Electrochemical generation of hydrogen peroxide using surface area-enhanced Ti-mesh electrodes

Article abstract of DOI:10.1016/j.electacta.2007.04.021

Ti-mesh electrodes coated with Ti were obtained by using an electrophoretic deposition (EPD) method. The Ti coating was porous and showed a good adherence to the Ti-mesh surface, due to sintering of Ti particles during thermal treatment at 900 °C. The Ti-coated mesh electrode has a BET surface area of 3.5 m²/g, about four times larger than that of the bare electrode. The surface area-enhanced Ti-mesh electrode was applied in electrical generation of hydrogen peroxide. It was shown that the rate of hydrogen peroxide generation increased drastically compared to the fresh electrode, since the larger electrode surface area enhanced not only current density, but also the oxygen mass transfer rate.

Full text of DOI:10.1016/j.electacta.2007.04.021

Electrochimica Acta 52 (2007) 6304–6309
Electrochemical generation of hydrogen peroxide using surface
area-enhanced Ti-mesh electrodes
∗
Kyung Shik Yang, Guido Mul , Jacob A. Moulijn
Reactor and Catalysis Engineering, DelftChemTech, Faculty of Applied Sciences, Delft University of Technology,
Julianalaan 136, 2628 BL Delft, The Netherlands
Received 17 November 2006; received in revised form 8 March 2007; accepted 3 April 2007
Available online 7 April 2007
Abstract
Ti-mesh electrodes coated with Ti were obtained by using an electrophoretic deposition (EPD) method. The Ti coating was porous and showed
◦
a good adherence to the Ti-mesh surface, due to sintering of Ti particles during thermal treatment at 900 C. The Ti-coated mesh electrode has a
2
BET surface area of 3.5 m /g, about four times larger than that of the bare electrode. The surface area-enhanced Ti-mesh electrode was applied in
electrical generation of hydrogen peroxide. It was shown that the rate of hydrogen peroxide generation increased drastically compared to the fresh
electrode, since the larger electrode surface area enhanced not only current density, but also the oxygen mass transfer rate.
©
2007 Published by Elsevier Ltd.
Keywords: Ti-mesh; Electrode; Electrophoretic deposition; Hydrogen peroxide; Oxygen reduction
1
. Introduction
oxide. The pathway of oxygen reduction is strongly determined
by the electrode materials [9].
Hydrogen peroxide (H2O2) is widely applied as a bleaching
Carbon is a well-known cathode for two-electron reduction of
oxygen to hydrogen peroxide in alkaline solution [7]. Graphite
has been widely used for the reaction but a gas diffusion elec-
trode (GDE) is more common, to overcome the problem of the
low solubility of oxygen, causing mass transfer limitations at
the cathode surface. However, several problems such as elec-
trode stability, and the cost, are still associated with using a
GDE.
Recently several techniques were investigated to enhance the
O2 electroreduction to H2O2. Gyenge and Oloman [10,11] tried
to investigate electrochemical reactors with three-dimensional
cathodes, consisting of graphite felt (GF) or reticulated vitre-
ous carbon (RVC). The influences of surfactants and pH were
examined for enhancing electroactivity and stability.
Applying an anthraquinone coating to carbon electrodes by
novel approaches has also been suggested. Anthraquinone can
chemically bind to the surface of the carbon electrode, and func-
tion as an organic catalyst [12]. The advantage of the quinone
modified electrodes is that a high peroxide selectivity can be
obtained.
agent and disinfectant in the paper and textile industry. It is con-
sidered an environmentally friendly chemical since it leaves no
hazardous residues. It is industrially produced by the oxidation
of alkylhydroanthraquinones and by the electrolysis of ammo-
nium bisulphate [1]. On-site production of H2O2 has become of
great interest because of the cost and hazards associated with
the transport and handling of concentrated-H2O2. The indus-
trial process is not an option for the in situ production of H2O2,
because of the need of quinones, organic solvents and multi-
ple reaction steps. Different alternatives for the bulk process,
such as direct synthesis from oxygen and hydrogen [2,3], enzy-
matic [4,5], electrocatalytic [6] and electrochemical processes
[
7,8], have therefore recently been investigated. Electrochem-
ical synthesis is the most popular alternative among them for
the production of H2O2 on a small scale by the oxygen reduc-
tion reaction. On the electron level two different pathways can
be distinguished: (i) the four electron pathway, forming water or
hydroxyl groups, and (ii) the two electron pathway, forming per-
Titanium has long been used as an electrode material in the
∗
2 2
production of H O from dissolved oxygen, since it is electro-
Corresponding author. Tel.: +31 15 278 4381; fax: +31 15 278 5006.
E-mail address: G.Mul@tudelft.nl (G. Mul).
chemically stable. To obtain a satisfactory H2O2 yield, noble
0
013-4686/$ – see front matter © 2007 Published by Elsevier Ltd.
doi:10.1016/j.electacta.2007.04.021

