Novel organic redox catalyst for the electroreduction of oxygen to hydrogen peroxide

Article abstract of DOI:10.1016/j.electacta.2012.01.086

The organic redox catalysis of O₂ electroreduction to H ₂O₂ in acidic media has been investigated using several quinone and riboflavin catalysts supported on Vulcan XC72 carbon. The synthesis of a novel riboflavinyl-anthraquinone 2-carboxylate ester (RF-AQ) is reported. The activity and selectivity of organic redox catalysts (riboflavin, anthraquinone derivatives and riboflavinyl-anthraquinone 2-carboxylate ester) for the electrosynthesis of H₂O₂ were investigated by the rotating ring-disk electrode (RRDE) method and potentiostatic electrolysis. Electrodes with 10 wt% RF-AQ loading on Vulcan XC-72 showed excellent electrocatalytic activity toward the two-electron oxygen reduction coupled with very good catalyst layer stability. The reaction mechanism for the organic redox catalysis by RF-AQ is discussed. Electroreduction of O₂ dissolved in 0.5 M H₂SO₄ under potentiostatic conditions (0.1 V vs. RHE) at 21 °C using the composite RF-AQ/Vulcan XC72 catalyst (total loading 2.5 mg cm^-2) deposited on unteflonated Toray carbon paper, generated H₂O₂ with an initial rate of 21 μmol h^-1 cm_geo^-2 and a stable current efficiency of 70%.

Full text of DOI:10.1016/j.electacta.2012.01.086

Electrochimica Acta 66 (2012) 222–229
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Novel organic redox catalyst for the electroreduction of oxygen
to hydrogen peroxide
∗
Andrew Wang, Arman Bonakdarpour, David P. Wilkinson, El o˝ d Gyenge
Department of Chemical and Biological Engineering and Clean Energy Research Center, 2360 East Mall University of British Columbia, Vancouver, BC, V6T 1Z3, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
The organic redox catalysis of O2 electroreduction to H O in acidic media has been investigated
2
2
Received 24 October 2011
Received in revised form 21 January 2012
Accepted 23 January 2012
Available online 1 February 2012
using several quinone and riboflavin catalysts supported on Vulcan XC72 carbon. The synthesis of a
novel riboflavinyl–anthraquinone 2-carboxylate ester (RF–AQ) is reported. The activity and selectiv-
ity of organic redox catalysts (riboflavin, anthraquinone derivatives and riboflavinyl–anthraquinone
2
-carboxylate ester) for the electrosynthesis of H2O2 were investigated by the rotating ring-disk elec-
trode (RRDE) method and potentiostatic electrolysis. Electrodes with 10 wt% RF–AQ loading on Vulcan
XC-72 showed excellent electrocatalytic activity toward the two-electron oxygen reduction coupled with
very good catalyst layer stability. The reaction mechanism for the organic redox catalysis by RF–AQ is
discussed. Electroreduction of O2 dissolved in 0.5 M H2SO4 under potentiostatic conditions (0.1 V vs. RHE)
Keywords:
Electrosynthesis of H2O2
O2 reduction
Modified electrodes
Anthraquinone
Riboflavin
◦
−2
at 21 C using the composite RF–AQ/Vulcan XC72 catalyst (total loading 2.5 mg cm ) deposited on unte-
®
−1
−2
flonated Toray carbon paper, generated H2O2 with an initial rate of 21 ␮mol h cmgeo and a stable
current efficiency of 70%.
©
2012 Elsevier Ltd. All rights reserved.
1
. Introduction
Bonakdarpour et al. reported that Co C based electrocatalysts can
be effective in acidic media [7]. The electrocatalysis of O electrore-
2
Hydrogen peroxide has an expanding market with a wide
duction to H O has been intensely investigated by research groups
2
2
range of applications in industries including pulp and paper (50%),
propylene oxide production (40%), textile, food and environmental
under the CLETEPEG Project and the European Union Framework
Program [8–14].
sectors (10%) [1]. The global demand for H O2 is over two million
The Riedl–Pfleiderer process, where an anthraquinone com-
pound is shuttled between reduction and oxidation (i.e. redox
catalysis), served as an inspiration for electrocatalysis as well.
