nm for undoped, Ni-, Fe-, and Ag-doped PbO2 electrodes,
respectively. It can be concluded that the undoped PbO2
electrode has the largest true surface area, followed by the
Ni-doped, Fe-doped and Ag-doped PbO2 electrode.
2.3 Accelerated test life
Fig.3 shows the curves of the potential versus time for
the 4 Ti/SnO2-Sb/PbO2 electrodes in which all the curves
have a similar pattern. There is a first platform period
around 2.5~3.1 V. In this period, the potential is relatively
stable which can be contributed to the steady consumption
process of the PbO2 layer. When the potential rises to 6~8 V,
the second platform period is shown. In the second platform
period, the potential can no longer be maintained within a
small interval, and a significant potential variation was observed instead. This may be due to the inner layer consumption. If the coating on the titanium base is used up, it
will result in a rapid increase of the potential, which indicates the inactivation of the electrode. As we set the 10 V
(vs Ag/AgCl) as the failure criteria, the service life is 116,
100.5, 99.1 and 52 h for Fe-, undoped, Ni-, and Ag-doped
PbO2 electrodes, respectively.
18000
c
Intensity/cps
β(101)
d
14000
10000
β(110)
β(211)
6000
α(200)
d
c
b
a
2000
50 µm
20
30
40
β(310)
50
β(301)
α(222)
60
2θ/(º)
Fig.1
SEM images of the 4 PbO2 electrodes: (a) undoped PbO2,
(b) Ni-doped PbO2, (c) Fe-doped PbO2, and (d) Ag-doped
PbO2
Fig.2
XRD patterns of the 4 PbO2 electrodes: (a) undoped PbO2,
(b) Ni-doped PbO2, (c) Fe-doped PbO2, and (d) Ag-doped
PbO2
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Xu Hao et al. / Rare Metal Materials and Engineering, 2013, 42(5): 0885-0890
9
Current Density/mA·cm-2
Potential/V(vs Ag/AgCl)
10
8
7
6
5
d
4
a
c
b
3
2
0
40
80
120
160
0
–10
a
–20
d
–30
c
–40
–50
1.500
200
1.750
Accelerated life tests of various PbO2 electrodes in 3
2.250
Fig.4
Linear scanning voltammetry curve of various PbO2 elec-
mol·L H2SO4 solution under 1 A·cm : (a) undoped PbO2,
trodes in 1 mol·L-1 H2SO4 solution at a scan rate of 50
(b) Ni-doped PbO2, (c) Fe-doped PbO2, and (d) Ag-doped
mV·s-1: (a) undoped PbO2, (b) Ni-doped PbO2, (c)
PbO2
Fe-doped PbO2, and (d) Ag-doped PbO2
0.8
0.6
1-Undoped
2-Ni-doped
3-Fe-doped
4-Ag-doped
a
2
1
3
4
0.4
0.2
0.0
35
0.0
0.5
1.0
ACE/%
1.5
2.0
b
1-Undoped
2-Ni-doped
3-Fe-doped
4-Ag-doped
30
25
20
15
2
1
3
10
4
5
0.5
1.0
1.5
200
EC/kWh·kg COD
It is noted that the Fe-doped PbO2 electrode has the
longest first platform period among the 4 tested electrodes.
The Fe-doped PbO2 electrode has a relatively lower electrode (surface) area due to its larger grain size, which reduces the electrolysis diffusion to the titanium base. In addition, the first platform period of the Ni-doped PbO2 electrode is longer than that of the undoped PbO2 electrode
which means the former’s stability is better than the latter.
This may be explained that the Ni-doped PbO2 electrode
has a denser surface than the undoped PbO2 electrode,
which can prevent the electrolysis diffusion and then reduces the PbO2 layer consumption rate.
