Fabrication of a novel PbO2 electrode with a graphene nanosheet interlayer for electrochemical oxidation of 2-chlorophenol

Article abstract of DOI:10.1016/j.electacta.2017.04.114

A novel PbO₂ electrode with a graphene nanosheet interlayer (marked as GNS-PbO₂) was prepared combining electrophoretic deposition and electro-deposition technologies. The micro morphology, crystal structure and surface chemical states of GNS-PbO₂ electrodes were characterized using scanning electronic microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Their electrochemical properties and stability were determined using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), ·OH radicals test and accelerated life test, and compared with traditional PbO₂ electrodes. Besides, their potential application in the electrochemical degradation of 2-chlorophenol (2-CP) was investigated. The GNS-PbO₂ electrode possessed perfect octahedral β-PbO₂ microcrystals, and its grain size was much smaller than that of traditional PbO₂ electrode. It exhibited higher electrochemical activity than traditional PbO₂ electrode due to its larger electrochemical active surface area and stronger ·OH radicals generation ability. The service lifetime of GNS-PbO₂ electrode (107.9?h) was 1.93 times longer than that of traditional PbO₂ electrode (55.9?h). The electrochemical degradation rate constant of 2-CP on GNS-PbO₂ electrode (k_app?=?2.75?×?10^?2?min^?1) is much higher than for PbO₂ electrode (k_app?=?1.76?×?10^?2?min^?1). 2-CP oxidation yielded intermediates including aromatic compounds (catechol, phenol and ortho-benzoquinone) and organic acids (oxalic acid, maleic acid and glutaconic acid), and a possible degradation pathway for 2-CP was proposed accordingly.

Full text of DOI:10.1016/j.electacta.2017.04.114

Electrochimica Acta 240 (2017) 424–436
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Fabrication of a novel PbO2 electrode with a graphene nanosheet
interlayer for electrochemical oxidation of 2-chlorophenol
Xiaoyue Duana,b, Cuimei Zhaoa, Wei Liua,b, Xuesong Zhaob, Limin Changa,
*
a
Key Laboratory of Preparation and Application of Environmental Friendly Materials, Ministry of Education, Jilin Normal University, Siping, 136000, China
b
Key Laboratory of Environmental Materials and Pollution Control, Education Department of Jilin Province, Siping, 136000, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A novel PbO2 electrode with a graphene nanosheet interlayer (marked as GNS-PbO2) was prepared
combining electrophoretic deposition and electro-deposition technologies. The micro morphology,
crystal structure and surface chemical states of GNS-PbO2 electrodes were characterized using scanning
electronic microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray
photoelectron spectroscopy (XPS). Their electrochemical properties and stability were determined using
Received 21 February 2017
Received in revised form 27 March 2017
Accepted 21 April 2017
Available online 23 April 2017
Keywords:
cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS),
accelerated life test, and compared with traditional PbO2 electrodes. Besides, their potential application
in the electrochemical degradation of 2-chlorophenol (2-CP) was investigated. The GNS-PbO2 electrode
ꢀ
OH radicals test and
Electrochemical oxidation
PbO2 electrode
Graphene nanosheet
Interlayer
possessed perfect octahedral
traditional PbO2 electrode. It exhibited higher electrochemical activity than traditional PbO2 electrode
due to its larger electrochemical active surface area and stronger OH radicals generation ability. The
b-PbO2 microcrystals, and its grain size was much smaller than that of
2-Chlorophenol
ꢀ
service lifetime of GNS-PbO2 electrode (107.9 h) was 1.93 times longer than that of traditional PbO2
electrode (55.9 h). The electrochemical degradation rate constant of 2-CP on GNS-PbO2 electrode
(kapp = 2.75 ꢁ10ꢂ2 minꢂ1) is much higher than for PbO2 electrode (kapp = 1.76 ꢁ10ꢂ2 minꢂ1). 2-CP
oxidation yielded intermediates including aromatic compounds (catechol, phenol and ortho-benzoqui-
none) and organic acids (oxalic acid, maleic acid and glutaconic acid), and a possible degradation
pathway for 2-CP was proposed accordingly.
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
[5–9]. Although the PbO2 electrode has good chemical stability,
release of toxic Pb2+ ions in strongly acid solution at a high current
Chlorophenols (CPs) have been widely used in the production of
pesticide, herbicide, bactericide and wood-preservative, so they
are frequently present in environment [1,2]. However, CPs have
high toxicity to organisms and difficult biodegradability, some of
them have been identified as priority pollutants by the US
Environmental Protection Agency [3], and conventional biologic
methods are insufficient to completely mineralize CPs. Neverthe-
less, the electrochemical oxidation process has been developed to
degrade the CPs because of its easy operation, low cost and high
efficiency [4].
density is possible. If released, serious threats to human health.
Therefore, lots of studies in past decades have been performed
highly to improve its electro-catalytic activity and stability by
modification with adding electrochemical active materials into the
b
-PbO2 active layer, such as Fꢂ anion [10,11], metal ions and oxides
[12–16], surfactant [17,18], fluorine resin [9,19], polytetrafluoro-
ethylene [20,21]. Besides modification, there are other strategies
for improving the electro-catalytic activity and stability of PbO2
electrode. Mo changed Ti substrates to three-dimensional struc-
tured carbon fiber felt, and the obtained PbO2 electrode exhibited
excellent electro-catalytic oxidation performances and reproduc-
ibility [22]. Zhao prepared the porous Ti/SnO2-Sb2O3/PbO2
electrode on a porous Ti substrate, and the obtained porous Ti/
SnO2-Sb2O3/PbO2 electrode had high oxygen evolution over-
potential, large active surface area and high electrochemical
activity, and its lifetime was 3.69 times higher than that of the
planar Ti/SnO2–Sb2O3/PbO2 electrode [23]. The TiO2 nanotube
PbO2 electrode has been widely used as effective anode in the
electrochemical oxidation processes due to its low cost, good
conductivity, chemical stability, as well as high catalytic activity
* Corresponding author.
E-mail address: changlimin2139@163.com (L. Chang).
http://dx.doi.org/10.1016/j.electacta.2017.04.114
0013-4686/© 2017 Elsevier Ltd. All rights reserved.