Since it is well established that hydrogenated 9,10-anthraquinone
derivatives spontaneously react with molecular oxygen to form
hydrogen peroxide [15], Gyenge and Oloman used an emulsion of
2-ethylanthraquinone with a cationic surfactant, and reticulated
vitreous carbon or graphite felt as the cathode to produce up to
2
ton per annum. The major commercial production method is the
Riedl–Pfleiderer process, which accounts for over 90% of the global
H O production. This process involves an alkyl anthraquinone,
2
2
dissolved in a mixture of polar and nonpolar solvents (such as
polyalkylated benzenes and alkyl phosphates), cycled between
hydrogenation, catalyzed by suspended Raney Ni or Pd, and auto-
oxidation in air to generate H O . Further purification by extraction
2
2
−
0.6 M of H O in 2 M Na SO electrolyte at 800 A m at about 50%
2 2 2 4
2
and distillation produces H O up to 70 wt% with over 95% yield [1].
2
2
The capital and energy intensive Riedl–Pfleiderer process is advan-
tageous only with the economy of scale, and is not applicable to
current efficiency [16,17].
Much effort has been made to carry out O2 electroreduction
smaller scale synthesis of H O .
to H O2 mediated by surface immobilized quinones. Tammeveski
2
2
2
Alternative methods of H O synthesis include reduction of O
et al. investigated the quinone mediated two-electron O2 elec-
troreduction in alkaline solutions [9,12–14]. However, the quinone
reduction potential shifts towards more negative values with
increasing pH [8], leading to high cathodic overpotentials.
2
2
2
to H O2 via electrochemical routes, such as trickle-bed reactors
2
using alkaline electrolyte and high-surface graphite cathodes [2,3],
and in H –O fuel cells operated in either alkaline or acidic media
2
2
[
4–6]. Under acidic or neutral conditions, to overcome the electrode
Methods of surface immobilization included: irreversible
adsorption [18–20], covalent attachment of diazonium salts
of quinones [8–14], and anodic polymerization of amino-
anthraquinone over the electrode surface [21–23]. Berchmans and
Vijayavalli attached riboflavin to glassy carbon electrodes through
covalent linkage of the amine or alcohol groups to the surface
carboxylic groups of the carbon substrate [24]. Manisankar et al.
kinetic limitation of the two-electron O2 reduction, the electro-
catalysis of this reaction has been extensively studied. Recently,
∗ Corresponding author. Tel.: +1 604 822 3217; fax: +1 604 822 6003.
E-mail address: egyenge@chbe.ubc.ca (E. Gyenge).
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2012.01.086

A. Wang et al. / Electrochimica Acta 66 (2012) 222–229
223
modified the electrode described by Berchmans and Vijavavalli
with several 9,10-anthraquinones and reported that riboflavin was
beneficial to the quinone reduction and thus, the subsequent rate
of O2 reduction in buffered pH neutral electrolytes increased as
well [25,26]. The majority of these investigations involved modifi-
cation of glassy carbon electrodes, where the presence of quinones
Riboflavinyl–anthroquinone-2-carboxylate ester (RF–AQ) was
synthesized as shown in Eq. (1), by first chlorinating CAQ with
thionyl chloride (Fluka) refluxed at 80 C for three hours. After
distilling off the excess thionyl chloride, the acyl chloride was dis-
solved in pyridine (Fisher Scientific) and allowed to react with
riboflavin. The excess pyridine was distilled off, and the final prod-
uct was purified by recrystallization in hexanes (Fisher Scientific),
followed by vacuum drying.
◦
−9
−2
or riboflavin (on the order of 10 mol cm ) shifts the potential of
(1)
The structures of CAQ, RF and the product RF–AQ were analyzed
by Fourier transform infrared (FTIR) spectroscopy. These measure-
ments were performed by a Bruker IFS 55 system using the KBr
pellet technique. IR spectra were collected in the mid IR range
O2 reduction to more positive values. Lobyntseva et al. prepared
a membrane-electrode assembly with Vulcan XC72 grafted with
diazonium-anthraquinone as the cathode, but the H O2 current
2
efficiencies were less than 1% [10].
−1
−1
.
