Effect of trialkylammonium salts and current density on the electrosynthesis of hydrogen peroxide from oxygen in a gas-diffusion electrode in acid solutions

Article abstract of DOI:10.1134/S1070427206050107

The influence exerted by the nature of cation of a supporting electrolyte and by the current density on the electroreduction of oxygen to hydrogen peroxide in acid K₂SO₄ solutions (pH 0.9-1.4) in gas-diffusion hydrophobized carbon black electrodes with varied electrolyte porosity was studied. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S1070427206050107

ISSN 1070-4272. Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 5, pp. 746 751. Pleiades Publishing, Inc., 2006.
Original Russian Text G.A. Kolyagin, V.L. Kornienko, 2006, published in Zhurnal Prikladnoi Khimii, 2006, Vol. 79, No. 5, pp. 757 761.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Effect of Trialkylammonium Salts and Current Density
on the Electrosynthesis of Hydrogen Peroxide from Oxygen
in a Gas-Diffusion Electrode in Acid Solutions
G. A. Kolyagin and V. L. Kornienko
Institute of Chemistry and Chemical Technology, Siberian Division, Russian Academy of Sciences,
Krasnoyarsk, Russia
Received June 21, 2004
Abstract The influence exerted by the nature of cation of a supporting electrolyte and by the current density
on the electroreduction of oxygen to hydrogen peroxide in acid K2SO4 solutions (pH 0.9 1.4) in gas-diffusion
hydrophobized carbon black electrodes with varied electrolyte porosity was studied.
DOI: 10.1134/S1070427206050107
It is known that the rate of electrochemical proc-
esses is strongly affected by the supporting electrolyte
and various additives [1]. For example, addition of
tetraalkylammonium salts affects the electroreduction
gen to hydrogen peroxide in an acid medium on por-
ous hydrophobized carbon black electrodes strongly
depends on the extent to which an electrolyte pene-
trates into the pore space [14, 15]. An increase in
pH makes lower the rate of further reduction of H O
of CO and CO [2], acetophenone [3], 1,1,2-trifluoro-
2
2 2
trichloroethane [4, 5], and trichlorofluoromethane [6],
electrodeposition of gold [7] and copper [8], and elec-
troreduction of oxygen to H O [9 11] and of H O to
to water. Addition of a salt with a common anion into
an acid solution diminishes the diffusion- and migra-
tion-related hindrance in proton transfer deep into the
electrode, which leads to an increase in the pH of the
catholyte and in the current efficiency. Raising the
rate of agitation of a solution makes higher the con-
tent of protons in the near-electrode layer and, con-
sequently, leads to an increase in the rate of reduction
of hydrogen peroxide to water. The rate of oxygen
reduction is also strongly affected by the cathode cur-
rent density [16].
2
2 2
water [12]. Changes in the reduction kinetics of oxy-
gen and H O in the presence of tetraalkylammonium
2
2
cations are due to their adsorption on the electrode
and to changes in parameters of the electrical double
layer [1, 9].
In [9], a 0.26 M solution of H O was prepared on
2
2
a hydrophilic porous glassy-carbon electrode from
oxygen dissolved in a 0.1 M H SO solution under
2
4
atmospheric pressure with addition of trioctylmethyl-
ammonium chloride ([CH (CH ) ] CH NCl) with a
In this context, it is of scientific and practical in-
terest to study the influence exerted by the nature of
electrolyte cations and current density on the electro-
synthesis of H O on porous hydrophobized elec-
3
2 7 3
3
current efficiency of up to 55% at a geometric current
2
density of 30 mA cm . In [10, 11], a three-phase sys-
2
2
tem constituted by an acid aqueous electrolyte, organ-
ic phase, and oxygen gas with 2-ethyl-9,10-anthraqui-
none as electron-transfer catalyst and addition of tetra-
butylammonium perchlorate and trioctylmethylammo-
nium chloride was used to accelerate the electrosyn-
thesis of H O on electrodes of this type.
trodes of varied structure. Also, mention should be
made of the close attention to electrosynthesis of acid
solutions of H O , associated with their use for in situ
oxidation of various organic compounds both for syn-
thesis and for mineralization of wastewater [17, 18].
2
2
2
2
In this study we examined the influence exerted by
addition of tetraalkylammonium salts to electrolytes
on the kinetics of accumulation of hydrogen peroxide
in acid aqueous solutions in electroreduction of oxy-
gen on gas-diffusion hydrophobized carbon black
electrodes with varied electrolyte porosity.
It is known that porous gas-diffusion hydro-
phobized electrodes are widely used for intensifying
electrosynthesis with gas reagents poorly soluble in
water [13].
It has been shown that the electroreduction of oxy-
746

