2018 - Chemically reduced graphene oxide paper as positive electrode for advanced Zn_Ce redox flow battery

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Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Chemically reduced graphene oxide paper as positive electrode for advanced
Zn/Ce redox flow battery
Zhipeng Xiea,b,∗, Baolu Liua, Chenfan Xiec, Bin Yanga, Yunfen Jiaoa, Dingjian Caia, Liang Yanga,
Qi Shua, Anhong Shia
a Engineering Research Institute, Jiangxi University of Science and Technology, Ganzhou, 341000, China
b State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China
c School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin, 124221, China
H I G H L I G H T S
• The RGOC paper possesses good flexibility with a porous structure.
• The C/O ratio of RGOC is different from that of RGOT.
• The activity of RGOC towards Ce3+/Ce4+ is better than that of RGOT.
A R T I C L E I N F O
Keywords:
Graphene oxide
Redox flow battery
Cerium
New energy
Energy storage
A B S T R A C T
Zn/Ce redox flow battery (ZRFB) is emerging as a promising technology to store large amount of energy eco-
nomically and efficiently, wherein a highly efficient positive electrode with a continuous and fast electronic and
ionic transportation path is urgently desired. The unique nanostructure of reduced graphene oxide (RGO) paper
electrode is prepared by a simply chemical reduction method, which facilitates transference of the electron and
Ce3+/Ce4+ at the electrolyte/electrode interface. Thus ZRFB exhibits superior extent of charge (81.0%) and
energy efficiency (71.3%). The results show RGO paper is a good candidate for positive electrode of ZRFB.
1. Introduction
Energy is of importance to all of us. It is necessary to develop new
energy resources [1–3] for reducing environmental pollutants [4] and
improving human life level. However, the more widespread use of them
is dependent upon the development of an affordable and reliable energy
storage system. Redox flow batteries (RFBs) have received increasing
attention for their storing enormous amount of electrical energy
friendly and efficiently [5,6]. For a typical RFB, its electroactive species
are dissolved in two electrolytes stored in separate tanks instead of in
electrodes, which is different from traditional batteries. The cell reac-
tions occur when electrolyte flows through electrode with the help of
pump. The electrode only offers a place where electrode reaction occurs
without undergoing any deformation, which is helpful to prolong the
service life of the battery. The capacity of RFB is determined by the
amount of electroactive species in electrolyte, while the power output
by the size of the electrode. For example, the greater the amount of
Ce3+/Ce4+ in positive electrolyte, the greater the capacity of ZRFB.
The architecture characteristic of separation of power output from ca-
pacity gives RFB considerable design flexibility.
It is of fundamental importance to select suitable electrode material
for optimization and upgrading of battery performance. The factors to
be considered in selection of electrode materials for RFB application
include conductivity, mechanical strength, chemical stability and
electrochemical activity. Generally, selection of electrode material for
RFB application is the process of finding a balance point in the above
mentioned factors based on the specific use. The kinetic characteristics
of Ce3+/Ce4+ electrode reaction on glassy carbon, platinum, platinized
titanium, carbon felt, graphite, porous carbon, and graphene oxide/
graphite composite electrodes were investigated by different re-
searchers [7–18]. Although some achievements have been made in the
research of electrode, it is still necessary to search for alternative ma-
terials with better performance toward effective positive electrode for
advanced ZRFB application.
Graphene-based materials have attracted significant attention for
their excellent mechanical and electrical properties [19–24], which can
https://doi.org/10.1016/j.matchemphys.2018.08.082
Received 29 September 2017; Received in revised form 8 April 2018; Accepted 24 August 2018
∗ Corresponding author. No.86, Hongqi Ave., Ganzhou, Jiangxi, China.
E-mail address: zhpxie_06@126.com (Z. Xie).
Materials Chemistry and Physics 220 (2018) 208–215
Available online 27 August 2018
0254-0584/ © 2018 Elsevier B.V. All rights reserved.
T
http://www.sciencedirect.com/science/journal/02540584
https://www.elsevier.com/locate/matchemphys
https://doi.org/10.1016/j.matchemphys.2018.08.082
https://doi.org/10.1016/j.matchemphys.2018.08.082
mailto:zhpxie_06@126.com
https://doi.org/10.1016/j.matchemphys.2018.08.082
http://crossmark.crossref.org/dialog/?doi=10.1016/j.matchemphys.2018.08.082&domain=pdf
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Fig. 1. a)Working principle of Zn/Ce redox flow battery. b)Assembled ZRFB. 1. negative electrode, 2. positive electrode, 3. Nafion 115 membrane.