(
4000–400 cm ) with a resolution of 0.5 cm
.2. Catalyst ink and electrode preparation
The catalyst in this paper refers to the composite of the organic
This paper presents results using organic redox catalysts sup-
ported on Vulcan XC72 for the electrosynthesis of H O2 in acidic
2
2
media. A carboxylate ester of anthraquinone and riboflavin was
synthesized and deposited on the Vulcan XC72 support, and inves-
tigated for the first time for electrocatalytic activity by the rotating
ring-disk electrode (RRDE) method. Potentiostatic batch electroly-
sis experiments were performed using gas diffusion cathode coated
with the novel organic redox catalyst to demonstrate the long
redox catalyst and Vulcan XC72 carbon powder. The appropriate
amount of quinone or riboflavin redox catalyst and Vulcan XC72
powder was sonicated in 280 ␮L of DI water and 70 ␮L of ethanol
for 15 min, followed by the addition of 100 ␮L of 5 wt% Nafion solu-
tion (Alfa Aesar) and sonication for 30 min to make a well-mixed
suspension.
term H O production rate and the electrode durability. The effort
2
2
of this paper aims to contribute to the development of an on-
site H O electrosynthesis unit for drinking water treatment that
2
2
For cyclic voltammetry (CV), 7 ␮L of the ink was placed on the
could eliminate the cost and risks involved in transporting, storing
and handling concentrated H O . This work is part of the Res’eau
◦
glassy carbon (GC) disk electrode and air dried at 21 C to achieve
2
2
−2
a catalyst loading of 0.66 mg cm geo. The typical dry weight ratio
WaterNET, a nationally funded research network in Canada, which
is dedicated to solving challenges faced by remote communities
with respect to drinking water supply. The Res’eau WaterNET iden-
of the catalyst layer components is 1:9:4.4 in the order of redox
catalyst:Vulcan XC72:Nafion.
For electrolysis measurements, Toray® paper with no Teflon®
tified the UV/H O2 disinfection processes as very promising for
2
treatment (TGP-H-060 Fuel Cell Earth) was cut into 2 cm × 2 cm
drinking water treatment.
squares with a 0.5 cm stem for electrical connection. The catalyst
2
ink was deposited on the 4 cm electrode in aliquots of 25 ␮L, fol-
◦
2
. Experimental methods
lowed by air drying at 21 C. After the entire area of the electrode
was covered with the desired amount of the catalyst, it was dried in
◦
2.1. Chemicals
an oven at 80 C for 12 h. Total composite catalyst (the organic redox
−
2
catalyst, Vulcan XC72 and Nafion) loadings of 1.7 to 3.5 mg cm geo
were determined by weighing the electrode before and after the
deposition.
The quinone compound and riboflavin were purchased from
Sigma–Aldrich at the highest purity available and used directly
without further purification: anthraquinone-2-carboxylic acid
(
CAQ) 98%, and riboflavin (RF) 98%. Vulcan XC72 carbon pow-
2.3. Electrochemical measurements
der was purchased from Cabot Inc. All solutions were prepared
−
1
◦
with deionized water of 18.2 Mꢀ cm resistivity at 21 C (Siemens
Cyclic voltammetry (CV) measurements were performed using
the CBP Bipotentiostat system (Pine Instruments) in a single com-
partment electrochemical cell. The working electrode consisted
of a Pine (AFE6R2GCPT) rotating-ring disk electrode (RRDE) with
Purelab). Cyclic voltammetric measurements were performed in
.1 M H SO (GFS Chemicals). Oxygen (Praxair, 99.998%) was
purged in the electrolyte through a 5 ␮m porous dispersion tube
Ace Glass) during the measurements.
0
2 4
2
(
a glassy carbon disk (0.2376 cm ) and a platinum ring electrode

2
24
A. Wang et al. / Electrochimica Acta 66 (2012) 222–229
Fig. 1. Mid-Infrared spectra of (a) CAQ, (b) RF and (c) RF–AQ.
2
(
0.2356 cm ) with a collection efficiency of about 38%. A platinum
chamber connected to the cathode chamber by a porous ceramic
frit.
wire counter electrode and a mercury/mercurous sulfate reference
electrode (Radiometer Analytical) were used for all measurements,
all immersed in about 100 mL of 0.1 M H SO .