X. Duan et al. / Electrochimica Acta 240 (2017) 424–436
425
array template was used as the substrate of PbO2 electrode, which
2.3. Electrode preparation
significantly enhanced electrocatalytic activity and lifetime of
PbO2 electrode [7,24]. The SnO2-Sb2O3 interlayers were also
modified using CNT [6] and Nb2O5 [25] to increase the
electrochemical activity and stability of PbO2 electrodes. In these
studies, the electrocatalytic properties and stability of PbO2
electrodes were proved to be much dependent on their composi-
tion and structure.
The GNS-PbO2 electrodes consisted of Ti substrate, SnO2-Sb2O3
bottom layer, -PbO2 intermediate layer, -PbO2 inner layer, GNS
interlayer, and -PbO2 outer layer, as described in Fig. 1. The
preparation processes of SnO2-Sb2O3 bottom layer and -PbO2
intermediate layer were same performed with the traditional PbO2
electrode [36]. The parameters of electrodeposition of -PbO2
inner layer and -PbO2 out layer were also same with the
traditional PbO2 electrode other than electrodeposition time. The
electrodeposition time of -PbO2 inner layer and -PbO2 outer
layer was 30 min and 5–15 min, respectively. More details about
the preparation of SnO2-Sb2O3 bottom layer, -PbO2 intermediate
layer, -PbO2 inner layer and -PbO2 outer layer can be found
elsewhere [36].The GNS interlayer was fabricated using electro-
phoretic deposition in GNS aqueous suspension. 200 mg of GNS
were dispersed in 100 mL aqueous solution of 0.05 g/L sodium
dodecylbenzenesulfonate by ultrasonic vibration for 0.5 h, and the
2.0 g/L GNS aqueous suspension was obtained. A Ti substrates with
a
b
b
a
b
Graphene nanosheet (GNS), with distinctive 2-dimensional
sheet structure consisting of a few layers of sp2-hybridizaed carbon
lattice [26,27], has received considerable attention recently from
many fields such as energy storage [28], sensing [29] and catalyst
[30] due to its high electrical conductivities, outstanding catalytic
properties, large surface area (theoretical specific area of 2620 m2/
g), and corrosion resistance properties [31]. Especially, GNS has
great potential application in electrocatalysis as supports or
composites of electrocatalysts. For example, Pd nanocubes/
reduced graphene oxide composites showed good electrocatalytic
activity and stability towards the electro-oxidation of ethanol [27];
Pd0.5Fe0.5/graphene catalyst exhibited high electrocatalytic activity
for accelerating the two-electron reduction of O2 to H2O2 [3]; a
quite good catalytic activity for the oxidation of Ascorbic Acid was
obtained at a metalloporphyrin–graphene composite modified
electrode [32]; Ag particles were introduced to the graphene
surface as a function of the applied current, and the electrochemi-
cal performance of the Ag-coated graphene electrode was two
times higher than that of the crude graphene electrode [33]. In
these studies, GNS significantly enhanced dispersity of metal
nanocrystals and electron transfer rate. Thus, it is expected to
improve electrocatlytic activity and stability of PbO2 electrode
through introducing a GNS layer into PbO2 electrode.
b
b
b
a
b
b
b-PbO2 inner layer was used as anode and a stainless steel sheet
with same area as cathode with a distance of 3 cm. Electrophoretic
deposition was conducted at a constant voltage of 30 V for a
deposition time of 10 min. After electrophoretic deposition, the
samples were dried under vacuum at a temperature of 50 ꢃC for 1 h.
For comparison, the traditional PbO2 electrodes were also
prepared according to the procedure described in our previous
study [36].
2.4. Electrode characterization
The crystal structure of coatings was investigated using X-ray
In the present work, we attempt to introduce a GNS interlayer
into the PbO2 electrode using electrophoretic deposition. Herein,
the effect of the GNS interlayer on the morphology and structure of
PbO2 electrode was investigated using scanning electron micros-
copy (SEM), transmission electron microscopy (TEM), X-ray
diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).
Further the electrocatalytic activity and stability of the PbO2
electrode with the GNS interlayer (GNS-PbO2) were compared with
traditional PbO2 electrode using electrochemical experiments. In
addition, simulated wastewater of 2-chlorophenol (2-CP) was
electrolyzed using an electrochemical system including GNS-PbO2
as anode and stainless steel as cathode, and the degradation
pathway for 2-CP was proposed.
diffraction (XRD, Rigaku D-max/3C) with Cu Ka radiation (45 kV,
30 mA). The surface morphology and microstructure were
analysed using scanning electron microscopy (SEM, JEOL JSM-
6510) and transmission electron microscope (TEM, TECCNAI F20),
respectively. X-ray photoelectron spectroscopy (XPS) was per-
formed on an ESCALAB250XI spectrometer using Al K
a radiation
for excitation to ascertain the surface chemical states of coatings.
2.5. Electrochemical measurement
The cyclic voltammetry (CV) and electrochemical impedance
spectroscopy (EIS) were both performed using an electrochemical
workstation (PGSTAT302) in a three-electrode system with the
tested electrode as working electrode, a platinum sheet as auxiliary
electrode and a saturated calomel electrode (SCE) as reference
electrode. EIS measurements were carried out in a frequency range
from 100 kHz to 0.01 Hz with an applied sine wave of 10 mV
amplitude at open circuit potential (OCP). All the experiments
were done at ambient temperature.
2. Experimental
2.1. Materials
All chemicals were of analytical grade and were used without
further purification. All solutions were prepared using deionized
water.
ꢄ
2.6. OH radicals generation measurement
The quantity of ꢄOH radicals generation during electrochemical
processes were estimated by means of a method using terephthalic
2.2. GNS preparation
GNS was prepared according to the reported methods with
some modification [34,35]. First, flexible graphite sheets (3 cm
ꢁ 5 cm ꢁ 0.02 cm) were washed with anhydrous ethanol and
deionized water. Then, the electrochemical re-intercalation of
flexible graphite sheets was performed using a potentiostat with
an electric field of 8 V/cm and in the electrolyte containing 0.4 wt
% (NH4)2S2O8. Thus, a large number of floccules exfoliated from
anode surface. Finally, the GNS was obtained through centrifuga-
tion, washing with deionization water and drying in a vacuum
oven at 60 ꢃC.
Fig. 1. Structure schematic of GNS-PbO2 electrode. 1 Ti substrate, 2 SnO2-Sb2O3
layer, 3
a-PbO2 layer, 4 b-PbO2 inner layer, 5 GNSinterlayer, 6 b-PbO2 outer layer.