The actual life of the modified electrodes can be calculated from the relationship between the electrode service
life (SL) and the current density [26],
SL ~ 1 / in
(4)
where n ranges form 1.4 to 2.0. Assuming an average n of
1.7 for the electrode, the service lives are predicted to be
8.91, 8.64, 14.1 and 4.58 for the undoped, the Ni-doped, the
Fe-doped and the Ag-doped PbO2 electrode, respectively,
under the conditions of 20 mA·cm-2 current density in the
actual conditions.
2.4 Linear scanning voltammetry
Oxygen evolution potential is a critical parameter for the
anode activity[27]. Fig.4 shows the linear scanning voltammetry
curve of the 4 PbO2 electrodes in 1 mol·L-1 H2SO4 solution at a
scan rate of 50 mV·s-1. It is found that the doped PbO2 electrodes have higher oxygen evolution potentials than the undoped PbO2 electrode. Among the doped PbO2 electrodes, the
Ni-doped PbO2 electrode has the highest oxygen evolution potential of 2.10 V at 1.0 mA·cm-2. Therefore, the Ni-doped
PbO2 electrode theoretically should have a better performance
in the electrochemical degradation of organic contaminants in
water and/or wastewater.
2.5 Phenol degradation
Fig.5a shows the COD removal efficiency for the elec-
COD Removal Effciency/%
-2
2.0
4 c
-1
-1
2.000
Potential/V(vs Ag/AgCl)
Time/h
Fig.3
b
1-Undoped
2-Ni-doped
3-Fe-doped
4-Ag-doped
150
100
50
3
1
2
0.5
1.0
1.5
2.0
Time/h
Fig.5
Phenol degradation by the 4 PbO2 electrodes: (a) COD
removal efficiency, (b) ACE, and (c) EC
trodes during the electrolysis. The Ni-doped PbO2 electrode
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Xu Hao et al. / Rare Metal Materials and Engineering, 2013, 42(5): 0885-0890
has the best performance by removing 75% of COD from
the solution in 2 h, then followed by the undoped, the
Fe-doped and the Ag-doped PbO2 electrodes. The best performance of Ni-doped PbO2 electrode is likely resulted
from the combination of its larger surface area and the
highest oxygen evolution potential. It is interesting to note
that the undoped PbO2 electrode has a second-best performance in phenol degradation. This suggests that the
oxygen evolution potential is not the only parameter responsible for the organic degradation, and the anode surface
area, for example, is another critical factor to determine the
overall performance of the electrolysis process. This presumption can be further verified by the poor performance of
the Fe-doped and Ag-doped PbO2 electrodes, where the
lower surface areas are observed, as shown in Fig.1.
In addition, electrode properties like the average current
efficiency (ACE) and the energy consumption (EC) are
found useful in evaluating the electrodes’ performance for
the degradation of phenol. Fig.5b and 5c show the ACE
curves and EC curves, respectively. The Ni-doped PbO2
electrode has the highest ACE and the lowest EC during the
phenol degradation. These results (i.e. curves of Fig.5b and
5c) are comparable to the COD decay curves with good
correlations for all the 4 electrodes. The descending of ACE
and uprising of EC curves (with reaction time) suggests the
reactivity of electrolysis of the intermediates resulting from
phenol oxidation is not as high as the parent compound
(phenol) itself.
moving 75% of COD from the solution in 2 h, then followed by the undoped, the Fe-doped and the Ag-doped
PbO2 electrodes. The ACE and EC analyses results are
comparable to the COD decay tendency with good correlations for all the 4 electrodes.
