Effect of the thickness and porosity of carbon-black gas-diffusion air electrodes on the effi ciency of hydrogen peroxide synthesis from atmospheric oxygen

Article abstract of DOI:10.1134/S1070427210020126

Effect of the porosity and thickness of hydrophobized gas-diffusion air electrodes composed of A 437-E acetylene black or its mixture with P 702 and P 701 furnace blacks on the effi ciency of electroreduction of atmospheric oxygen to H₂O₂ in 0.5 M NaOH was studied.

Full text of DOI:10.1134/S1070427210020126

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2010, Vol. 83, No. 2, pp. 253−257. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © G.V. Kornienko, N.V. Chaenko, I.S. Vasil’eva, G.A. Kolyagin, V.L. Kornienko, 2010, published in Zhurnal Prikladnoi Khimii, 2010,
Vol. 83, No. 2, pp. 255−258.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Effect of the Thickness and Porosity of Carbon-Black
Gas-Diffusion Air Electrodes on the Efficiency of Hydrogen
Peroxide Synthesis from Atmospheric Oxygen
G. V. Kornienko, N. V. Chaenko, I. S. Vasil’eva, G. A. Kolyagin, and V. L. Kornienko
Institute of Chemistry and Chemical Technology, Siberian Branch, Russian Academy of Sciences, Krasnoyarsk, Russia
Received April 22, 2009
Abstract—Effect of the porosity and thickness of hydrophobized gas-diffusion air electrodes composed of
A 437-E acetylene black or its mixture with P 702 and P 701 furnace blacks on the efficiency of electroreduction
of atmospheric oxygen to H O in 0.5 M NaOH was studied.
2
2
DOI: 10.1134/S1070427210020126
Air electrodes are presently used in numerous fields
of technology: manufacture of fuel cells and chemical
power cells, and electrosynthesis processes, e.g.,
limitations on the delivery of oxygen into the reaction
zone [8].
The goal of this study was to examine the effect
of the porosity and thickness of the working layer of
hydrophobized gas-diffusion air electrodes composed
of A 437-E acetylene black or its mixture with P 702
and P 701 furnace blacks on characteristics of hydrogen
peroxide electrosynthesis from atmospheric oxygen in
synthesis of H O from O , which can be, in particular,
2
2
2
used for indirect electrochemical oxidation of organic
and inorganic substances [1].
It is known [2, 3] that the electroreduction of O
2
–
on carbon blacks occurs via formation of the HO ion,
2
which is a stable product in a wide range of potentials.
The use of electrodes fabricated from a mixture of carbon
blacks with different physicochemical characteristics [4,
0.5 M NaOH: process selectivity and kinetics of H O2
accumulation.
2
5
] as catalysts for electrosynthesis of H O from oxygen
EXPERIMENTAL
2
2
in gas-diffusion electrodes (GDEs) made it possible
to obtain alkaline H O solutions with concentrations
exceeding 30 g l at a current density of 500Am , with
The experimental conditions, electrolyzer design,
and fabrication conditions of single- and double-layer
electrodes were described in [4, 9]. The following
brands of carbon blacks were chosen as objects of study:
hydrophobic A 437-E acetylene black and hydrophilic
P 702 and P 701 furnace blacks. The blacks differ in
the specific surface area (37–101 m g ), particle size
(30–100 nm), and ratio between surface groups. The
physicochemical characteristics of the blacks are listed
in Table 1 [10].
2
2
–
1
–2
a current efficiency by H O of 96–98%. The difference
2
2
in the hydrophilic-hydrophobic properties of the carbon
blacks used in the active mass of the working electrode
[
6, 7] resulted in better conditions of H O extraction
2 2
2
–1
from the pore space of the electrodes, which created
prerequisites for development of electrodes with stable
electrochemical characteristics and a prolonged service
life.
It is known that the operation efficiency of gas-
diffusion air electrodes strongly depends on their struc-
tural characteristics, which is primarily due to diffusion
The electrode pastes were prepared from the
acetylene black and its mixtures with P 702 and P 701
furnace blacks (A 437-E : P 701, A 437-E : P 702 = 1 : 1)
2
53