426
X. Duan et al. / Electrochimica Acta 240 (2017) 424–436
acid as a fluorescence probe according to a procedure reported in
literature [37] with minimal modification. 200 mL of aqueous
solution containing 0.5 mM terephthalic acid, 0.5 g/L NaOH, and
0.25 M Na2SO4 was used as electrolyte. The anode was the GNS-
PbO2 electrode or the traditional PbO2 electrode described above
and the cathode was a stainless steel sheet with same area. The
electrolyte was performed with a current density of 30 mA/cm2
and a temperature of 30 ꢃC. During the experiments, samples were
drawn from the reactor every 5 min and diluted 20 times with
deionized water, then analyzed by a fluorescence spectrophotom-
eter (Cary Eclipse). Fluorescence spectra were recorded in the
range of 370–520 nm, using an excitation wavelength of 315 nm.
♣
graphene
♥(301)
♥ β-PbO2
♥
(101)
♥
♥
(211)
♥
Traditional PbO2
♥
♥
(200)
15 min
10 min
♥
(110)
♥
♥
♥
♥
♥
♥
♣
♥
5 min
GNS
♣
2.7. Accelerated lifetime tests
10
20
30
40
θ (degree)
50
60
70
In normal wastewater treatment, a good PbO2 electrode should
be able to work effectively for several years [38]. It is time
consuming to test electrode stability under normal conditions.
Thus, the accelerated life tests were conducted to estimate the
lifetime of electrode according to the methods described in Ref.
[39]. The accelerated life time tests were conducted in a three-
electrode system. The fabricated PbO2-based electrode was used as
anode, a stainless steel sheet as counter electrode and a SCE as
reference electrode. The electrolyte solution was 2 M H2SO4 and
the cell temperature was ꢅ60 ꢃC. A DC power supply was used to
provide a constant current density of 1 A/cm2. The anode potential
was monitored during tests, the service lifetime of an electrode is
defined as the operation time at which the anodic potential
increased rapidly by 5 V (vs. SCE).
2
Fig. 2. XRD of GNS and GNS-PbO2 electrodes deposited b-PbO2 outer layer for 5, 10
and 15 min.
and GNS-PbO2 electrodes deposited
different time measured in a range of 2
be seen that there is only one broad peak at
b
-PbO2 outer layer for
from 10ꢃ to 90ꢃ. It can
u
2u
= 26.7ꢃ
corresponding to C (002) plane for GNS. The absence of other
diffraction peaks indicates the even structure of graphene and the
uniform packing of GNS [41]. For GNS-PbO2 electrode deposited
-PbO2 outer layer for 5 min, apart from the diffraction peak of C
(002) existed at 26.7ꢃ, the diffraction peaks observed at 2 = 25.4ꢃ,
32.0ꢃ, 36.2ꢃ, 49.0ꢃ and 62.5ꢃ are assigned to the (110), (101), (200),
(211) and (301) crystal faces of -PbO2, respectively, which shows
that part of GNS was exposed or the formed -PbO2 outer layer was
b
u
b
b
2.8. Electrochemical oxidation of 2-CP
too thin. After prolonging the deposition time to 10 and 15 min, the
diffraction peak of C (002) disappeared, indicating that the GNS
The electro-catalytic oxidation of 2-CP was carried out using
batch process. In combination with the anode of PbO2 electrodes
described above, a stainless steel sheet with the same area was
used as cathode. Electrolyte was 50 mg/L 2-CP solution with a
volume of 200 mL, and 0.05 M Na2SO4 was used as supporting
electrolyte. The electro-catalytic oxidation was performed at a
current density of 50 mA/cm2 and at a temperature of 30 ꢃC. The
detection of 2-CP and intermediates formed during oxidation
degradation was carried out using high-performance liquid
chromatography (HPLC, Shimadzu, Japan), equipped with an
interlayer was completely covered by
can be noticed that, with prolonging deposition time of
outer layer, the predominantly crystal phases were still
but the diffraction intensity of (200) planes decreased, while the
diffraction intensities of (301) plane increased significantly. This
may be related to the sheet like morphology of GNS. The
grew with (200) plane preferentially oriented parallel to the
graphene sheets near the GNS interlayer, and this effect became
b-PbO2 outer layer. It also
b
-PbO2
b
-PbO2,
b-PbO2
less with increasing the thickness of
b-PbO2 film. This effect also
has been described by other authors [42–44]. From their studies, it
can be concluded that the intensities of (200) diffraction peak of
the modified PbO2 electrodes (e.g., Bi-doped PbO2, La-Y-codoped
PbO2, Al-doped PbO2) were stronger than those of pure PbO2
electrode, and these modified PbO2 electrodes exhibited higher
electrocatalytic activity than pure PbO2 electrode. Therefore, the
GNS interlayer should helpful to increase the electrocatalytic
activity of PbO2 electrode.
Above XRD results show that the preferred orientation was
different for the GNS-PbO2 electrodes and traditional PbO2
electrode, which maybe result from the different grain size and
crystal morphology for the GNS-PbO2 electrodes and traditional
PbO2 electrode. Thus, the average grain sizes of PbO2 crystals were
estimated from the half height width of strong diffraction peaks
using the Debye-Scherrer equation [45]:
Agilent TC-C18 column (250 mm ꢁ 4.6 mm, 5
mm) and a variable
wavelength UV–vis detector. The mobile phase was 55/45
methanol/H2O with a flow rate of 1.0 mL/min, and at a column
temperature of 30 ꢃC. The analytical wavelength was set at 265 nm.
The identification of intermediates was performed through
comparing the chromatographic retention time with those of
commercially available standard compounds. TOC was measured
using a TOC analyzer (ELAB, China). On the basis of the TOC data,
the current efficiency (CE) for the electrochemical treatment of 2-
CP was calculated according to the following formula [40]:
ðTOCÞ0 ꢂ ðTOCÞt
CE ¼ 2:67FV
ꢁ 100%
ð1Þ
8IDt
where (TOC)0 and (TOC)t are the total organic carbon (g/L) at time
zero and t (s), respectively, I is the current (A), F is the Faraday
constant (96,487C/mol), and V is the electrolyte volume (L).
k
l
D ¼
ð2Þ
b
cos
where D is the crystallite size,
full width at half maximum of the peak, and
u
3. Results and discussion
l
is the X-ray wavelength,
b
is the
u
is the diffraction
3.1. Morphological and chemical characterization of GNS-PbO2
electrodes
angle. The result is shown in Table 1. It can be seen that the GNS-
PbO2 electrode deposited -PbO2 outer layer for 5 min (denoted as
b
GNS-PbO2–5 min) had the smallest grain size of 40.16 nm, and it
increased with prolonging deposition time. When the deposition
XRD was carried out to identify the components present in the
films of GNS-PbO2 electrodes. Fig. 2 shows the XRD patterns of GNS