K.S. Yang et al. / Electrochimica Acta 52 (2007) 6304–6309
6305
metals were doped on titanium. For example, Baez and Pletcher
13] tried to develop gold coatings on titanium as alternative
cathodes to replace carbon.
[
In this work, we investigated surface areas-enhanced Ti-
mesh electrodes instead of Au/Ti electrode, employing an
electrophoretic deposition (EPD) method. Titanium powder was
selected as deposition material to increase the surface area of the
Ti-mesh. It will be reported herein that Ti-coated wire-mesh has
a highly porous layer with a large surface area. An increase of the
production rate of H2O2 is reported. The data will be discussed
based on calculations of the oxygen mass transfer rate.
2
. Experimental
2
.1. Preparation of Ti-coated wire-mesh electrodes
In previous work, we investigated the EPD method to
deposit aluminum as catalyst support material on stainless steel
wire-mesh [14,15]. Similar experiments were performed with
suspensions of Ti metal powder in ethanol. We used Ti wire-
mesh as a substrate instead of stainless steel in this case, since
we would apply this as electrode. Since the density of tita-
nium is higher than that of aluminum, more ultrasonification
and mechanical stirring is required for suspension of Ti par-
ticles. There are several other EPD variables, such as the Ti
amount, additive concentration, applied voltage and deposition
time. The concentration of additive is in particular critical for
EPD, since it creates particle charges and stabilizes the suspen-
sions. Therefore, we first fixed the Ti concentration to 0.5 wt.%,
the DC voltage to 50 V and the deposition time to 5 min, refer-
ring to the previous result [14]. Then we changed the amount of
Ti-isopropoxide to determine the optimal amount for the EPD
procedure of Ti.
Fig. 1. A schematic diagram of the electrochemical cell, including representa-
tion of the applied mesh electrodes.
2.2. Electrode characterization
Polarization curves were obtained by cyclic voltammetry
using a three-electrode potentiostat (Hokuto Denkdo HSV-100).
A saturated Ag/AgCl (HS-205C, TOA Electronics) electrode
was used as the reference electrode. A platinum wire was
employed as the counter electrode. All potentials are quoted
versus the saturated Ag/AgCl reference electrode. The cathodic
potential was swept from 0 to −1.0 V at a linear scan rate of
10 mV/s. The specific surface area of the various samples was
determined by nitrogen adsorption in a constant volume adsorp-
tion apparatus (Micrometrics, ASAP 2021C). X-ray diffraction
(XRD, Mac Science Co., M18XHF) was employed to identify
Titanium powder (99.5%, 325 mesh, Alfa Aesar) was used
the phase of the coated-samples. The surface structure and the
cross section of the coated wire were studied by means of scan-
ning electron microscopy (SEM, Hitachi, S-2460N), equipped
with an EDX system.
as a deposition material, with a specific BET surface area of
2
0
.92 m /g. Titanium isopropoxide (Aldrich) was used as addi-
tive, which was expected to control the suspension conductivity
and enhance the dispersion.
Commercial wire mesh, made of titanium grade 1 (Cleveland
Wire Cloth and Manufacturing Co., 24 Taylor mesh screen) was
used as a substrate. It has an open pore size of around 0.8 mm
2.3. Electrochemical cell for hydrogen peroxide generation
and a wire diameter (dw) of 0.25 mm. The mesh weighs about
Fig. 1 shows the schematic diagram of the reaction system.
The cathodic and anodic compartments had a volume of 300
and 900 ml, respectively. During the experiments, 200 ml of
catholyte and 300 ml of anolyte were used, and the catholyte was
thoroughly mixed by a magnetic stirrer. The distance between
the cathode and anode was kept at 2 cm. A nafion membrane was
usedtoseparatethetwocompartments. Thismembraneprohibits
the diffusion of anions and H2O2 molecules, but allows cations
to freely penetrate. As a result, H2O2 generated at the cathode
will be confined in the catholyte, avoiding its decomposition
at the anode. Moreover, protons generated at the anode will be
electrically driven to the catholyte, partially supplementing the
protons consumed for H2O2 synthesis. 0.1 M of sodium per-
chlorate (NaClO4) was used as the inert supporting electrolyte.
The developed Ti-mesh with enhanced surface area and Ti-mesh
were used as cathode and anode, respectively. Compressed air
2
0
.055 g/cm , on average. The surface of the substrate was treated
with acetone to remove dirt and grease.
−
2
First, a suitable amount of additive (1.0 × 10
to
−3
5
.0 × 10 M) was added to ethanol (99.9%, Aldrich), under
stirring, and then titanium powder (0.5 wt.%) was added to the
suspension. The slurry solution was well mixed, using an ultra-
sonic bath for 5 min. The apparatus used for the electrophoretic
deposition is described elsewhere [14]. For particle deposition at
the cathode, the wire mesh was cut into pieces (1.2 cm × 7 cm).
A titanium plate was used as an anode and the size was the same
as that of the cathode. The distance between the two electrodes
was kept at 10 mm and the DC voltage was changed from 50 to
3
00 V. The titanium-coated samples were dried at room temper-
◦
◦
ature for 12 h, sintered at 900 C (heating rate was 5 C/min),
under 10% H2 in N2 flow for 3 h.