254
KORNIENKO et al.
Table 1. Physicochemical parameters of carbon blacks
2
–1
Specific surface area, m g
Brand of
pH of aqueous
suspension
Particle size, nm
Ash content, %
a carbon black
external
adsorption
A 437-E
P 702
30–39
60–100
60–100
101
37
ПО
35
9.2
8.0
11
0.2
0.5
P 701
39
70
0.35
Table 2. Results of hydrogen peroxide electrosyntheses from atmospheric oxygen on single-layer electrodes
Porosity, vol %
Brand of a carbon
black
Electrode
Current efficiency
С
–1
Run no.
Н O , g l
2
2
thickness, mm
by H O , %
2
2
total
70
electrolyte
8.3
1
A 437-E
0.40
50
28
2
3
4
5
6
7
8
0.90
0.40
0.90
0.30
0.60
0.70
0.75
70
60
60
60
60
70
60
8.3
18
10
14
13
18
14
40
35
50
32
41
46
63
22
20
29
18
23
26
35
A 437-E + P 701
A 437-E + P 702
9
0.98
60
10
61
34
with a hydrophobizer (FP-4D) content of 20 wt %. The
single-layer electrodes had the following parameters:
diameter 33 mm, thicknesses 0.3 and 0.8 mm, total
porosity 60 and 70 vol %.
exchange membrane.
A 1 M H SO solution was used as the anolyte,
2
4
and a platinum plate, as the anode. The electrolysis
was performed in the galvanostatic mode at a current
–
2
In double-layer electrodes with a working layer
composed of the mixture (A 437-E : P 701 = 1 : 1), the
content of the FP-4D hydrophobizer was also 20 wt %.
The blocking layer contained 50 wt % A 437-E and 50
wt % FP-4D. The thickness of the blocking layer was
density of 500 A m in 0.5 M NaOH at 20°C. The
current density was calculated per unit apparent front
2
surface area of the electrode (5 cm ). The electrode
potential was measured relative to a saturated silver
chloride electrode. Air was fed into the electrode from
its backside via the gas chamber under atmospheric
0
0
.5 mm, and the thicknesses of the working layer, 0.2,
.3, and 0.4 mm.
–
1
pressure at a rate of 100 ml min . The electrodes were
tested for selectivity in the reaction of O reduction to
the HO2 ion by determining by gasometry the current
The electrode paste was prepared by joint coagulation
2
–
of suspensions of FP-4D and a carbon black under
mechanical agitation [11]. The amount of the paste
necessary for fabrication of electrodes with certain
porosity was calculated by the known formula [12]. The
electrodes were sintered in a die mold under a pressure
at a temperature of 360°C. The electrolyte porosity of
the electrode was determined by hydrostatic weighing
fraction γ expended for synthesis of hydrogen peroxide
in electrolyses with oxygen supplied from a cylinder.
Potentiometric polarization curves were measured at
–
1
a potential sweep rate of 10 mV s . The concentration
of hydrogen peroxide in solution was found using
permanganatometry.
The experimental data obtained in electrosynthesis
of hydrogen peroxide on single-layer electrodes are
listed in Table 2.
[
13]. Electrolyses were performed in a three-chamber
cell whose design was described in [14]. A gas-diffusion
electrode in a fluoroplastic shell was placed between
the cathode and gas chambers. The cathode and anode
chambers were separated with an MF-4SK-100 cation-
It can be seen in Table 2 that 03–0.6-mm single-
layer electrodes are less efficient in electrosynthesis
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 2 2010

EFFECT OF THE THICKNESS AND POROSITY OF CARBON-BLACK ELECTRODES
255
Table 3. Results of hydrogen peroxide electrosyntheses from atmospheric oxygen on double-layer electrodes
Brand of a carbon
black
Working layer
thickness, mm
Current efficiency by
С
, g l–1
Н2O2
Run no.
1
Total porosity, vol %
70
H O , %
2
2
A 437-E
0.2
65
24.3
2
3
4
5
6
7
8
9
437-E + P 701
A 437-E
437-E + P 701
A 437-E
437-E + P 701
A 437-E
437-E + P 701
A 437-E
70
50
50
60
60
70
70
70
70
0.2
0.3
0.3
0.3
0.3
0.3
0.3
0.4
0.4
63
38
68
65
70
71
80
32
30
23.4
16.0
24.1
22.0
25.1
27.2
29.1
7.5
1
0
437-E + P 701
7.1
of hydrogen peroxide. On thicker electrodes, the
efficiency of the process is noticeably higher, the
produced concentration of hydrogen peroxide is as high
produced hydrogen peroxide into the electrolyte
bulk. Experimental data on operation of double-layer
electrodes are listed in Table 3.
–1
as 35 g l , their operation is more stable in the course
of time, but the service life is comparatively short.
In addition to the lower current efficiencies by H O
It can be seen in Table 3 that variation of the total
porosity within the range under study (50–70 vol%)
has no pronounced effect on the efficiency of hydrogen
peroxide production, whereas a decrease in the working
layer thickness from 0.4 to 0.2–0.3 mm at a 70% total
porosity of the electrodes leads to an increase in the
2
2
–1
(
18–23 g l ), a phenomenon of electrode “wetting-
through” was noticed, i.e., the electrolyte appeared on
the electrode backside (Table 2, run nos. 3, 5, and 6).
This is presumably due to flooding of gas-supplying
pores by the electrolyte, with the result that the stability
of the three-phase boundary is disturbed, the efficiency
of oxygen supply into the electrode decreases, and the
current efficiency by H O becomes lower.
concentration of produced hydrogen peroxide to 23.4–
–1
2
9 g l .
The electrochemical activity of the electrodes was
also determined on the basis of the polarization curves
in Fig. 1. The polarization curves of reduction of pure
oxygen and atmospheric oxygen in double-layer gas-
diffusion cathodes composed of a mixture of carbon
blacks show that replacement of oxygen with air
noticeably enhances the polarization of the electrode.
2
2
It has been noted previously [4] that, in operation
of double-layer mixed-carbon-black electrodes on pure
O , the produced concentration of hydrogen peroxide
2
grows because of the weaker diffusion limitations on
the removal of the peroxide from the pore volume of an
electrode. Probably, introduction of a more hydrophobic
blocking layer into double-layer electrodes operating
on atmospheric oxygen must favor solution of several
problems. Primarily, the hydrophobic layer not only
improves the mechanical stability of electrodes, but
also serves as a good transport channel for supply of
atmospheric oxygen across the diffusion barrier of inert
gases. At the same time, an intermediate layer, more
hydrophobic than the working one, is spontaneously
created between the blocking and working layers in
the stage of electrode fabrication because of the mutual
penetration of active masses, which must stabilize the
boundary of the three-phase contact. In the process,
the thickness of the working layer decreases, and
this must affect the efficiency off removal of the
–
E, V
Fig. 1. Polarization curves of electroreduction of (1) pure
oxygen and (2) atmospheric oxygen on double-layer electrodes
in 0.5 M NaOH. (i) Current density and (E) potential.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 2 2010