All measurements were performed at 21 ± 0.5 C and 1 bar. The
Pt ring electrode was cleaned by cycling between 0 and 1.2 V vs. RHE
until steady state CVs were obtained. During the measurements, the
All the potential measurements reported are referenced to
the reversible hydrogen electrode (RHE). The RHE scale for the
Hg/Hg2SO4 reference electrode was established by purging H2 in
the electrolyte and recording the open circuit potential difference
between the reference electrode and a Pt wire electrode in the same
chamber.
2
4
◦
ring electrode was held constant at 1.2 V vs. RHE to record the H O2
2
oxidation current.
First, a CV in the N -purged electrolyte was recorded to char-
2.4. H2O2 analysis by UV–vis spectrophotometry
2
acterize the redox behavior of the quinone based catalysts. Next,
O2 was purged for 30 min in the electrolyte, and its saturation in
the electrolyte was maintained by allowing an O2 blanket above
the solution for the short-term duration of the oxygen reduction
reaction (ORR). After the ORR, a second CV measurement in the N2-
purged electrolyte was again performed to determine the adhesion
of the quinone-based catalysts on the carbon support. ORR was per-
formed with the RRDE assembly rotating at fixed rotation speeds
of 100, 400 and 900 rpm.
The concentration of H2O2 was determined by the UV spec-
trophotometric method described by Klassen et al. [27]. The UV
absorption measurements were performed with a Varian Cary 100
UV–vis spectrophotometer. The sample pH had a significant impact
on the absorption [28], and therefore, the reference measurements
were conducted using the fresh electrolyte (0.5 M H2SO4) with the
corresponding pH. This method is sensitive in the ␮M range.
Bulk electrolysis experiments were performed using an H-
cell connected to the same CBP Bipotentiostat system. The anode
and cathode were housed in chambers separated by Nafion® 112
3. Results and discussion
3.1. Structural characterization of
(
Dupont) membrane, which was pretreated by a three step boiling
riboflavinyl–anthroquinone-2-carboxylate ester
treatment in 3 wt% H O , DI water, and 0.5 M H SO . The cathodic
2
2
2
4
potential was fixed at 0.1 V vs. RHE throughout the 24 h dura-
tion of the electrosynthesis. O2 was purged for 30 min before the
experiments and continued during the experiments to maintain
O2 saturation in the electrolyte. The cathodic chamber contained
The structures of anthraquinone-2-carboxylic acid (CAQ),
riboflavin (RF) and riboflavinyl–anthraquinone-2-carboxylateester
(RF–AQ) are shown in Eq. (1). The mid IR spectra of CAQ, RF and
−
1
RF–AQ are presented in Fig. 1. The broad peak at about 3330 cm
(Fig. 1b and c) is due to the O H stretch of H-bonded alcohol groups
1
0
36 mL of the electrolyte (0.5 M H SO ) with constant stirring.
2 4
−
1
.5 M H SO was used to provide sufficient ionic conductivity and
observed in both RF and RF–AQ. Peaks at 1730, 1240 and 1080 cm
2
4
protons for the reaction. Moreover, the flooded batch electroly-
sis cell setup will provide a qualitative indication of the catalyst’s
performance in solid polymer fuel cells employing a Nafion® mem-
brane. Samples of 3.5 mL were periodically taken from the cathode
(Fig. 1c) are generated by one C O stretch and two C O stretch
modes of an unsaturated ester, which in this case is a characteristic
feature of RF–AQ. The peak at about 1720 cm observed in Fig. 1b
−1
is due to the C O stretch of the ketone groups in RF. This peak
in Fig. 1c is likely merged with the more prominent C O stretch of
chamber and replaced with fresh electrolyte. A Hg/Hg SO4 refer-
2
−
1
ence electrode was used and located in the reference electrode
the ester at 1730 cm in the RF–AQ spectrum. Another C O stretch

A. Wang et al. / Electrochimica Acta 66 (2012) 222–229
225
Fig. 2. Organic redox catalyst screening by cyclic voltammetry in N2 and O2 purged electrolytes: (a) Vulcan XC-72 (support baseline), (b) 10 wt% RF–AQ, (c) 10 wt% CAQ, and
−
2
(
d) 10 wt% RF. The experiments were performed using a total composite catalyst (organic redox catalyst and Vulcan XC-72) loading of 0.66 mg cm on the RRDE tip, 0.1 M
◦
H2SO4 electrolyte, at 21 C and 1 bar pressure.
mode is observed at 1680 cm−1 for both RF–AQ (Fig. 1c) and CAQ
loadings on a weight basis. It was also observed that RF–AQ (MW
−
1
(
Fig. 1a), due to the ketone groups of anthraquinone. C N stretch
610 g mol ) with a much lower molar loading showed almost
as large a reduction peak current density as CAQ; however, its
reduction peak potential was the same as that of RF. This could
be attributed to the reduction of the RF constituent in RF–AQ, and
subsequently mediating the reduction of the AQ constituent.