X. Duan et al. / Electrochimica Acta 240 (2017) 424–436
427
Table 1
Crystalline grain diameter.
Electrode
Crystallite size/nm
(101) plane
(200) plane
(211) plane
(301) plane
Average value
GNS-PbO2–5 min
GNS-PbO2–10 min
GNS-PbO2–15 min
Traditional PbO2
43.06
45.98
58.40
54.95
45.98
48.69
55.13
48.58
36.31
41.21
43.19
45.39
35.31
40.04
36.76
41.91
40.16
43.98
48.37
47.71
Fig. 3. (a) SEM image of GNS interlayer. (b, c, d) SEM images of GNS-PbO2 electrodes deposited
b-PbO2 outer layer for 5, 10 and 15 min, respectively. (e) SEM image of
traditional PbO2 electrode. (f) SEM cross-section for GNS interlayer.

428
X. Duan et al. / Electrochimica Acta 240 (2017) 424–436
time was 15 min, the GNS-PbO2 electrode had the similar crystal
size with the traditional PbO2 electrode. So, the smaller -PbO2
grains can be formed near GNS interlayer. This phenomenon might
be explained by the electrochemical deposition mechanism of
PbO2 electrode described as following: [45]
Fig. 3a shows the top view of GNS interlayer, and a random and
homogeneous GNS film was formed on the -PbO2 inner layer. The
formation of GNS film on the -PbO2 inner layer was ascribed to
the strong adsorption between GNS and negatively charged LAS in
electrolytes solution and some possible functional groups such as
epoxide, hydroxyl, carbonyl, and carboxyl groups on the surface of
GNS, which allows the GNS to have a negatively charged surface
and be readily dispersed in the deposition solution [46]. The
negatively charged GNS in electrolyte solution moved toward the
anode surface under the applied field, where they were deposited
b
b
b
H2O !
ꢀ
OH + H+ + eꢂ
(3)
(4)
(5)
Pb2+
+
ꢀ
OH ! Pb(OH)2+
on the
-PbO2 inner layer can be improved after drying [47]. Evolution of
-PbO2 outer layer on GNS interlayer was followed by SEM
b-PbO2 inner layer. The adherence of the GNS film to the
b
b
Pb(OH)2+ + H2O ! Pb(OH)22+ + H+ + eꢂ
observation of samples at different electrodeposition time.
Representative images of the samples obtained at 5, 10 and
15 min are shown in Fig. 3b–d. After 5 min, lots of spherical
particles were formed on GNS interlayer, but not a compact film.
From the partial enlargement of particles shown in the inset of
Fig. 3b, it can be seen that the particles were composed of perfect
Pb(OH)2 ! PbO2 + 2H+
(6)
2+
In the initial stage of electrodeposition of
b
-PbO2 outer layer,
more
GNS interlayer due to its larger surface area, and then more
crystal nucleus can be formed via eqn (4) to (6), which inhibited
the -PbO2 crystal growth. Besides, graphene nanosheets may
have the confinement effect for the growth of -PbO2 crystals.
However, with the prolonging of electro-deposition time, the
graphene nanosheets were completely covered by the -PbO2
crystals, and then the grain size of -PbO2 increased.
ꢀ
OH groups generated from eqn (3) would be absorbed on the
b
-PbO2
octahedral
0.5 m, which is only approximately one-tenth of that of
traditional PbO2 electrode. After 10 min, a uniform and compact
-PbO2 film was formed and the average size of -PbO2 crystals
b-PbO2 microcrystals, whose average size is about
m
b
b
b
b
significantly increased. With further prolonging the electrodepo-
sition time to 15 min, and the crystal size was nearly the same as
b
b
Fig. 4. TEM of GNS (a), GNS-PbO2 film (b,c) and traditional PbO2 film (d).