Z. Xie et al. Materials Chemistry and Physics 220 (2018) 208–215
209
be prepared in large scale by modifying graphene oxide (GO) with re-
duction reagent and/or thermal treatment method. There are wide
range of oxygen functional groups on both basal planes and edges of
GO, which provides more possibilities for applications in materials
science and nanocomposites [25]. Graphene oxide paper is a novel free-
standing paper-like material with good mechanical properties [25–28].
Herein, we report the first use of reduced graphene oxide paper as
positive electrode in advanced ZRFB. These results show that the re-
duced graphene oxide paper is an excellent candidate in the application
of ZRFB.
2. Experimental
2.1. Electrode preparation
The preparation of GO was carried out on the basis of the improved
Hummers’ method [28]. Briefly, graphite powders (99.95%, 1200
mesh, aladdin®) were oxidized by the concentrated H2SO4/H3PO4 and
KMnO4 to produce precursor, reoxidized by 30%-H2O2, then washed
using H2O, HCl and C2H5OH, respectively. The centrifugalization of the
mixture was performed at 12000 rpm for 10min and obtains graphite
oxide. Graphite oxide was further diluted in deionized water, ultra-
sonication for 30min to obtain graphene oxide aqueous solution [18].
Then, sodium borohydride solution was added into the GO aqueous
solution, which was accompanied by a violent reaction and the gen-
eration of reduced graphene oxide (i.e., chemically reduced graphene
oxide, RGOC). RGOC dispersion requires washing using H2O no less
than 3 times, then followed by freeze-drying under vacuum for 24 h.
The RGOC paper is obtained by pressing the RGOC powder at a pressure
of 10MPa. All the chemicals are AR grade, purchased from Shanghai
Aladdin Bio-Chem Technology Co., LTD (aladdin®, China).
2.2. Sample characterization
The characterization of RGOC paper was carried out by field emis-
sion scanning electron microscopy (HITACHI S-4800, Shanghai Yong
Ming Automation Equipment Co. LTD), X-ray diffraction patterns
(Bruker D8 Focus powder XRD, Xiamen Mingda Technology Co. LTD),
Raman scattering (Renishaw inVia spectrometer system, Suzhou Ruice
Precision Instrument Co. LTD) and X-ray photoelectron spectroscopy
(QUANTUM 2000 surface analysis system, Physical Electronics USA).
2.3. Electrochemical measurements
Several electrochemical analytical methods (cyclic voltammetry,
linear sweep voltammetry and impedance spectroscopy) were selected
for characterizing the electrochemical behaviour of Ce3+/Ce4+ redox
couple in a solution of 2mol/L CH3SO3H at RGOC paper electrode at
room temperature with a computer-controlled CHI660 (USA). A pla-
tinum mesh of 5.3 cm2 was used as counter electrode, and a saturated
calomel electrode (SCE) as the reference electrode with a salt bridge
eliminating the liquidjunction potential [29]. In the experiment, GO
paper, thermally reduced graphene oxide (RGOT) paper, and platinum
electrodes were compared with RGOC paper electrode.
2.4. Battery test
The working principle of ZRFB is illustrated in Fig. 1a and an as-
sembled cell is shown in Fig. 1b. The positive electrode and negative
electrode of zinc-cerium redox flow test cell are separated by an ion
exchange membrane (Nafion 115, Dupont) in positive half-cell (18mL)
and negative half-cell (18mL), where the electrochemical reactions
occurred when the electrolytes (each ca. 200mL in two external tanks)
flowed through with the help of pumps (12.0 mL/min) [18]. The po-
sitive electrode is reduced graphene oxide paper (3× 3×0.2 cm3)
with flexible graphite sheet served as current collector. The negative
electrode is zinc sheet (3×3×0.2 cm). The measurement of Zn/Ce
RFB was carried out under constant current of 180mA at the voltage
range between 2.6 and 0.5 V.