In the O2-saturated electrolyte all three catalysts generated
reduction peaks at potentials virtually equal to the charac-
teristic catalyst reduction peak potential in the absence of
O2. The corresponding reduction peak current densities are
composed of two electrode reactions: net O2 electroreduction
and redox catalyst electroreduction, respectively (Fig. 2b–d).
RF–AQ generated the highest reduction peak current density,
modes of aromatic amines are observed for both RF–AQ (Fig. 1c)
−1
and RF (Fig. 1b) at about 1270 and 1340 cm
.
The spectral features of CAQ that are unique to the carboxylic
acid group (Fig. 1a) are not observed in RF–AQ (Fig. 1c), validating
therefore that the esterification reaction (1) with RF has occured
and the RF–AQ product was well purified. Thus, the very broad band
−1
between 3400 and 2400 cm due to the O H stretch of carboxylic
acid is only observed for CAQ (Fig. 1c). Moreover, the peaks at 1700,
−1
1
280 and 930 cm representing the stretch modes of C O, C O
and O H respectively of the carboxylic acid group, are observed
only in the CAQ spectrum (Fig. 1c). Thus, the IR analysis supports
the structure of RF–AQ shown in Eq. (1).
−
2
−2
−1.6 mA cm followed by RF with −1 mA cm and lastly CAQ with
−
2
−
0.75 mA cm
.
Fig. 3a shows the net faradaic current due to O electroreduction,
2
3.2. Electroanalytical and electrolysis measurements
after correction for the charging and discharging of the double layer
capacitance and the intrinsic catalyst reduction in the absence of
3.2.1. Screening of organic redox catalysts
O2 (measurements in N -purged electrolyte). Fig. 3b presents the
2
Organic redox catalysts supported on Vulcan XC72 with a
oxidation current of H O collected at the ring electrode (collection
2
2
loading of 10 wt% redox catalyst were examined by cyclic voltam-
metry in 0.1 M H SO , including riboflavinyl–anthroquinone-2-
efficiency equal to 38%). The selectivity of the catalysts towards
H O formation, shown in Fig. 3c was calculated from the disk and
2
4
2
2
carboxylate ester (RF–AQ), anthraquinone-2-carboxylic acid (CAQ),
and riboflavin (RF). Fig. 2 shows the cyclic voltammograms for the
ring currents as follows:
investigated catalysts on the RRDE electrode, in N -purged and
2
200 × Iring
Selectivity =
(2)
O -purged 0.1 M H SO , respectively. The oxygen reduction onset
2
2
4
N × |Idisk| + Iring
−2
potential defined at −0.1 mA cm is about 0.2 V vs. RHE on the bare
Vulcan XC72 (Fig. 2a). In de-aerated 0.1 M H SO , the formal redox
where N, Iring and Idisk refer to the collection efficiency of the ring,
ring current, and disk current respectively [29].
The standard potential of oxygen reduction to H O is 0.70 V vs.
SHE [30]. Modifications of the Vulcan XC72 by RF–AQ (10 wt%) or RF
(10 wt%) increased the onset potential of the O reduction to H O
2
4
potential for both RF–AQ (Fig. 2b) and RF (Fig. 2d) is about 0.2 V
whereas for CAQ, it is about 0.15 V (Fig. 2c). All the organic redox
catalysts are electrochemically active.
2
2
−
1
It was observed that RF (molecular weight 376 g mol ) showed
2
2
2
−
1
a lower reduction peak current density than CAQ (MW 252 g mol
)
by up to 50 mV compared to Vulcan XC72 and CAQ (Fig. 3a). Further-
more, for disk electrode potentials between 0.25 V and -0.1 V the
in the de-aerated electrolyte, for the same redox catalyst

2
26
A. Wang et al. / Electrochimica Acta 66 (2012) 222–229
Fig. 4. RF–AQ redox waves with 10 wt% RF–AQ loading. (a) Redox waves of the
RF–AQ couple, and (b) averaged peak current density as a function of potential sweep
rate. The experiments were performed using a total composite catalyst loading of
−
2
0
.66 mg cm on the RRDE tip, 0.1 M H2SO4 electrolyte, at 295 K and 1 bar pressure.