References
1
Marco Panizza, Giacomo Cerisola. Chemical Reviews[J],
2009, 109: 6541
2
Carlos A Martinez-Huitle, Sergio Ferro. Chem Soc Rev[J],
2006, 35:1324
3
Song Shuang, Fan Jiaqi, He Zhiqiao et al. Electrochim Acta[J],
2010, 55: 3606
4
Panakoulias T, Kalatzis P, Kalderis D et al. J Appl Electrochem[J], 2010, 40: 1759
5
Zhou Minghua, He Jiangjian. J Haza Mater[J], 2008, 153: 357
6
Marco Panizza, Giacomo Cerisola. J Haza Mater[J], 2008,
153: 83
7
Fukunaga M T, Guimaraes J R, Bertazzoli R. Chem Eng J[J],
2008, 136: 236
8
Cheng Xueming, Chen Guohua, Yue Po Lock. Chem Eng
Sci[J], 2003, 58: 995
9
Wang Bo, Kong Wuping, Ma Hongzhu. J Haza Mater[J], 2007,
146: 295
10
Zhu Xiuping, Ni Jinren, Wei Junjun et al. J Haza Mater[J],
2011, 189: 127
11
Zhao Guohua, Li Peiqiang, Nong Fuqiao et al. J Phys Chem
C[J], 2010, 114: 5906
3 Conclusions
12
1) 4 Kinds of PbO2 electrodes are prepared by thermal
decomposition and electrochemical deposition including an
undoped PbO2 electrode and Ni-, Fe-, and Ag-doped PbO2
electrodes.
2) The surfaces of the electrodes are mainly covered
with quadrangular β-PbO2 crystals and a small amount of
α-PbO2. The average grain sizes of the undoped PbO2 and
Ni-doped PbO2 electrodes are similar and smaller than that
of the Fe-doped and Ag-doped PbO2 electrodes.
3) The service life is 116, 100.5, 99.1 and 52 h for Fe-,
undoped, Ni-, and Ag-doped PbO2 electrodes, respectively.
The Fe-doped PbO2 electrode has the longest service life
due to its dense surface.
4) The LSV test shows that the oxygen evolution potentials of the doped electrodes are higher than that of the
undoped electrode. The Ni-doped PbO2 electrode has the
highest oxygen evolution potential which is 2.10 V at 1.0
mA·cm-2.
5) During the process of the phenol electrolysis, the
Ni-doped PbO2 electrode has the best performance by re-
13
Panizza M, Kapalka A, Comninellis C. Electrochim Acta[J],
2008, 53: 2289
Xu Hao, Yan Wei, Tang Chengli. Chin Chem Lett[J], 2011, 22:
354
14
Wu Tao, Zhao Guohua, Lei Yanzhu et al. J Phys Chem C[J],
2011, 115: 3888
15
Li Peiqiang, Zhao Guohua, Cui Xiao et al. J Phys Chem C[J],
2009, 113: 2375
16
Zhao Guohua, Cui Xiao, Liu Meichuan et al. Environ Sci
Tech[J], 2009, 43: 1480
17
Liu Yang, Li Zhiying, Li Jinghong. Acta Mater[J], 2004, 52:
721
18
Chen Xueming, Chen Guohua. Electrochim Acta[J], 2005, 50:
4155
19
Chahmana N, Matrakova M, Zerroual L et al. J Pow Sour[J],
2009, 19: 51
20
Amadelli R, Armelao L, Velichenko A B et al. Electrochim
Acta[J], 1999, 45: 713
21
Song Yuehai, Wei Gang, Xiong Rongchun. Electrochim
22
Zhou Minghua, Dai Qizhou, Lei Lecheng et al. Environ Sci
Acta[J], 2007, 52: 7022
Tech[J], 2005, 39: 363
889
Xu Hao et al. / Rare Metal Materials and Engineering, 2013, 42(5): 0885-0890
23
Yang Xiupei, Zoua Ruyi, Huo Feng et al. J Haza Mater[J],
26
2009, 164: 367
24
Cong Y Q, Wu Z C. J Phys Chem C[J], 2007, 111: 3442
25
Kong J T, Shi S Y, Kong L C. Electrochim Acta[J], 2007, 53:
Cheng Xueming, Chen Guohua, Yue Po Lock. J Phys Chem
B[J], 2001, 105: 4623
27
Huang Aisheng, Zhao Guohua, Li Hongxu. Chin Chem Lett[J],
2007, 18: 997
2048
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