3. Results and discussion
3.1. XRD and Raman characterization
As shown in Fig. 2a, there is a big difference among the XRD pat-
terns of GO powder, RGO powder (RGOT, thermally reduced graphene
oxide; RGOC, Chemically reduced graphene oxide) and graphite
powder. The graphene oxide shows a characteristic diffraction peak
(001) at 10.02° with an interplanar spacing of 0.88 nm, which is caused
by the atomic scale roughness and the generation of oxygen-containing
functional groups attached on GO nanosheets [18]. When the graphene
oxide is reduced by thermal treatment under a nitrogen atmosphere at
200 °C for 2 h (corresponding to RGOT) or by NaBH4 (corresponding to
RGOC), the disappearance of the characteristic diffraction peak at
10.02° is accompanied by the emergence of a weak wide bulge centered
at 22.51° for RGOT and at 24.04° for RGOC. As for graphite powder, it
has a strong peak (002) at 26.54° with an interlayer spacing of 0.34 nm
and a weak peak (004) at 54.66° with an interlayer spacing of 0.17 nm,
which means that the graphite powder was partly oxidized [18].
Raman spectroscopy, a nondestructive and fast method for char-
acterization of the crystal structure, is then used to characterize the
disorder and defects in the as-prepared samples. The graphite is also
employed for comparison, which consists of fused aromatic ring
structures along direction in the plane. As seen in Fig. 2b, the graphite
crystal shows two bands: a strong one positioned at a Raman frequency
of 1569.9 cm−1 and a much weaker one at 1346.4 cm−1. The former
was assigned to the E2gC-C stretching mode, called G band and the
latter D band representing a disorderly network of sp2-coordinated
clusters. Generally, graphene of high quality also shows a weak D band.
In theory, the greater the quantity of the ring near a graphene edge or
the more the structure defect of a graphene, the greater the intensity of
D band. The material without any defect has a very weak, even no D
band. Hence, the disorder in a graphene can be reflected from the D
band intensity, and its measurement is often based on the ratio of D to G
band intensity. In this work, we prepared two kinds of reduced gra-
phene oxide, which are graphene with many defects. One is chemically
reduced graphene oxide (RGOC) prepared by the reduction of GO with
NaH4B solution, the other is thermally reduced graphene oxide (RGOT)
prepared by thermal treatment of GO under a nitrogen atmosphere at
200 °C for 2 h. GO is prepared by the oxidation of graphite. However,
the reduction degree of GO and the oxidation degree of graphite can not
be reflected from the ratio of D to G band intensity which can be the
result of interaction of other factors such as edges, defects and ripples.
The intensity ratios of D to G for GO, RGOT and RGOC are 0.84, 0.89
and 1.03, respectively. The increment of intensity ratio of D to G band is
also observed in other study on reduction of graphene oxide [29].
Therefore, the following conclusions can be obtained. The reduction of
GO, whether thermal ruduction or chemical reduction, can introduce a
large number of edges, defects and ripples to graphene. Furthermore,
the amount of edges, defects and ripples introduced by chemical re-
duction is more than that of by thermal reduction.
3.2. SEM
As shown in Fig. 3a, there is a big difference in colour between GO
paper and RGOC paper. GO paper is golden, while RGOC paper is black
(Fig. 3b). After reduction, the RGOC paper can still possess good flex-
ibility. It should be noted that dense and disordered aggregation and
stack problems are generally observed in most of the free standing
RGOC papers. Consequently, the favourable properties of individual
graphene sheets such as high surface area can not be achieved in RGOC
paper. In the present work, when GO dispersion is reduced using
Z. Xie et al. Materials Chemistry and Physics 220 (2018) 208–215
210
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Fig. 2. a)XRD patterns of GO, RGOT, RGOC and graphite powders. b)Raman stpectra of GO, RGOT, RGOC and graphite powders.
Z. Xie et al. Materials Chemistry and Physics 220 (2018) 208–215
211
Fig. 3. a) Digital images of GO paper, b) digital images of RGOC paper, c) and d) SEM images of RGOC paper.
Fig. 4. XPS spectra for the GO, RGOT and RGOC.
Z. Xie et al. Materials Chemistry and Physics 220 (2018) 208–215
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NaBH4, the obtained RGOC paper can still maintain a porous structure
(Fig. 3c and d), which could contribute to the formation of conductive
network in the paper electrode prepared by pressing RGOC powder. The
large surface area and easy pathway within RGOC paper electrode for
fast electrolyte ions diffusion partly result in fast kinetics of Ce3+/Ce4+
electrode reaction.