3
.2.2. Reaction mechanism of O2 reduction to H O catalyzed by
2 2
RF–AQ
Fig. 4a shows cyclic voltammograms as a function of sweep rate,
recorded in N -saturated 0.1 M H SO electrolyte using the 10 wt%
2
2
4
RF–AQ catalyst deposited onto the RRDE GC tip with an effective
−
7
−2
RF–AQ coverage of 1.07 × 10 mol cm geo (calculated from the
composite catalyst composition). A reversible wave is observed
with a formal redox potential at 0.2 V vs. RHE. The reduction peak
current density was a linear function of the potential sweep rate
(Fig. 4b). Thus, assuming Nernstian behavior of the surface modi-
fied electrode, the number of electrons can be estimated from the
peak current densities [31]:
2
2
∗
n F vA ꢁ
4RT
RF−AQ
Fig. 3. Oxygen electroreduction study on rotating ring-disk electrode (RRDE) using
0 wt% organic redox catalyst loading on Vulcan XC-72. (a) O2 reduction measure-
jp =
(4)
1
ments obtained from the rotating disk electrode, (b) ring current densities, and (c)
Selectivity of the catalysts for reduction of O2 to H2O2. The experiments were per-
where n, ꢂ, A and ꢁ *RF–AQ represent the number of electrons trans-
ferred, potential sweep rate, geometric area of the electrode, and
the coverage of RF–AQ, respectively.
−
2
formed using a total composite catalyst loading of 0.66 mg cm on the RRDE tip,
◦
0
.1 M H2SO4 electrolyte, at 21 C and 1 bar pressure.
The one electron reduction of RF–AQ in the N -purged elec-
2
trolyte raises the question whether the anthraquinone structural
unit or the riboflavin constituent participate in the electroreduc-
tion and mediation of the O2 reduction. It is evident that RF has
a pronounced effect in the mediated O2 reduction reaction. The
mechanism by which the anthraquinone constituent improved the
H O yield and selectivity (Fig. 3b and c) is unknown and deserves
H O generation rate was the fastest in case of the RF–AQ catalyst
2
2
as shown by the higher H O oxidation ring current densities (up
2
2
−2
to 0.5 mA cm , Fig. 3b) compared to the other catalysts. For disk
electrode potentials below 0.1 V, a selectivity for H O of 85 ± 5% is
2
2
2
2
further investigation.
observed for the RF–AQ catalyst, which is 20% higher than for the
baseline Vulcan XC72 (Fig. 3c). The selectivity is influenced by the
A reaction scheme for the mediated O reduction is proposed in
2
Fig. 5, where the radicals (b or c) formed by the one-electron reduc-
tion of RF (a) is responsible for the electrocatalytic activity for O2
reduction, as supported by reports of mediated O2 reduction by
competing secondary reaction, which is the reduction of H O2 to
2
H O:
2
many quinoid compounds [19]. The radicals (b or c), stabilized by a
number of resonant structures, reacts thermochemically with O2 to
form peroxyl radical adducts (d or e). The formation of the adducts
V
H O + 2H + 2e− → 2H O, E0 = 0.53
+
(3)
2
2
2
V vs. SHE
has been proposed as the rate determining step [34,35]. H O2 is
2
Riboflavin (RF) itself shows pronounced activity towards O2
thereafter formed via the subsequent electro-reduction of adduct
(d or e) to adduct (f or g) followed by its molecular rearrange-
ment with simultaneous regeneration of the riboflavin structural
unit (referred to also as redox mediation).
reduction to H O , as well as assistance to the quinone reduction
2
2
(
Fig. 3a–c). The constitutional groups of the redox catalyst have
profound impact on the catalyst’s activity towards O2 electrore-
duction. Among the investigated organic redox catalysts, RF–AQ
Riboflavin has been reported to assist the reduction of quinones
[25,26]. In the present study, it is evident that the RF structural
has been found to be the most active and selective for O reduction
2
to H O . Therefore it was down selected for further studies.
