Recent Advances in Energy, Environment and Economic Development
Chemical modification of graphite felts for efficient H2O2 production:
Influence of operational parameters
LEI ZHOU, MINGHUA ZHOU*
College of Environmental Science and Engineering
Nankai University
Weijin Road 94
CHINA
[email protected]
Abstract: - Electro-Fenton process (EF) is a promising method for degradation of refractory pollutants in
aquatic environment. The suitable cathode which produces hydrogen peroxide by oxygen reduction still plays a
key role in the efficiency improvement of the EF. In this study, hydrazine hydrate-ethanol system was used to
modify graphite felts, and the influence of the concentration of hydrazine hydrate on the production and
efficiency of hydrogen peroxide was discussed based on the SEM and CV characterizations. The results
showed that at a constant potential of -0.65 V (vs. SCE) the modified electrodes had much higher catalytic
activity than the unmodified one. Operational parameters such as cathodic potential, pH and aeration amount
were also investigated. The maximum yields of H2O2 obtained at -0.75 V (vs. SCE) in 0.05M Na2SO4 aqueous
solution with oxygen mass flow rate at 0.4 L/min was 198.5 mg/L after 90 min. Thus, this chemical
modification was a promising approach to promote the H2O2 electro-generation of carbon cathodes.
Key-Words: - Graphite felts; Hydrazine hydrate; Chemical modification; H2O2; Oxygen reduction reaction;
Electro-Fenton
carbonaceous electrodes by changing their surface
functional groups.
In the present work, hydrazine hydrate-ethanol
system was used to modify graphite felts, and the
influence of the concentration of hydrazine hydrate
on the production and efficiency of hydrogen
peroxide was discussed based on the SEM and CV
characterizations. Operational parameters such as
cathodic potential, pH and aeration amount were
also investigated, and the experimental results were
presented.
1 Introduction
As an environmentally friendly electrochemical
technology, Electro-Fenton process (EF) is a
promising method for degradation of refractory
pollutants in aquatic environment [1,2]. The EF
process is based on the continuous in-situ
electrogeneration of H2O2, which can eliminate
acquisition, shipment and storage, along with the
addition of iron catalysts to produce oxidant ·OH.
Therefore, the major concern with the EF system
relates to the improvement of H2O2 production.
Carbonaceous materials are widely used as cathodes
due the advantages such as nontoxic, good stability,
conductivity and chemical resistance, high
overpotential of hydrogen evolution, and low
catalytic activity of H2O2 decomposition [2].
Recently, several carbonaceous electrodes were
reported such as graphite [3], reticulated vitreous
carbon [4], activate carbon fiber [5,6], carbon
sponge [7], carbon/ graphite felt [8-11],
carbonaceous PTFE combined electrode [12-14],
and metal-modified carbonaceous electrode [15,16].
The carbon/ graphite felt electrode has a high
specific surface area favoring the fast generation of
H2O2, mechanical integrity and easily acquisition,
which make it a promising cathode material.
Chemical modification is an efficient way to
improve the electrochemical activity of the
ISBN: 978-1-61804-139-5
2 Experimental
2.1 Preparation of cathode materials
All chemicals used in this study were analytical
grade and used without further purification. The
commercial graphite felts (Shanghai Qijie Carbon
material Co., LTD) with a specific surface area of
about 0.6 m2·g-1 were degreased in an ultrasonic
bath with acetone and deionized water in sequence,
dried at 80 °C for 16 h, and then annealed at 150 °C
for 2 h. These pretreated materials were marked as
CF. A series of modified electrodes were prepared
as follow. The pretreated graphite felts were
immersed in 100 mL mixture of ethanol and
hydrazine hydrate, and after refluxing at 60 °C for
6h, the samples were annealed at 150 °C for 2 h.
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Recent Advances in Energy, Environment and Economic Development
Since the volume concentration of the hydrazine
hydrate in the mixture were 5%, 10%, 15% and
20%, the modified electrodes were marked as CFHA-5%, CF-HA-10%, CF-HA-15% and CF-HA20%, respectively.
3 Results and discussions
3.1 Morphologies and properties of graphite
felts
a
2.2 Characterization
For a morphology characterization of graphite felt
electrodes, a field-emission scanning electron
microscopy (FE-SEM, LEO1530VP) was used. The
contact angle of water on the electrode surface is
examined by a contact angle meter (OCA15,
Dataphysics). Electrochemical measurements were
carried out with CHI660D workstation (CH
Instruments, Chenhua, Shanghai, China) in a threeelectrode cell system at ambient temperature.
b
2.3 Electro-generation of H2O2
The H2O2 electro-generation experiments were
performed in a 0.13 L undivided three-electrode cell
using CHI660D electrochemical workstation as
power supply. The prepared cathode (5 cm×2
cm×0.5 cm) was used as working electrode, a
platinum wire as counter electrode and a saturated
calomel electrode (SCE) as reference electrode. The
distance between the working electrode and counter
electrode was 3.5 cm. Prior to the electrolysis,
oxygen (96% purity) was bubbled near the cathode
through the 0.05 M Na2SO4 aqueous solutions for 10
min, and then oxygen was reduced at a desired
potential (vs. SCE) on the working electrode for
90min, with a constant magnetic stirring of 300 rpm.