X. Duan et al. / Electrochimica Acta 240 (2017) 424–436
429
that of traditional PbO2 electrode (Fig. 3e). This result is consistent
with the findings of XRD, which was ascribed to the larger surface
area and nanostructure of graphene nanosheets. Further informa-
tion about micro morphology and thickness of GNS interlayer can
be obtained from the SEM image of cross section shown in Fig. 3f.
referencing the C 1 s peak at 285.0 eV. The general XPS scan spectra
of two electrodes (Fig. 5a) both show sharp XPS peaks for O and Pb.
In addition, the C was also be detected during the elemental survey
scans, which could be originated from the organic impurities in
electrodeposition solution. Fig. 5b shows Pb 4f core level XPS
spectra. The Pb 4f spectra display two splitting peaks of Pb 4f7/2 and
Pb 4f5/2 at 136.3 and 141.2 eV, respectively, due to the electron spin
and orbit coupling, identified with the spectral values for PbO2
[48]. Fig. 5c shows the high resolution XPS spectra of O 1s. The O 1 s
spectra revealed two peaks at 528.4 and 530.5 eV: the narrow one
at the lower binding energy was assigned to the lattice oxygen
species (Olat) which were incorporated into PbO2 crystal lattice,
while the broad one at higher energy was attributed to weakly
bound oxygen species (Oads) (e.g. hydroxyl group) [8,43,49]. The
presence of Oads was attributed to the occurrence of lead cation
disorder in the crystallographic structure. Each missing Pb4+ ion
would be compensated by Pb2+ and ?=ꢂ ions, and the chemical
Obviously, there were significant boundaries between
inner layer, GNS interlayer and -PbO2 outer layer. The thickness of
GNS interlayer was about 5 m. It’s worth noting that the -PbO2
inner layer, GNS interlayer and -PbO2 outer layer were formed
without any gaps, and the GNS interlayer and -PbO2 outer layer
didn’t detached from the -PbO2 inner layer, even we cut off the
films to prepare the sample of cross section (Fig. 3f), which indicate
that the GSN layer can exist stably between the -PbO2 layers. It
can be explained that there were a large number of -PbO2
microcrystals were formed between graphene nanosheets in the
process of the electrodeposition of -PbO2 outer layer, which
-PbO2 inner layer, GSN layer and -PbO2
b-PbO2
b
m
b
b
b
b
b
b
b
tightly connected the
outer layer.
b
b
composition
is
described
by
the
formula
of
Pbð41þꢂxꢂyÞPb2ð2þꢂ4xꢂ2yÞOHꢂð4xþ2yÞ[49]. The stronger intensity of Olat
More details can be observed in the TEM images of GNS, GNS-
PbO2 film and traditional PbO2 film. As shown in Fig. 4a, the GNS
had a wrinkled sheet like morphology with high transparency,
indicating that the delaminated GNS was very thin. A large number
of nanoscale PbO2 particles with obvious grain boundaries were
presented in the GNS-PbO2 film (Fig. 4b and c). However, the grain
size of traditional PbO2 was so large that no grain boundary was
observed in PbO2 film (Fig. 4d). This result once again proved that
and weaker intensity of Oads for GNS-PbO2 electrode imply that the
GNS-PbO2 electrode electrode has less deviation of PbO2 crystals
than traditional PbO2 electrode and should possess higher electro-
catalytic activity. The result is probably due to the higher degree of
crystallinity and higher order degree of atomic lattice for GNS-
PbO2 electrode than for traditional PbO2 electrode.
smaller
b-PbO2 crystals were easily attained on GNS layer due to
3.2. Electrochemical properties of GNS-PbO2 electrode
the large surface area and nanostructure of graphene nanosheets.
XPS analysis of GNS-PbO2 electrode and traditional PbO2
electrode were also conducted to obtain the surface chemical
states. The binding energy (BE) for each sample was corrected by
The above mentioned results of SEM and XRD showed that the
b-PbO2 crystals were fine and compact when the deposition time
of outer layer was 10 min. Thus, the electrochemical properties and
a
Traditional PbO2
GNS-PbO2
1200 1000
800
600
400
200
0
Binding Energy (eV)
528.4
530.5
c
b
136.3
141.2
Traditional PbO2
Traditional PbO2
GNS-PbO2
GNS-PbO2
132
136
140
144
148
152
525
530
535
540
545
Binding energy (eV)
Binding energy (eV)
Fig. 5. X-ray photoelectron spectroscopy (XPS) analysis of GNS-PbO2 and traditional PbO2 electrodes. (a) general scan; (b) Pb 4f spectra; (c) O 1s spectra.

430
X. Duan et al. / Electrochimica Acta 240 (2017) 424–436
a
b
0.04
0.02
0.02
GNS-PbO2
Traditional PbO2
0.01
0.00
0.00
10 mV/s
20 mV/s
30 mV/s
40 mV/s
50 mV/s
-0.02
-0.04
-0.06
-0.01
-0.02
-0.03
10 mV/s
20 mV/s
30 mV/s
40 mV/s
50 mV/s
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Potential (V vs. SCE)
Potential (V vs. SCE)
0.7
c
25
d
0.6
0.5
0.4
0.3
0.2
0.1
Traditional PbO2
GNS-PbO2
Traditional PbO2
20
15
10
5
GNS-PbO2
0
0.12
0.16
0.20
0.24
0.28
0.32
0
10
20
30
40
50
60
70
80
v-0.5(mV0.5/s0.5)
Zʹ (Ω)
Fig. 6. (a, b) Cyclic voltammograms obtained on GNS-PbO2 and traditional PbO2 electrodes in 0.5 M H2SO4 solution. (c) Relationship of voltammetric charge quantity (q*)
versus the square root of scan rate. (d) Nyquist plots of GNS-PbO2 and traditional PbO2 electrodes. (e) Equivalent circuit and obtained electrochemical parameters in the
analysis of experimental EIS data.
service lifetime of the GNS-PbO2 electrode deposited
layer for 10 min were further studied and compared with
traditional PbO2 electrode below.
The electrochemical activity of PbO2 electrode is related to its
real surface area and the number of active sites accessible to the
electrolyte. The voltammetric charge quantity (q*) is considered to
be related to real surface area and the number of active sites of
electrodes [45,50–52]. Thus, the q* of GNS-PbO2 electrode was
b
-PbO2 outer
evaluated by integration of the cyclic voltammetric curves over the
whole tested potential range from 0.5 to 2.0 V (vs. SCE). Fig. 6a and
b show cyclic voltammograms of GNS-PbO2 and PbO2 electrodes,
respectively. The relationship of q* against the reciprocal of square
root of scan rate is shown in Fig. 6c. The q* values of GNS-PbO2
electrode were over 2 times those of traditional PbO2 electrode at
each scan rate, indicating a remarkable higher electrochemical
activity of GNS-PbO2 electrode than traditional PbO2 electrode.