6
306
K.S. Yang et al. / Electrochimica Acta 52 (2007) 6304–6309
was used to dissolved oxygen (DO). A flow of 150 ml/min was
sparged into the catholyte through a porous pipe-diffuser. The
catholyte was pre-saturated with DO by purging air for 30 min
beforeelectrolysis was initiated. The electro-generation of H2O2
was carried out under a constant potential mode.
The concentration of H2O2 was determined by the indometric
titration method [16] with subsequent UV absorbance measure-
ment. The iodide solution was prepared by mixing 2.5 ml of
solution A (potassium iodide 66 g/l, ammonium heptamolybdate
0.2 g/l, sodium hydroxide 2 g/l), and 2.5 ml solution B (potas-
sium hydrogenphtalate 20 g/l). When a sample containing H2O2
is added, the iodide reacts with H2O2 and forms a triiodide solu-
tion that has a yellow color at low concentrations to dark red
at high concentrations. Triiodide can be analysed in low con-
centrations using the UV absorption at 350 nm. For the UV
measurement a Thermo Spectronics Unicam UV Series 5000
spectrometer was used. Measurements were done using the deu-
terium arc lamp of the spectrometer. The absorbance of light at
Fig. 3. A schematic diagram of electrophoretic deposition of Ti particles.
3
50 nm has a linear dependence on the concentration of triiodide
and thus on the concentration of H2O2 in the sample.
For the Ti suspensions, the deposited weight increased
sharply with increasing additive concentration in the range up to
3
. Results and discussion
−3
1
.0 × 10 M. The deposited weight decreased when the con-
−3
centration of the additive was higher than 1.0 × 10 M. This
3
.1. Electrophoretic deposition of Ti particles
can be explained as follows. When the concentration of Ti iso-
−
3
propoxide was over 1.0 × 10 M, the adherence of the deposit
became poor and the deposition layer on the wire surface became
non-uniform. Since for the Ti particles at the point, where full
adsorption was reached, extra ions, that were not adsorbed,
move also towards the cathode, competing with the charged
titanium particles under the electrical field. This phenomena is
exactly corresponding to the result of Al deposition previously
reported [15]. Therefore there exists an optimal concentration
Fig. 2 shows the deposition weight of Ti particles versus
Ti isopropoxide additive concentration at constant time (5 min)
and voltage (50 V). No noticeable deposition took place with-
out any additives. When titanium isopropoxide is added to the
suspension, the following reaction might occur:
+
−
Ti(OC3H7)4 ↔ [Ti(OC3H7)3] + OC3H7
−3
The positively charged titanium alkoxides are adsorbed on the
surface of Ti particles (Fig. 3). As a result, titanium particles
become positively charged, and move towards the cathode under
the applied field.
of Ti isopropoxide, which is in the range from 1.0 × 10 to
−
3
5.0 × 10 M for good adherence.
After deposition of Ti particles, the Ti-coated wire-meshes
were thermally treated with a H gas purge for particle sintering.
2
The adherence of the metal-coatings has been evaluated accord-
ing to the ultrasound method described in the literature [17]. Ti
coatings were easily detached from the metallic substrate when
◦
the temperatures were less than 700 C, since titanium parti-
cles do not sinter at those temperatures. It was shown that the
detached amount of the Ti coatings was below 1 wt.% at sinter-
◦
ing temperature over 850 C. The coating were hardly detached
◦
from the metallic substrate at 1000 C. However, the Ti coat-
ing layer was highly sintered at that temperature, so it had a
non-porous surface structure with a low surface area. The opti-
◦
mal temperature range, therefore, is around 900 C for thermal
treatment.
3.2. Physical and electrochemical properties
The surface areas of fresh Ti wire-mesh, Ti powder and Ti-
coated samples were measured by the BET method (Table 1).
The surface areas of Ti wire-mesh and Ti powder were found
Fig. 2. Effect of additive concentration on the weight of Ti particles deposited
2
to be very small, 0.01 and 0.9 m /g, respectively. The titanium-
(
closedcircle). TheeffectofadditiveconcentrationonpHisindicatedbytheopen
2
triangles. Solvent: ethanol; additive: Ti-isopropoxide; deposition time: 5 min.
coated wire-mesh has a surface area of 3.5 m /g, about four