3.3. XPS
The characterization of reduction degree of GO can be carried out
by analysing the C/O ratio on the surface of GO using X-ray photo-
electron spectroscopy which is a surface-sensitive quantitative
spectroscopic technique requiring high vacuum or ultra-high vacuum
conditions. The C/O ratio would become bigger with the reduction of
GO. That is to say, the greater the C/O ratio, the greater the reduction
degree of GO. The XPS C1s spectra before and after GO reduction are
shown in Fig. 4. There are various oxygen-containing functional groups
on the surface and edge of GO including carboxylate, carboxyl, car-
bonyl, hydroxyl and epoxide. The carbon content in GO, RGOT and
RGOC is 67.33%, 68.5% and 84.75%, respectively. So, the reduction
degree of RGOC is greater than that of RGOT. It is observed that the
peak at about 286.6 eV became weaker after thermal treatment of GO,
even disappeared after chemical reduction of GO. The weakening or
disappearance of the peak at about 286.6 eV is cosistent with the
Fig. 5. Electrochemical analysis of Ce3+/Ce4+ redox couple. a) CV curves of the Ce3+/Ce4+ redox pair, test conditions: aqueous solvent, 20mM Ce(CH3SO3)3, 2 M
CH3SO3H as the supporting electrolyte; RGOC paper as working electrode (1.0 cm2), and Pt mesh and SCE as the counter electrode and the reference electrode,
respectively. b) Anodic peak current against the square root of scan rate for the Ce3+/Ce4+ redox pair. c) CV curves of the Ce3+/Ce4+ redox pair at GO, RGOT, RGOC
and Pt electrodes(each of 1.0 cm2), scan rate of 50mV s−1. d)Overpotential-current curves of 10mM Ce3+ + 10 mM Ce4+ + 2 M CH3SO3 at 1mV s−1.e)EIS curves
and the equivalent circuit used to fit the EIS results.
Z. Xie et al. Materials Chemistry and Physics 220 (2018) 208–215
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decrease or depletion of hydroxyl and epoxide on the surface and edge
of GO.
3.4. Electrochemical analysis
The evaluation of kinetics of Ce3+/Ce4+ redox reaction at RGOC
paper electrode is firstly carried out by cyclic voltammetry which is a
reversal technique and is the potential-scan equivalent of double po-
tential step chronoamperometry. Fig. 5a shows CV curves of 20mM Ce
(CH3SO3)3 in aqueous solvent with 2M CH3SO3H as supporting elec-
trolyte under air atmosphere. Regardless of the scan rate, every CV
curve of Ce(CH3SO3)3 shows an anodic peak corresponding to the oxi-
dation of Ce3+ to Ce4+ and a cathodic one corresponding to the re-
duction of Ce4+ to Ce3+. Both the anodic peak current and cathodic one
increased with the scan rate, and the diffusion coefficient of Ce3+ was
evaluated by employing the Randles-Sevcik equation [30] for reversible
systems. Fig. 5b shows the fitting of the anodic peak current against the
square root of the scan rate, giving a diffusion coeffient of
3.6× 10−5 cm2 s−1 for Ce3+ ion.
The separation of peak potentials is one of the two important
parameters of cyclic voltammogram, the smaller separation of peak
potential indicating the faster kinetics of electrode process. As seen in
Fig. 5c, the cyclic voltammogram of Ce3+/Ce4+ couple showed a
356mV of separation of peak potentials at GO paper, while those at
RGOT, RGOC and Pt electrodes are 145mV, 101mV and 159mV, re-
spectively. Another important parameter of cyclic voltammogram is the
ratio of peak currents. For a nernstian wave with stable product, the
ratio of peak current is unity regardless of scan rate. In this work, de-
viation of the ratio of peak currents from unity was observed, indicating
the existence of other complications in the electrode process. The sig-
nificant increase of peak current was also observed after the reduction
of GO. Since the experiment is carried out under the same conditions as
other conditions, for example, temperature, scanning speed, electrode
composition and active substance concentration, it can be concluded
that the order of electrochemical activity toward Ce3+/Ce4+ couple
from high to low is RGOC, RGOT, Pt and GO. In addition, the same
conclusion can be obtained from the data showed in Fig. 5d and e.