The concentration of H2O2 during electro-generation
process (C) was monitored by UV-vis
spectrophotometer (UV759, Shanghai instrument
analysis instrument Co., LTD) using the potassium
titanium (IV) oxalate method [17]. The current
efficiency (CE) for H2O2 production was defined as
follow [18]:
nFCH O V
CE = t 2 2 ×100 %
Fig. 1 SEM images of (a) CF and (b) CF-10%
Fig. 1a and 1b show the SEM images of CF and
CF-10%. It was observed that the graphite felts were
composed of an entangled network of carbon
microfilaments with diameter around 20 µm, and
this structure form could contribute to the large gasliquid contact
interfaces.
After
chemical
modification, the longitudinal etching trace on the
fiber increased, and many carbon particles and
clusters with an average diameter of about 500 nm
were appeared on the surface. The changes in the
microstructures of modified samples could
effectively increase the specific surface areas and
the number of active sites, which were considered to
be conducive to the catalytic process.
∫ Idt
0
Where n is the number of electrons transferred for
oxygen reduction to H2O2, F is the Faraday constant
(96 486 C/mol), CH2O2 is the concentration of H2O2
(mg/L), V is the bulk volume (L), I is the current
(A), and t is the time (s).
ISBN: 978-1-61804-139-5
3.2 Effect of the concentration of hydrazine
hydrate
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Recent Advances in Energy, Environment and Economic Development
160
140
80
120
60
100
40
CE (%)
Concentration (mg/L)
To further investigate the effects of chemical
modification on electrocatalytic activity of cathodes
toward ORR, cyclic voltammetry was carried out at
graphite felts before and after modification as Fig. 3
shown. Standard voltammograms with total
irreversibility were obtained for all samples, and the
modified cathodes exhibited stronger current
responses and more negative hydrogen evolution
potentials than unmodified one, suggesting that
modified samples had higher activities for oxygen
reduction, which promoted the H2O2 production.
Moreover, with the increasing concentration of
hydrazine hydrate the current response for cathodes
became stronger. It was also seen that the
modification not only encouraged the two-electron
transfer process towards ORR (i.e. H2O2 electrogeneration, Eq. 1), but also enhance the other ORR
processes, which might be competition for the H2O2
generation (Eqs. 2, 3), resulting in a drop of current
efficiencies for modified electrodes.
100
80
20
60
40
0
0
5
10
15
20
Concetration of hydrazine hydrate (%)
Fig. 2 The effects of the precursors on H2O2
production. Conditions: E= -0.65 V vs. SCE, 0.05M
Na2SO4, pH=7, O2 flow rate at 0.4L/min.
The concentrations of hydrogen dioxide and
current efficiencies for cathodes before and after
modification were shown in Fig. 2. After 90min, the
concentrations of H2O2 for CF, CF-HA-5%, CFHA-10%, CF-HA-15% and CF-HA-20% were 52.7,
122.4, 144.7, 139.0 and 134.5 mg/L, respectively.
The enhanced yields of H2O2 after modification
indicated the positive effects of precursors on H2O2
electro-generation. In addition, the optimum
concentration of the hydrazine hydrate was detected,
and among the modified electrodes the CF-HA-10%
showed the highest yield of H2O2. When the
concentration of the hydrazine hydrate exceeded the
optimum, a little decrease in the catalytic activity
was observed under the constant potential of -0.65
V vs. SCE. The current efficiencies of CF, CF-HA5%, CF-HA-10%, CF-HA-15% and CF-HA-20%
were 89.1%, 86.8%, 85.1%, 80.3% and 73.7%,
respectively. It was seen that the current efficiencies
declined with the increasing concentration of
hydrazine hydrate used in chemical modification.
O 2 + 2H + + 2e − → H 2 O 2
O 2 + 4H + + 4e − → 2H 2O
H 2 O 2 + 2H + + 2e − → 2H 2O
(1)
(2)
(3)
With the increasing concentration of hydrazine
hydrate in modification, the ORR tended to the
higher number electron transfer for each oxygen
molecule under the same potential. In addition, the
enhanced current in the system promoted the
parasitic reactions at anode (Equ. 4, 5), which
resulted in a decreased H2O2 accumulation and
lower current efficiency [19].
H 2 O 2 → HO 2 ⋅ + H + + e −
(4)
HO 2 ⋅ → O 2(g) + H + + e −
(5)
3.3 Effect of cathodic potential
0
100
200
-1
80
Concentration (mg/L)
2
-2
CF
CF-HA-5%
CF-HA-10%
CF-HA-15%
CF-HA-20%
-3
-4
-1.6
60
120
40
80
20
40
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0
-0.9
Potential (V)
-0.8
-0.7
-0.6
-0.5
-0.4
0
-0.3
Potential (V)
Fig. 3 Cyclic voltammograms of the cathodes
obtained in the potential range from -1.4 to 0 V (vs.