−
2
2
unit is active for the 2e O2 reduction in the RF–AQ catalyst. The

A. Wang et al. / Electrochimica Acta 66 (2012) 222–229
227
Fig. 5. Proposed reaction mechanism scheme of RF–AQ mediated O2 electroreduction to H2O2.
molecular and electronic interaction of the RF and AQ is unknown at
present but it could involve activation of the thermochemical reac-
and after performing the oxygen reduction experiments. The redox
waves of the immobilized RF–AQ showed no decrease in peaks,
suggesting good adhesion to the GC tip of the RRDE during the
entire time of measurements. Furthermore, the ORR voltammo-
grams were also stable for several cycles. The immobilization is
attributed to the ␲–␲ interaction between the polycyclic aromatics
and the carbon support [33].
tion between the semiquinol radical and O and/or stabilization of
2
the adduct (d or e).
From the RRDE experiments of the RF–AQ modified Vulcan XC72
carbon with 5 and 10 wt% RF–AQ contents, and the unmodified Vul-
can XC72 catalysts at rotation speeds of 100, 400, 900 and 1200 rpm,
the Koutecky–Levich relationship at 0.1 V vs. RHE was generated
and it is shown in Fig. 6:
1
jF
1
1
1
1
= j + j = −nFkC
− 0.62nFD2
(5)
/3
O2
k
dl
O2,b
v
−1/6CO2,bω1/2
where jF, j , j , k, C , F, D, n, ꢂ and ω represent the total faradaic
k
dl
b
current density, kinetic current density, diffusion limited current
density, electrochemical rate constant for O2 reduction, bulk con-
centration of the analyte (dissolved oxygen), Faraday’s constant,
diffusivity of the analyte, number of electrons transferred, viscos-
ity of the electrolyte, and angular velocity of the rotating electrode,
respectively.
Using
the
literature
reported
values
of
−1
CO
=
2,b
−
6
−3
−5
2
1
.26 × 10
mol cm
,
DO2 = 1.93 × 10
cm s ,
and
−2
2
−1
ꢂ = 1.009 × 10 cm s
[32], and the reciprocal of the slopes
in Fig. 6, n is estimated to be about 2, confirming the two electron
reduction of O2 mediated by the catalysts. From the reciprocal
intercept of the Koutecky–Levich plots, the electrochemical rate
−
6
constants (at 0.1 V vs. RHE) are estimated to be 1.51 × 10
,
−
6
−6
−1
4
.53 × 10 , and 6.91 × 10 cm s
for the Vulcan XC72 carbon
baseline, 5 wt% RF–AQ modified Vulcan XC72, and 10 wt% RF–AQ
modified Vulcan XC72 catalysts, respectively. Thus, the RF–AQ
modification significantly improved the kinetics of O reduction.
2
3
.2.3. Adhesion and monolayer coverage of RF–AQ on Vulcan
XC-72
The adhesion of RF–AQ to Vulcan XC72 and the RF–AQ/Vulcan
Fig. 6. Koutecky–Levich plot for O2 reduction on the unmodified Vulcan XC72, 5 wt%
and 10 wt% RF–AQ on Vulcan XC72 catalysts. The experiments were performed using
XC72 catalyst to the GC disk were examined by comparing the
−2
◦
a total composite catalyst loading of 0.66 mg cm , 0.1 M H2SO4 electrolyte, at 21 C
and 1 bar pressure. Oxygen saturated electrolyte.
cyclic voltammograms recorded in the N purged electrolyte before
2

2
28
A. Wang et al. / Electrochimica Acta 66 (2012) 222–229
indicating desorption of the RF–AQ from the carbon support at high
loadings.
3
.2.4. Chronoamperometric batch H O electrosynthesis
2 2
Fig. 7 shows the H O2 yield and current efficiency of the Vul-
2
can XC72 carbon baseline and a number of RF–AQ modified Vulcan
XC72 catalyst in the electrolysis cell described in Section 2.3 using
O purged 0.5 M H SO . The H O generation is expressed both as
2
2
4
2
2
accumulative concentration in parts per million (ppm) and in molar
quantities normalized by the electrode’s geometric surface area
−
␮mol cm ). The catholyte volume was kept constant (136 mL)
geo
2
(
throughout the electrolysis experiments, such that the two y-axes
correspond in value.