The change of oxygen-containing functional groups is observed on
the surface and edge of GO during thermal reduction or chemical re-
duction. The descending order of C-O functional groups content is GO,
RGOT and RGOC. The obvious increase of the peak potential separation
at RGOT and RGOC papers will be observed if Ce3+/Ce4+ redox reac-
tions could be catalysed by the C-O functional groups. But the data
obtained in this work are just reversed, indicating these functional
groups are of no moment to Ce3+/Ce4+ electrode processes. The fact
that the activity of RGOC toward Ce3+/Ce4+ is better than that of
RGOT also confirm the above conclusions. Furthermore, the removal of
C-O functional groups leads to the decrease of steric hindrance and the
introduction of defects in RGOC and RGOT papers. The decrease of
steric hindrance is beneficial to the acceleration of adsorption kinetics.
The introduction of defects would bring the increase of reactive points
for the Ce3+/Ce4+ electrode process. Therefore, RGOC is better than
RGOT, and RGOT is better than GO, in terms of electrochemical activity
towards Ce3+/Ce4+ electrode reaction.
3.5. Battery performance
The improvement of utilization rate of active material is beneficial
to the improvement of battery energy density, which is an important
parameter of battery performance. As shown in Fig. 6a, the Zn/Ce RFB
with RGOC paper positive electrode exhibited a charge capacity of
2167.2 mAh, corresponding to a 81.0% utilization rate of active mate-
rial, while it was 168.6 mAh with GO paper positive electrode and the
utilization rate of active material was only 6.3%. An obvious decrease
Fig. 6. Performance of zinc-cerium redox flow cell. a) Cell-voltage profile with respect to capacity during a typical charge/discharge process. b) Coulombic efficiency
variety as a function of the cycle number. c) Voltage efficiency variety as a function of the cycle number. d) Energy efficiency variety as a function of the cycle
number.
Z. Xie et al. Materials Chemistry and Physics 220 (2018) 208–215
214
of overpotential with RGOC paper as positive electrode can be re-
sponsible for the improvement of utilization rate of active material in
the charge-discharge process of Zn/Ce RFB. Under the condition that
the charge-discharge voltage range is certain, the smaller the over-
potential is, the greater the charge capacity and the utilization rate of
active material will be.
The Zn/Ce RFB with RGOC paper as positive electrode exhibited an
average coulombic efficiency (CE) of 89.6% (Fig. 6b), an average vol-
tage efficiency (VE) of 79.6% (Fig. 6c), and an average energy effi-
ciency (EE) of 71.3% (Fig. 6d), while they were 23.8% (Fig. 6b), 55.7%
(Fig. 6c) and 13.3% (Fig. 6d) for the cell with GO paper electrode. In
addition, the coulombic efficiency, voltage efficiency and energy effi-
ciency of Zn/Ce RFB with RGOC paper electrode has no obvious decay
over 50 cycles, indicating the stability of RGOC paper electrode is very
good over repetitive cycling. While those of Zn/Ce RFB with GO paper
electrode exists obvious decay over 5 cycles. High coulombic efficiency
of RGOC paper should be attributed to low oxygen functional group in
materials (Fig. 4) because oxygen functional group can lead to serious
oxygen evolution reaction [3]. According to the above result, it could
be deduced that RGOC paper could be a candidate of positive electrode
material for advanced Zn/Ce RFB.
4. Conclusions
RGOC paper with good voids and strength was prepared by a simple
method that reduction of GO was carried out using NaBH4. The im-
proved kinetics of Ce3+/Ce4+ electrode process at RGOC paper is at-
tributed to the introduction of more defects into graphitic structure of
RGOC paper and the removal of C-O functional groups minimizing the
space steric hindrance. RGOC paper is a candidate toward an excellent
electrode material for Zn/Ce RFB.
Acknowledgements
The authors acknowledge the financial support from the National
Natural Science Foundation of China (No.21361010), Province Natural
Science Foundation of Jiangxi (No.20161BAB206144 and
No.20171BAB206001), China Scholarship Council (No.201708360025)
as well as JXPCOD-JXSTA (No. 100RYH2017).
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ary.assuncao
Destacar
	Chemically reduced graphene oxide paper as positive electrode for advanced Zn/Ce redox flow battery
	Introduction
	Experimental
	Electrode preparation
	Sample characterization
	Electrochemical measurements
	Battery test
	Results and discussion
	XRD and Raman characterization
	SEM
	XPS
	Electrochemical analysis
	Battery performance
	Conclusions
	Acknowledgements
	References