EFFECT OF TRIALKYLAMMONIUM SALTS AND CURRENT DENSITY
EXPERIMENTAL
747
2
i, mA cm
The experimental conditions and the electrolyzer
design were similar to those described in [13 15, 19].
The preparative electrolysis was performed in the
galvanostatic mode at a geometric current density of
2
50 100 mA cm at 20 C in the course of 6 7 h
in an electrolyte of composition 0.2 M K SO
+
2
4
0.087 M H SO (pH 1.4) with and without addition of
2
4
tetrabutylammonium bromide (0.01 M). The current
density was calculated per unit apparent frontal sur-
2
face area (5 cm ). The catholyte volume was 0.023 l.
Oxygen was delivered into the electrode from its
backside under atmospheric pressure. The catholyte
was agitated by bubbling argon via a tube 1.5 in diam-
1
eter permanently at a rate of 10 ml min (agita-
E, V
Fig. 1. Polarization curves for electroreduction of oxygen
tion mode no. 1) or hourly for 1 min at a rate of
38 ml min before sampling the solution for analysis
for H O (agitation mode no. 2). The catholyte and
1
on gas-diffusion carbon black electrode containing
a
60 wt % fluoroplastic in solutions of various compositions.
(i) Current density and (E) electrode potential; the same
for Fig. 2. Solution composition: (1) 0.11 M H SO ,
2
2
anolyte were separated by an MF-4SK-100 cation-
exchange membrane. The electrodes were fabricated
from an electrode paste containing an A-437E acetyl-
ene black and 8 or 60 wt % 4D fluoroplastic (Teflon).
The electrode paste was preliminarily heated to 300 C,
and electrodes were sintered in air at 360 C in the
course of 10 min. To eliminate hindrance to oxygen
supply into the electrode, the electrodes were fabri-
cated with a high porosity of 65 vol %. The electrolyte
porosity was measured by hydrostatic weighing [20].
It was, on the average, 7.6 and 1.3 vol % for elec-
trodes with 8 and 60 wt % fluoroplastic, respectively.
The fraction of current expended for the synthesis of
hydrogen peroxide was determined by gasometry [19].
The electrode potential was measured relative to a
silver chloride reference electrode (EVL-1M1). Polari-
zation curves were measured at a potential sweep rate
2
4
(2) 0.025 M K SO + 0.12 M H SO , (3) 2 + 0.05 M
2
4
2
4
TEAC, (4) 2 + 0.01 M DEDBAB, and (5) 2 + 0.01 M
TBAB.
2
i, mA cm
1
of 4 mV s under continuous agitation of the electro-
lyte. A solution of 0.025 M K SO + 0.12 M H SO
2
4
2
4
(pH 0.9) served as the main electrolyte, to which
the following salts were added: tetrabutylammonim
bromide (TBAB), tetrabutylammonim iodide (TBAI),
diethyldibenzylammonium bromide (DEDBAB), tetra-
ethylammonium chloride (TEAC), and allyltrimethyl-
ammonium bromide (ATMAB). Also, in some experi-
ments K SO was replaced with LiCl or CsNO .
E, V
Fig. 2. Polarization curves for electroreduction of oxygen
on a gas-diffusion carbon black electrode containing 8 wt %
fluoroplastic in (1) 0.1 M H SO solution, (2) solution of
0.025 M K SO + 0.12 M H SO , and (3 5) that with
TBAB added in various concentrations. TBAB concentra-
2
4
2
4
2
4
tion (M): (3) 0,001, (4) 0.01, and (5) 0.1.
2
4
3
The polarization curves obtained are shown in
Figs. 1 and 2. It should be noted that the fraction of
the current expended for the synthesis of H O in all
oxygen by the kinds of anions of the salts used or of
alkali metal cations was insignificant. The values of
and the polarization curves for electroreduction of
O , obtained in solutions of 0.025 K SO + 0.12 M
2
2
of the electrolytes studied, except for acid solutions
without additives, is high. This indicates that the
curves refer virtually only to the oxygen reduction to
H O . The influence exerted on the reduction of
2
2
4
H SO , 0.05 M LiCl + 0.12 M H SO , and 0.05 M
2
4
3
2
4
CsNO + 0.12 M H SO are identical. The reduction
2
4
of oxygen begins in these solutions at +0.05 V. Simi-
2
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 5 2006