X. Duan et al. / Electrochimica Acta 240 (2017) 424–436
431
This is most probably attributed to the smaller crystals and larger
specific surface area of GNS-PbO2 electrode, which can provide
more active sites for electrochemical oxidation.
of consecutive electrochemical degradation of 2-chlorophenol
were conducted over GNS-PbO2 anode. After each run, the GNS-
PbO2 anode was rinsed by deionized water and dried naturally, and
then directly used in the next run. As shown in Fig. 8a, no obvious
decrease of activity was observed, suggesting the excellent
stability of GNS-PbO2 electrode for electrocatalytic oxidation
process. In order to estimate the stability of GNS-PbO2 electrode in
a short time, the accelerated life tests also were conducted under
strongly acidic conditions with a large current density of 1 A/cm2.
Fig. 8b shows the variation of anode potential with electrolysis
time in the accelerated life test for GNS-PbO2 and PbO2 electrodes.
The service lifetime of an electrode is designed the operation time
at which the anodic potential increased rapidly by 5 V (vs. Ag/
AgCl). According to Fig. 8b, we can find that the GNS interlayer can
obviously extend the lifetime of PbO2 electrodes. The accelerated
lifetime of GNS-PbO2 electrode is 107.9 h, which is about 1.93 times
as long as that of PbO2 electrode (55.9 h). Interestingly, it also was
found that from Fig. 8b, the increase of potential for GNS-PbO2
electrode took place in two stages. The first stage with a small but
significant increase occurred at the time range from 55 to 60 h, and
the second stage with a significantly large increase occurred at 104
to 108 h. The increase of anode potential of metal oxide electrodes
in accelerated lifetime tests is the consequence of the rapid
decrease of oxide loading and the growth of an insulating TiO2
phase on the coating [38,53]. Therefore, the first increase of
potential may be caused by the dissolution, detachment and
The charge transfer resistance of GNS-PbO2 and PbO2 electrodes
was also investigated using EIS. The Fig. 6d displays the
electrochemical impedance spectra obtained in 0.5 M H2SO4
solution. It can be clearly seen one semicircle appeared in the
electrochemical impedance spectra for both GNS-PbO2 and PbO2
electrodes. The equivalent circuit used to fit the EIS data is shown
in Fig. 6e. In this Rs(RctCPE) circuit, Rs, the intersection of plots with
the real axis at the high-frequency end, represents the resistance of
electrolyte and active material, Rct, the diameter of the semicircle
size, stands for charge-transfer resistance [30], and CPE introduced
to replace the electric double layer capacitance. The calculated
values of Rs, Rct, and CPE are listed in Fig. 6e. The values of Rct of
GNS-PbO2 and PbO2 electrodes were 26.32 and 69.94
V, indicating
a faster transfer of electrons on GNS-PbO2 electrode than for PbO2
electrode. In addition, the Rct also reflects the oxygen evolution
reaction activity, which depends on the active sites of electrodes
[23]. Hence, the lower Rct of GNS-PbO2 electrode further proves the
larger electrochemical active surface area of GNS-PbO2 electrode.
It is well known that the OH radicals generation ability can also
ꢀ
be used to evaluate the electro-catalytic activity of PbO2 electrode,
because the pollutants are mainly degraded by the indirect
oxidation by
ꢀ
OH radicals [23]. Hence, the OH radicals generation
ꢀ
ability of GNS-PbO2 and PbO2 electrodes was determined using
fluorescent spectrometry. During the electrochemical reaction, the
passivation of
should be attributed to the stripping of
inner layer and SnO2-Sb2O3 layer. Thus, we deduce that there are
two reasons for the extension of lifetime of GNS-PbO2 electrode.
First, GNS interlayer reduced
b
-PbO2 outer layer, while the second increase
-PbO2 inner layer, -PbO2
terephthalic acid was added into electrolyte as
agent, which can readily react with the generated
ꢀ
OH radicals capture
OH radicals to
b
a
ꢀ
produce highly fluorescent product of 2-hydroxyterephthalic acid.
The fluorescence intensity of 2-hydroxyterephthalic acid repre-
b-PbO2 particle size, a compact and
sents the amount of generated
the fluorescence intensity of 2-hydroxyterephthalic acid around
425 nm gradually increased with the increase of electrochemical
reaction time, demonstrating
generated during electro-catalytic oxidation. Comparing the
fluorescence intensity for GNS-PbO2 and PbO2 electrodes, it can
be found that the fluorescence intensity for the GNS-PbO2
electrode was nearly 2 times that for traditional PbO2 electrode,
which demonstrates once again that the GNS-PbO2 electrode had
higher electro-catalytic activity of than the traditional PbO2
electrode. This may be ascribed to the higher active surface of
GNS-PbO2 electrode than the traditional PbO2 electrode, which
ꢀ
OH radicals. As can be seen in Fig. 7,
fine surface layer was made which is observed from SEM image in
Fig. 3c. The compact surface of GNS-PbO2 electrode can baffle the
penetration of the electrolyte toward the titanium substrate
through the cracks and pores and inhibit the formation of
ꢀOH radicals were constantly
insulating TiO2 film [23]. Second, the
b
-PbO2 outer layer and
b-PbO2 inner layer of GNS-PbO2 electrode provide double
protection. Once the
b-PbO2 outer layer broke off, the b-PbO2
inner layer could continue to protect substrate to be corroded,
which can be proved by the two stages of potential increase.
3.3. Electrochemical oxidation for 2-chlorophenol
provided more active sites to generate more
Fig. 7 High electrochemical stability of electrodes is very
important for industrial application. Regarding this issue, a series
ꢀ
OH radicals.
Based on the above results, the GNS-PbO2 electrode should
exhibit a higher catalytic activity than traditional PbO2 electrode in
electrochemical oxidation process. Therefore, the electrochemical
b
30min
500
400
300
200
100
0
250
200
150
100
50
30min
25min
a
25min
20min
15min
10min
20min
15min
10min
5min
0min
5min
0min
0
380 400 420 440 460 480 500 520
Walvelength (nm)
380 400 420 440 460 480 500 520
Walvelength (nm)
Fig. 7. Fluorescence spectra observed during electro-catalytic oxidation processes using GNS-PbO2 (a) and traditional PbO2 (b) electrodes as anode.