256
KORNIENKO et al.
The current fraction γ determining the selectivity
of hydrogen peroxide synthesis (Fig. 2, curves 1–
) is 0.98–0.95 in the first 2 h of electrolysis. The
4
experimental results of the study and the theoretically
calculated points in the H O accumulation curves are
2
2
in good agreement.
It can be seen in Fig. 2 that the process of H O2
2
accumulation (curve 4) is less efficient on single-layer
electrodes fabricated from A 437-E acetylene black
(FP-4D content 20 wt %). The absence of differences in
τ, h
operation between double-layer (curves 1, 3) and single-
layer (curve 2) electrodes can be attributed to the short
operation of the electrodes (7 h).
Fig. 2. Curves of hydrogen peroxide accumulation in
electrolysis with hydrophobized gas-diffusion carbon-black
electrodes in 0.5 M NaOH. (c) H O concentration in the
electrolyte bulk and (τ) electrolysis duration. Electrodes: (1,
) double-layer and (2, 4) single-layer. Electrode composition
wt %): (1, 2) mixture of carbon blacks A 437-E + P 701 (1 : 1)
and (3, 4) A 437-E + 20 FP-4D.
Naturally, it would be expected that, on passing to
prolonged endurance tests, the advantages in operation
of double-layer gas-diffusion electrodes fabricated from
a mixture of A 437-E and P 701 carbon blacks should
become more clearly pronounced. In [4], gas-diffusion
electrodes fabricated from a mixture of A 437-E and
P 702 operated on pure oxygen for 220 A h without any
noticeable decrease in the electrochemical activity.
2
2
3
(
It is known that O is transported by a flowing gas in
2
electrodes operating on pure oxygen and via diffusion
in those operating on air, and the transport hindrance
to the gas supply to the electrodes becomes more
important in air, as also noted in [8]. After electrolyses,
the electrochemical activity decreased for all types of
electrodes, with a change in the electrocatalytic activity
of the working layer also being a possible reason.
CONCLUSIONS
(
1) It was found that variation of the total porosity
of double-layer electrodes within the range under study
50–70 vol %) has no pronounced effect on the efficiency
of electrolysis.
(
Figure 2 shows kinetic curves of hydrogen peroxide
accumulation in electrolyses with single- and double-
layer electrodes.
(2) The optimal working layer thicknesses (0.2–
0
.3 mm) at a 70% total porosity of double layer
electrodes composed of a mixture of carbon black. At
these thicknesses, alkaline hydrogen peroxide solutions
The value of γ was determined to be 0.99–0.98 in the
reaction of O reduction to H O for all the electrodes
operating on pure oxygen. For electrodes operating
on atmospheric oxygen, γ was calculated from curves
of H O accumulation, using the equation reported in
2
2
2
–1
with concentrations of 24–29 g l can be produced from
atmospheric oxygen.
2
2
ACKNOWLEDGMENTS
[
15].
It is known that the kinetics of hydrogen peroxide
The study was supported by the Russian Foundation
for Basic Research (grant Sibir’ 09-03-98000).
accumulation affects the ratio between the rates of H O
electrosynthesis and decomposition [15]:
2
2
1
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