748
KOLYAGIN, KORNIENKO
Results of a preparative electrosynthesis of hydrogen peroxide and electrolysis parameters* (electrolysis time 6 h)
E, V
Equation
= b acH2O = b1 a1cH2O
2
Teflon
content,
wt %
cH2O
M 2
,
Run
no.
i,
2
mA cm
fraction units at
initial
final
cH2O = 0.9 M
2
2
1
2
3**
4
5
8
50
100
100
50
100
100
0.31
0.38
0.40
0.74
1.20
1.32
0.44
0.47
0.51
0.90
1.37
1.39
0.87
1.82
1.91
0.87
1.11
1.43
0.81
0.90
0.95
0.66
0.49
0.73
0.75
0.89
0.92
0.73
0.56
0.73
0.95 0.16cH2O
0.95 0.22cH2O
2
2
0.94 0.042cH2O
0.94 0.058cH2O
0.95 0.034cH2O
2
2
const = 0.95
2
60
0.99 0.37cH2O
0.94 0.23cH2O
2
2
0.89 0.43cH2O
0.89 0.37cH2O
2
0.88 0.16cH2O
2
2
0.86 0.15cH2O
2
6**
* i, E, and
are the current density, electrode potential, and integral current efficiency by H O , respectively.
2 2
** Electrolysis with addition of 0.01 M TBAB to the electrolyte.
lar results were obtained in solutions of 1.0 M CsNO ,
3
c, M
(a)
1.0 M LiCl, and 0.5 M K SO .
2
4
The enhancement of the electrode polarization at a
higher Teflon content (60 wt %) (see table, Fig. 1) is
due to a decrease in the volume of electrolyte-filled
pores at a higher content of fluoroplastic, which
makes thinner the zone of the oxygen electroreduc-
tion. The true working surface area of the electrode
decreases and the true current density increases, which
enhances the electrode polarization.
The polarization curves obtained in solutions with
TBAI and ATMAB additives are not shown in Fig. 1
because they coincided with the curves measured in
solutions with TBAB and TEAC, respectively. Fig-
ure 1 shows for comparison the curve obtained in
0.1 M H SO (pH 0.9). Figures 1 and 2 show that
, h
2
4
addition of tetraalkylammonium salts accelerates the
oxygen reduction; this effect is also observed upon
addition of these salts to 0.1 M solutions of H SO
c, M
(b)
2
4
and HCl. Addition of TBAB has the strongest effect,
and that of TEAC, the weakest. No specific effect of
the unsaturated bond present in the tetraalkylammo-
nium cation (ATMAB) or of the benzene ring in
DEDBAB on the oxygen reduction was observed. The
presence of tetraalkylammonium salts in an alkaline
electrolyte (0.5 M NaOH) has no effect on the polari-
zation characteristics of the oxygen reduction. The in-
fluence exerted by the salt concentration on the elec-
trode polarization is illustrated for TBAB as example
(Fig. 2, curves 3 5). Raising the TBAB concentration
from 10 to 10 M accelerates the process, but
further increase in this concentration to 0.1 M leads to
a decrease in the rate of oxygen reduction, which is
commonly the case in practice of electrosynthesis,
e.g., in dehalogenation of 1,1,2-trifluorotrichloroeth-
ane [4]. As the current density is raised, the effect of
tetraalkylammonium cations on the electrode polariza-
4
2
, h
Fig. 3. H O accumulation curves obtained on a gas-
diffusion carbon black electrode containing (a) 60 and
2
2
(b) 8 wt % fluoroplastic under varied conditions. (c) H O
concentration and ( ) time. Current density (mA cm ):
2
2
2
(1, 2) 100 and (3, 4) 50. Electrolyte: (2 4) 0.2 M K SO +
2
4
0.087 H SO and (1) the same with addition of 0.01 M
2
4
TBAB. Agitation mode: (1 3) no. 2 and (4) no. 1.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 5 2006