432
X. Duan et al. / Electrochimica Acta 240 (2017) 424–436
100
80
60
40
20
0
a
b 12
GNS-PbO2
Traditional PbO2
11
10
9
8
7
6
5
0
20
40
60
80
100
120
1
2
3
4
5
6
7
8
9
10
Run
Time (h)
Fig. 8. (a) Evolution of 2-CP removal efficiency during ten electrocatalytic running over GNS-PbO2 anode and (b) variation of anode potential with electrolysis time in the
accelerated life test.
0.0025
0.0025
b
a
1th cycle
2th cycle
3th cycle
4th cycle
5th cycle
0.0020
0.0015
0.0010
0.0005
0.0000
-0.0005
-0.0010
1st cycle
2nd cycle
3rd cycle
4th cycle
5th cycle
GNS-PbO2
0.0020
0.0015
0.0010
0.0005
0.0000
-0.0005
-0.0010
Traditional PbO2
0.6
0.8
1.0
1.2
1.4
1.6
0.6
0.8
1.0
1.2
1.4
1.6
Potential (V vs SCE)
Potential(V vs SCE)
Fig. 9. Cyclic voltammograms of GNS-PbO2 electrode (a) and traditional PbO2 electrode (b) measured in 0.5 mol/L Na2SO4 solution containing 200 mg/L 2-CP, scan rate:
50 mV/s.
b100
a 100
3.3
GNS-PbO2
Traditional PbO2
90
3.0
2.7
2.4
2.1
1.8
1.5
80
70
60
50
40
30
20
10
0
80
GNS-PbO2 y=0.0275x-0.1661 R2=0.9909
PbO2
y=0.0176x-0.1468 R2=0.9803
60
40
20
0
3
2
1
0
Traditional PbO2
GNS-PbO2
GNS-PbO2
Traditional PbO2
0
20
60 80
4T0ime (min) 100
120
0
20
40
60
80 100 120 140 160
Time (min)
0
30
60
90
120
Time (min)
Fig.10. (a) 2-CP removal ratio on GNS-PbO2 and traditional PbO2 electrodes. Inset shows reaction kinetics of 2-CP removal. (b) TOC removal on GNS-PbO2 and traditional PbO2
electrodes. Inset shows current efficiency.

X. Duan et al. / Electrochimica Acta 240 (2017) 424–436
433
oxidation of 2-CP on GNS-PbO2 anode was further studied using CV
and degradation tests.
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Fig. 9a shows the cyclic voltammetry curves for the electro-
chemical oxidation of 2-CP on GNS-PbO2 anode in 0.5 mol/L
Na2SO4 solution containing 200 mg/L 2-CP at a scan rate of 50 mV/
s. A significant oxidation peak presented at about 1.3 to 1.4 V
potential in the first scan, corresponding to the direct oxidation of
2-CP on the GNS-PbO2 electrode. However, like other phenolic
pollutants [54,55], the oxidation peak disappeared and the
reaction current of oxygen evolution was reduced gradually in
the subsequent scans because of the passivation of electrodes,
which was caused by the polymer formed in the first step of
oxidation. Fortunately, this passive film could be destroyed and the
electrode could be restored in an anodic polarization process in
aqueous solution at the water decomposition potential ( > 2.5 V)
with the same supporting electrolyte [56]. The similar phenome-
non occurred on traditional PbO2 electrode (Fig. 9b), but the
oxidation peak current on the GNS-PbO2 electrode was higher than
for traditional PbO2 electrode. It indicated that the GNS-PbO2
electrode had higher oxidation activity than traditional PbO2
electrode for the direct oxidation of 2-CP. In addition, an oxidation
peak at about 1.25 V and a corresponding reduction peak at about
0.9 V were observed from the cyclic voltammetry curves for both
electrodes, which corresponded to the PbSO4/PbO2 and PbO2/
PbSO4 transformation, respectively [57]. It also can be discovered
that the oxidation peak current of PbSO4/PbO2 was reduced and
oxidation peak shifted to more positive potential after the first scan
due to the passivation of electrode [58,59].
Fig. 10a shows the concentration evolution of 2-CP on GNS-
PbO2 and traditional PbO2 electrodes as a function of electrolysis
time. Clearly, the GNS interlayer significantly improved the 2-CP
removal efficiency. The 2-CP removal efficiency was 95.42% when
the GNS-PbO2 electrode was used as anode with an electrolysis
time of 120 min, while only 86.26% for PbO2 electrode. The
degradation processes were fitted with a pseudo first-order
reaction rate equation. As shown in the inset of Fig. 10a, the
straight lines of electrolysis time versus natural logarithm of the
concentration of 2-CP were obtained. According to the kinetics
curve data, we can conclude that the electrochemical oxidation of
2-CP is a pseudo-first order reaction. The apparent rate constant on
GNS-PbO2 electrode (kapp = 2.75 ꢁ10ꢂ2 minꢂ1) is much higher than
for traditional PbO2 electrode (kapp = 1.76 ꢁ10ꢂ2 minꢂ1). The TOC
reduction of degradation solution can reflect the mineralization
capability of the different electrodes. Thus, TOC removals of 2-CP
degradation solution for GNS-PbO2 and traditional PbO2 electrodes
were compared. As shown in Fig. 10b, TOC was removed more
rapidly on the GNS-PbO2 electrode than for traditional PbO2
electrode. For these two electrodes, 55.09% and 40.09% of TOC
removal were achieved after 2 h respectively. Besides, the CE for
the electrochemical oxidation on the two electrodes was also
calculated according to TOC values. In accordance with above
results, the CE for the GNS-PbO2 electrode (2.91%, 2 h) was higher
than for the traditional PbO2 electrode (2.16%, 2 h). The results
show that the GNS-PbO2 electrode was more efficient for 2-CP
oxidation than traditional PbO2 electrode. It is well known that the
Clꢂ anions could be released into electrolyte due to dechlorination
during electrocatalytic degradation of 2-CP, and then active
chlorines species such Cl2 and hypochlorous acid (HClO) are
formed, upon oxidation of Clꢂ anion at PbO2 by reactions (7) and
(8) [60].
0 min
15 min
30 min
60 min
90 min
120 min
150 min
180 min
200 225 250 275 300 325 350 375 400
Wavelength (nm)
Fig. 11. UV absorption spectra of 2-CP degradation solution at different electrolysis
time.
Thus, the generated Cl2 and HClO should beneficial to
enhancing the CE. However, Zhu et al. studied the influence of
NaCl concentration on the electrocatalytic degradation of BPA and
their experimental results showed that few ClOꢂꢂ and Cl2 was
generated when the NaCl concentration was less than 0.020 mol/L,
and the COD removal efficiency for 0.010 mol/L NaCl as supporting
electrolyte was nearly same with that for 0.020 mol/L Na2SO4 as
supporting electrolyte [61]. In our study, the initial concentration
of 2-CP was only 0.389 ꢁ10ꢂ3 mol/L, even if all the chlorine was
departed from 2-CP into electrolyte, the concentration of Clꢂ was
much less than 0.010 mol/L. Therefore, Clꢂ anions would not
influence the CE variation in our study.
3.4. The degradation mechanism of 2-CP
The ultraviolet (UV) absorption spectrum was used to monitor
the changes of 2-CP during the different periods of electrochemical
oxidation. As shown in Fig. 11, there were two obvious UV
absorption bands at 273 nm and 279 nm for 2-CP before treatment.
During the degradation process, the two absorption bands
decrease with the electrolysis time proceeded and almost totally
disappeared when electrolysis time reached 150 min, it inferred
that the aromatic ring of 2-CP was opened during electrolysis. It
also can be observed from spectra that a plateau appeared around
240–250 nm from 30 to 90 min of electrolysis, and a new band
around 265 nm appeared at 15 min, and then they also decreased
with the degradation time proceeded. The appearance of plateau
and new peak indicates that benzoquinone and other aromatic
intermediates were formed during the degradation process of 2-CP
[8]. Total disappearance of the all bands at the end of electrolysis
suggests that all of the 2-CP and its aromatic intermediates were
degraded during electrochemical oxidation process.
The probable intermediate products were analyzed using HPLC,
and the results are shown in Fig. 12. Only the peak of 2-CP with
retention time of 4.6 min presented in initial chromatogram
(electrolysis time equal to 0 min), and its intensity was continu-
ously reduced during the course of electrolysis. Prolonging the
degradation time, some new peaks appeared, indicating that some
intermediates were formed during the degradation process. The
intermediates were identified through comparison of retention
time with those of the standard compounds. The products with
retention time of 2.253, 2.724 and 3.407 min were aromatic
compounds of catechol, phenol and ortho-benzoquinone, respec-
tively, and other products with retention time of 1.286, 1.612 and
1.748 were organic acids of oxalic acid, maleic acid and glutaconic
acid, respectively. However, HCOOH weren’t detected in our study,
2Clꢂ ! Cl2 (aq) + 2eꢂ
(7)
Cl2 (aq) + H2O ! HClO + Cꢂ + H+
(8)