K.S. Yang et al. / Electrochimica Acta 52 (2007) 6304–6309
6307
Table 1
BET surface areas
Material
Thermal treatment
BET surface
area (m /g)
2
Titanium wire-mesh
×
×
0.01
0.92
3.5 (70 ␮m)
Fresh titanium powder
Ti-coated wire mesh (average
thickness of coating layer)
◦
900 C, 3 h with
10% H2 in N2
times larger than that of the fresh electrode. The result must
be caused by formation of micro-pores in the deposition layer
as Ti particles experience a certain extent of sintering during
the thermal treatment, as shown in the SEM images (Fig. 4).
We could estimate the thickness of the Ti coating being around
7
0 ␮m from the cross-section view of the SEM image.
Cathodic polarization of the surface area-enhanced Ti-mesh
Fig. 5. Polarization curves of the surface area-enhanced Ti-mesh and fresh
Ti-mesh. Sweeping rate = 10 mV/s, pH 12, air feed rate = 150 ml/min, 0.1 M
NaClO4.
and bare Ti-mesh electrodes are shown in Fig. 5. The I–E
curves were obtained by linear sweep voltammetry from 0 to
H2O2 generation in the range from −0.35 to −0.55 V. The cur-
rent increases sharply above −0.55 V, since side reactions, for
example H2O2 decomposition or H2 evolution, start to occur.
In the limiting current region, H2O2 generation is controlled
by mass transfer of dissolved oxygen (DO) through the cathode-
solution diffusion layer, rather than by the electron transfer
between DO and the cathode. Since the DO concentration at
the cathode surface rapidly approaches zero after electrolysis
starts, the limiting current under a steady-state condition can be
expressed by the following Eq. (1) for macroscopic electrodes
−
1.0 V at 10 mV/s in a saturated oxygen solution at pH 12.
The highly exposed surface area of the Ti-coated wire-mesh
electrode resulted in a much higher current density in this poten-
tial range than observed for the fresh Ti-mesh electrode. There
exists a plateau that represents the limiting current region for
[18].
IL = kmnFAeC∗
(1)
where IL represents the limiting current (A), km the mass transfer
coefficient (m/s), n the stoichiometric number (=2) of electrons
transferred, F the Faraday’s constant (96,485 C/mol), Ae the
2
*
effective cathode surface area (m ) and C is the DO concentra-
3
tion in bulk solution (mol/m ). We could estimate the limiting
current density from Fig. 5, and a C value of 0.55 mol/m at
0 C, therefore, the oxygen mass transfer coefficients of sur-
*
3
◦
2
face area-enhanced Ti-mesh and bare Ti-mesh could be obtained
by substituting DO and cathode surface area in Eq. (1). The
results are listed in Table 2. The oxygen mass transfer coeffi-
−
5
cientis9.2 ± 1.0 × 10 m/sforsurfacearea-enhancedTi-mesh.
It is more than twice that of bare Ti-mesh. Therefore, we could
expect that the surface area-enhanced Ti-mesh electrode would
be more effective than bare Ti-mesh for H2O2 generation since it
enhanced not only current density but also oxygen mass transfer
rate in this range. These are the result of an enlarged specific
electrode surface area and a reduced size of the stagnant layer,
Table 2
Calculated mass transfer coefficients
Materials
Limiting current
density (A/m )
Mass transfer
coefficient (m/s)
2
−
−5
5
Titanium wire-mesh
Surface area-enhanced
Ti-mesh
4.0 ± 0.6
9.8 ± 1.0
3.8 ± 0.6 × 10
9.2 ± 1.0 × 10
Fig. 4. SEM pictures of the surface of Ti-coated wire-meshes after thermal
sintering at 900 C with H2 gas purge: (a) low magnification and (b) high
magnification.
◦