EFFECT OF TRIALKYLAMMONIUM SALTS AND CURRENT DENSITY
tion tends to decrease. This is particularly noticeable
749
when performing a preparative electrolysis. The results
obtained in the preparative electrolysis are shown
in Figs. 3 and 4 and listed in the table. The data
in the table correspond to the conditions in which
curves 1 3 in Fig. 3 were measured.
An increase in the current density makes higher
the rate of accumulation and the concentration of hy-
drogen peroxide, but, in this case, the change in and,
consequently, in the current efficiency depends on
the electrolyte porosity of the electrode. For example,
2
upon an increase in i from 50 to 100 mA cm , the
slope of the dependence = f(c
) decreases by
H O
2
2
c, M
a factor of 3.8 on electrodes containing 8 wt % fluoro-
plastic and increases by a factor of approximately 1.2
on electrodes with 60% fluoroplastic (see table). As
the current density increases in the case of electrodes
containing 8 wt % Teflon, the H O concentration, ,
Fig. 4. Differential current efficiency vs. the concentra-
tion c of H O in an electrolyte (2, 4) without and (1,
3) with TBAB added. Teflon content, wt %: (1, 2) 8 and
(3, 4) 60.
2
2
2
2
and current efficiency become higher. Raising the Tef-
lon content to 60 wt % leads to a decrease in the ex-
tent to which the H O concentration grows, with a
The rate of hydrogen peroxide decomposition in
the presence of TBAB, as well as that in other acid
solutions, is described by a first-order equation. A cer-
tain decrease in the rate of H O decomposition in
the presence of TBA sulfate, reported in [12], was not
observed in this study.
2
2
decrease in and current efficiency observed. This is
accounted for by the fact that, as the fluoroplastic con-
tent decreases, the volume of electrolyte-filled pores
increases and the reaction zone extends deeper into the
electrode, away from its frontal surface. As a result,
the hindrance to proton transport into the electrode is
enhanced and thereby the rate of further reduction of
H O is lowered. To illustrate the effect of mass
2
2
The increase in upon addition of TBAB to the
electrolyte may be due to an increase in the rate of
oxygen reduction to H O and to a decrease in the
2
2
2
2
rate of further reduction of H O to water. The de-
2
2
transfer of protons, Fig. 3 shows curves of H O ac-
2
2
crease in the rate of H O reduction is due to a de-
2
2
cumulation in different catholyte agitation modes. In
an excess of water in neutral and alkaline solutions,
synthesis of H O involves water and, therefore, the
crease in the surface activity of protons in the pres-
ence of tetraalkylammonium ions. For example, it was
shown in [12] that, being adsorbed on the electrode,
tetrabutylammonium sulfate markedly diminishes the
rate of H O reduction to water. The calculations
2
2
effects observed above are not revealed [15].
The table shows that the addition of TBAB and an
increase in the current density have no effect on the
run of the dependence of and current efficiency on
the H O concentration [14, 15]. The accumulation
2
2
made in [9] for an acid solution containing trioctyl-
methylammonium chloride (TOMAC) show that tetra-
alkylammonium cations adsorbed on the electrode can
raise the pH of the diffuse part of the electrical double
layer from 0.9 to 9.4. Under these conditions, oxygen
is reduced on the electrode, with water molecules
2
2
curves obtained in the presence of TBAB (Fig. 3)
have the common shape [9, 14, 18] and are described
by the equation [14, 15]
involved, to give the peroxide ion (HO ) and OH ,
1
AI b V 1{exp[( AI a V
k) ]
1}
2
and the H O molecule is only formed after HO is
c =
,
2
2
2
1
AI b V
k
removed from the electrode surface and interacts with
a proton in the catholyte bulk. The rate constant of
oxygen reduction to H O increases in the presence of
where c is the concentration of H O in the catho-
2
2
2
2
lyte (M); A, electrochemical equivalent of H O
tetraalkylammonium cations. For example, the rate
2
2
1
1
13
(mol A h ); I, current (A); a and b, coefficients of
the equation = f(c ) (see table); V, catholyte
constant in 0.1 M H SO increases from 5.1 10
to 9.9 10
2
4
10
1
H O
m s in the presence of 0.017 M
2
2
volume (l); , electrolysis time (h); and k, rate con-
TOMAC. The TOMAC layer formed on the electrode
surface favors an increase in the content of oxygen in
1
stant of H O decomposition (h ).
2
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 5 2006

750
KOLYAGIN, KORNIENKO
the catholyte and makes lower its diffusion coefficient
[9]. The decrease in the diffusion coefficient of oxy-
gen, observed when a certain concentration of TBAB
of tetrabutylammonium bromide to an acid electrolyte
make higher the concentration of H O . In this case,
2
2
the change in the fraction of the current expended for
oxygen reduction to H O and in the current effi-
is reached, makes lower the rate of O reduction
2
2
2
ciency depend on the electrolyte porosity of the elec-
trode. An appropriate combination of the electrolyte
porosity, composition of the electrolyte, and mode
of its agitation can provide that the current efficiency
will increase with the current density. The effect of
the above-mentioned factors is based on a change
in the parameters of the electrical double layer and
in the pH in the pore space of the electrode and in
the near-electrode layer.
(Fig. 2).
The effect of TBAB on the H O accumulation
2
2
depends on the electrode structure. For example, in
6 h of electrolysis on electrodes containing 60 wt %
fluoroplastic in the presence of TBAB, the concentra-
tion of H O increased by 0.32 M and the slope of the
2
2
= f(c
) dependence decreased by a factor of 2.5,
H O
whereas 2on electrodes containing 8 wt % fluoroplastic,
2
the concentration of H O and the value of
in-
2
2
creased only slightly (see table). As demonstrated
above, the higher the electrolyte porosity of the elec-
trode, the stronger the hindrance to transport of pro-
tons into the electrode, and, hence, the weaker the
effect of TBAB and the change in the rate of H O
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2
2
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2
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2
2
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2
4
3
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