SCE), in 0.05M Na2SO4 solution, at the scan rate of
50mV·s-1
ISBN: 978-1-61804-139-5
CE (%)
j (mA/cm )
160
Fig. 4 The effects of the potentials (vs.SCE) on
H2O2 production. Conditions: using CF-HA-10% as
cathode, 0.05M Na2SO4, pH=7, O2 flow rate at
0.4L/min.
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Recent Advances in Energy, Environment and Economic Development
the higher level of hydrogen ion also promoted the
decomposition of H2O2 (Eq. 2). As a result, the
obtained results showed that pH did not
dramatically influence the accumulation of H2O2,
which was similar with the results of previous
studies [7,20], but the current efficiencies declined
due to the increasing side reactions in acid solution.
The effects of the potential on H2O2 production at
CF-HA-10% cathode were shown in Fig. 4. The
accumulations of H2O2 at the potential ranging from
-0.35 V to -0.85 V were 15.7, 46.4, 106.0, 144.7,
198.5 and 149.5 mg/L, respectively. It can be seen
that the H2O2 electro-generation initially enhanced
with the increasing cathodic potential (|E|), and then
declined when exceeded the potential of -0.75 V, at
which the maximum yield of H2O2 was obtained.
The current efficiencies of H2O2 production at the
potential ranging from -0.35 V to -0.85 V were
97.2%, 89.7%, 87.1%, 82.0%, 64.3% and 41.7%,
respectively. As CV curves confirmed, the
competing reactions were also enhanced after
modification, which could become more and more
remarkable with the cathodic potential increasing.
Hence, the current efficiencies went downhill
quickly, although the yields of H2O2 were improved.
When the cathodic potential became more negative
than -0.75 V, the side reactions corresponding to the
H2O2 decomposition and higher number electron
transfer became dominant, which resulted in a
decreased yields and current efficiencies of H2O2.
3.5 Effect of oxygen mass flow rate
160
140
80
Concentration (mg/L)
CE (%)
0
6
7
8
9
10
pH
Fig. 5 The effects of the pH on H2O2 production.
Conditions: using CF-HA-10% as cathode, E= -0.65
V vs. SCE, 0.05M Na2SO4, O2 flow rate at 0.4L/min.
The effects of the pH on H2O2 production at CFHA-10% cathode were shown in Fig. 5. When
pH=3~9, the concentrations of H2O2 were 158.9,
152.1, 144.7 and 138.0 mg/L, and the current
efficiencies were 68.9%, 75.5%, 85.0% and 86.1%,
respectively. There was a slight increase of the
electro-generated H2O2 with the increasing pH.
Since H2O2 was electro-generated at cathode surface
by reduction of dissolved oxygen in acidic medium,
as Equ. 1 shown, from this point of view, the lower
pH was beneficial to the H2O2 production. However,
ISBN: 978-1-61804-139-5
0.6
The effects of the aeration amount on H2O2
production at CF-HA-10% cathode were shown in
Fig.6. The accumulation of H2O2 at O2 flow rate of 0,
0.2, 0.4 and 0.6 L/min were 16.8, 100.2, 144.7 and
129.2 mg/L, and the current efficiencies were 35%,
77.5%, 85.1% and 94.0%, respectively. The
increasing aeration amount could enhance the
concentration of dissolved O2 and promote the mass
transfer rate of dissolved O2 in solution, which were
conducive to the H2O2 electro-generation. However,
when O2 flow rate reached to 0.6 L/min, the
resistance of the medium increased with a mass of
bubble in the constant potential system, and then the
current declined, inducing a little drop in the yields
of H2O2. The result indicated that too much aeration
could impede the H2O2 production.
20
5
0.4
Fig. 6 The effects of the aeration amount on H2O2
production. Conditions: using CF-HA-10% as
cathode, E= -0.65 V vs. SCE, 0.05M Na2SO4, pH=7.
110
4
0.2
Aeration Amount (L/min)
120
3
0
0.0
60
2
40
0
140
100
40
60
20
150
40
60
80
20
80
130
100
CE (%)
Concentration (mg/L)
120
3.4 Effect of pH
160
100
4 Conclusion
In this study, hydrazine hydrate-ethanol system
was used to modify graphite felts. After
modification, a larger specific surface area was
observed by SEM, which was considered to be
conducive to the catalytic process. The modified
samples had much higher electro-catalytic activity
of oxygen reduction than the bare one, and the
concentration of H2O2 generated was doubled after
modification. Besides, there was an optimum
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Recent Advances in Energy, Environment and Economic Development
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Spectrophotometric
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yields of H2O2 obtained at -0.75 V (vs. SCE) in
0.05M Na2SO4 aqueous solution with oxygen mass
flow rate at 0.4 L/min was 198.5 mg/L after 90 min,
and the results indicated a dramatically influence of
the applied potentials on the H2O2 production. There
was a slight increase of the electro-generated H2O2
with the increasing pH, but the current efficiencies
declined due to the increasing side reactions in acid
solution. The increasing aeration amount was
conducive to the H2O2 electro-generation, but too
much aeration would impede the H2O2 production.
Acknowledgements
This work was financially supported by Natural
Science Foundation of China (No. 51178225 and
21273120) and Fund for the Doctoral Program of
Higher Education of China (20110031110025).
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