6
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K.S. Yang et al. / Electrochimica Acta 52 (2007) 6304–6309
Fig. 6. Hydrogen peroxide generation at the potentials of −1.05 and −0.55 V
over the surface area-enhanced Ti-mesh and fresh Ti-mesh. pH 12, air feed
rate = 150 ml/min, 0.1 M NaClO4.
induced by the 3-D shape of the electrode structure and the
irregular shape of the electrode surface, respectively.
3
.3. Electrochemical generation of H2O2
Electrochemical generation of H2O2 was performed to eval-
uate the surface area-enhanced Ti-mesh electrode in a saturated
oxygen solution at pH 12 and 0.1 M NaClO4 at two different
applied potentials of −0.55 and −1.05 V. A concentration of
H2O2 was obtained as shown in Fig. 6. Regardless of the applied
potential, the amount of H2O2 produced increased more than
7
0%, as a result of the electroplating procedure. The reason
was that the developed-electrode enhances current intensity and
oxygen mass transfer as described in Section 3.2.
The H2O2 production rate gradually decreased at higher
potential of −1.05 V, since side reactions such as H2O2 reduc-
tion or hydrogen evolution occur at these potentials. Indeed the
current density increased at a potential of −1.05 V as time goes
on, but at a potential of −0.55 V it was constant (Fig. 7). The
selectivity of the cell for a certain reaction is given by the current
efficiency (η). This is the percentage of the total cell current that
is used for the desired reaction. It can be calculated by dividing
the amount of charge in Coulomb for a specific reaction by the
total charge that has passed the electrode (Eq. (2)):
Fig. 7. (a) Current intensity and (b) current efficiency at the potentials of −1.05
and −0.55 V over surface area-enhanced Ti-mesh and fresh Ti-mesh. pH 12, air
feed rate = 150 ml/min, 0.1 M NaClO4.
row, and shown in Fig. 8. The applied potential was kept at
−0.55 V in a saturated oxygen solution. It is obvious that the
amount of H2O2 generated increased with increasing number
of cathode electrodes, due to a higher overall current intensity.
The H2O2 production rate gradually decreased, especially for
large geometrical surface areas, since there is more chance for
side reactions to happen, due to the higher current intensity,
and H2O2 concentration. However, the initial production rate of
H2O2 showed a linear relationship as a function of geometri-
cal surface area of the cathodes as shown in Fig. 9. This means
that the main side reaction might be H2O2 reduction under these
conditions. We can maintain the initial H2O2 production rate if
we use a continuous reaction system, in order to keep the H2O2
concentration below a certain level. The surface area-enhanced
Ti-mesh electrode showed an optimized H2O2 production rate of
∗
nFC V
t
t0
η (%) = ꢀ
× 100
(2)
I dt
where n is the stoichiometric coefficient of electrons (=2), F
the Faraday’s constant (96,485 C/mol), C the concentration
*
of reactant in the bulk, V the electrolyte volume and I is the
cell current intensity. Current efficiency is higher at a cathode
potential of −0.55 V. Fig. 7 shows that current efficiency of the
surface area-enhanced Ti-mesh was almost constant around 60%
at −0.55 V.
2
The effect of the cathode geometrical surface area was inves-
tigated with increasing the number of cathodes, standing in a
0.3 mol/l h m . Future work will focus on the deposition of active
metals such as gold and palladium on the developed-Ti-mesh,

K.S. Yang et al. / Electrochimica Acta 52 (2007) 6304–6309
6309
to increase electrocatalytic activity and current efficiency. The
feasibility of a continuous reactor system will also be evaluated.
4. Conclusions
The present study shows that H2O2 generation benefits from
enhancement of the surface area of Ti-mesh electrodes. We suc-
cessfully prepared Ti-mesh coated with Ti particles employing
an electrophoretic deposition method, and determined the effect
of parameters such as concentration of additive, applied voltage
and time. The highly exposed surface area of Ti-coated mesh
enhanced not only the current density, but also the oxygen mass
transfer rate.
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