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X. Duan et al. / Electrochimica Acta 240 (2017) 424–436
3
2
1
0
3
2
1
0
0.3
a
b
180 min
0.2
0.1
0.0
0.3
0.2
0.1
0.0
0.3
0.2
0.1
0.0
0.3
0.2
0.1
0.0
0.3
0.2
0.1
0.0
90 min
60 min
120 min
90 min
60 min
3
2
1
0
3
2
1
0
3
2
1
0
3
2
1
0
3
2
1
0
3
2
1
0
catechol
30 min
phenol
30 min
15 min
maleic acid
15 min
ortho-benzoquinone
oxalic acid
5 min
5 min
Glutaconic acid
2-CP
0 min
0
1
2
3
4
5
6
7
1
2
3
4
Retation time (min)
Retation time (min)
Fig. 12. (a) HPLC chromatograms of 2-CP degradation at different electrolysis time, (b) partial enlarged HPLC chromatograms of 2-CP degradation at 30, 60, 90 and 120 min.
O
C
OH
O
OH
OH
Cl
OH
O
·OH
·OH
·OH
·OH
CO2
+
H2O
+
HCOOH
C
OH
O
2-chlorophenol
ortho-benzoquinone
glutaconic acid
formic acid
catechol
OH
H
O
·
O
C
OH
OH
HO
C
O
O
·OH
·OH
phenol
CO2 + H2O
C
HO
C
O
maleic acid
oxalic acid
Fig. 13. Proposed degradation pathway of 2-CP during electrochemical oxidation.
though frequently detected in the degradation of cholophenols
[1,62]. This may be due to that HCOOH, as one kind of small
molecule acid, was easily degraded into CO2 and H2O on GNS-PbO2
anode, and its accumulation in solution was too small to be
detected.
Based on the above intermediates, we proposed the main
reaction pathway for electrochemical degradation of 2-CP on the
GNS-PbO2 electrode as illustrated in Fig.13. Firstly, via the attack of
C—Cl bonds by hydroxyl radicals, part of 2-CP was dechlorinated
and changed to phenol, while others turned into catechol through

X. Duan et al. / Electrochimica Acta 240 (2017) 424–436
435
the hydroxyl radicals addition. The phenol is also subsequently
transformed to catechol by hydroxyl radicals attack. Further, the
catechol was converted to the ortho-benzoquinone, which would
be oxidized and ring cleavage small fragmented products of formic
acid and glutaconic acid were generated. Then the glutaconic acid
was oxidized into the short organic acids of oxalic acid and maleic
acid. Finally, oxalic acid and formic acid could be mineralized into
CO2 and H2O.
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The results of this study showed that the GNS-PbO2 electrode
had perfect octahedral and fine
b-PbO2 microcrystals. Comparing
with the traditional PbO2 electrode, the GNS-PbO2 electrode
demonstrated higher voltammetric charge quantity q*, lower
charge transfer resistance, stronger capacity of ꢄOH radicals
production. The accelerated lifetime test data exhibited that the
service lifetime of GNS-PbO2 electrode is 107.9 h, which is about
1.93 times as long as that of PbO2 electrode (55.9 h). During the
process of electrochemical degradation of 2-CP, the GNS-PbO2
electrode exhibited much higher apparent rate constant (kapp
=
2.75 ꢁ10ꢂ2 minꢂ1) than traditional PbO2 electrode (kapp = 1.76 ꢁ10
ꢂ2 minꢂ1). The intermediate products for 2-CP degradation were
aromatic compounds (catechol, phenol and ortho-benzoquinone)
and organic acids (oxalic acid, maleic acid and glutaconic acid),
which were mineralized into CO2 and H2O at the end of
electrochemical oxidation. These results indicate that PbO2
electrode with a GNS interlayer could be a good candidate as
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Acknowledgments
This project was supported by the Young People Fund of Jilin
Science and Technology Departmen (20150520079JH), the Science
and technology Project of Jilin Education Bureau (No. 2016225),
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