The RF–AQ modification improved both the H O yield and
2
2
the current efficiency. With a total composite catalyst loading of
−
2
2
.50 ± 0.05 mg cm
(10 wt% RF–AQ, Vulcan XC-72 and Nafion)
geo
®
◦
on the Toray carbon paper, 432 ppm H O was produced at 21 C
2
2
with 70 ± 5% current efficiency after 24 h. The initial H O produc-
2
2
−
1
−2
tion rate was 21 ␮mol h cm
during the first two hours. For
geo
the unmodified Vulcan XC72 only about 170 ppm H O was pro-
2
2
◦
duced at 21 C with 55 ± 5% current efficiency after 24 h (Fig. 7a and
−
2
c). A higher total composite catalyst loading of 3.40 mg cm
pro-
geo
duced about 500 ppm of H O after 24 h, but with about 10% lower
2
2
efficiency. This lower efficiency was likely due to the thicker porous
catalyst layer retaining the generated H O , which subsequently
2
2
underwent further reduction to H O.
2
◦
The elevated temperature of 50 C considerably improved the
H O production rate of the RF–AQ redox catalyst, by increasing
2
2
the rate of the limiting thermochemical reaction of the HRF–OO*
adduct formation (Fig. 5d or e). Much higher H O2 concentration,
2
6
28 ppm after 24 h, was achieved but at this temperature H O2
2
decomposition was also significant as shown by the drop of the
apparent current efficiency from 80 ± 5% initially to about 60 ± 5%
◦
after 24 h at 50 C (Fig. 7a and c). The effect of temperature on the
Vulcan XC72 catalyst was much less pronounced, and the H O con-
2
2
centration was below 200 ppm after 24 h. The current densities for
◦
all electrolysis experiments were stable at both 21 and 50 C, and
no noticeable desorption of the immobilized RF–AQ was observed
during the 24 h experiments.
Fig. 7. The effect of the RF–AQ redox catalyst on H2O2 generation in a batch electrol-
ysis cell. (a) current efficiency of H2O2 generation. (b) Cathodic current density, and
(
c) H2O2 accumulative production normalized by the cathode geometric area and
H2O2 concentration as a function of time. The experiments were performed using
RF–AQ 10 wt% on Vulcan XC72 carbon, 0.5 M H2SO4 electrolyte, at 0.1 V vs. RHE,
◦
◦
4. Conclusions
1
bar pressure and 21 C or 50 C as specified. The total composite catalyst loading
10 wt% RF–AQ and Vulcan XC72) on the unteflonated Toray carbon paper is given
in the legend.
(
Riboflavinyl–anthraquinone-2-carboxylate ester (RF–AQ) is
electrochemically active toward the two-electron reduction of O2
to H O2 in acidic media based on a redox catalysis mechanism.
2
To approximate the maximum RF–AQ loading on the carbon
support for complete monolayer coverage, the molecular volume of
Among the various derivatives of AQ and RF compounds studied,
the synthesized RF–AQ compound supported on Vulcan XC72 car-
bon showed the highest electrocatalytic activity followed by RF and
CAQ. RF–AQ at 10 wt% content showed irreversible adsorption over
the Vulcan XC72 carbon support, which was estimated to be equiva-
lent to a surface coverage of 0.56. Full monolayer of RF–AQ coverage
is estimated at 17.7 wt% loading on Vulcan XC72.
RF–AQ was estimated by total energy minimization in vacuum to be
3
6
85 A˚ . A trial version of CambridgeSoft ChemBio 3D software was
used to perform the computation using the Hartree–Fock method
with the Gaussian algorithm and 6-31G basis set [34,35]. For com-
parison, a van der Waals volume of 504 A˚ 3 was calculated from
−
the correlation developed by Zhao et al. [36], which is about 25%
less than that predicted by the computational chemistry method,
since the correlation only considers the atoms and number of bonds
present in the molecule. Assuming the active surface area of the
The RF–AQ modified electrode had a 2e O2 electroreduction
−
6
−1
◦
rate constant of 6.9 × 10 cm s (0.1 V vs. RHE, at 21 C). Batch
−
2
electrolysis using a composite catalyst loading of 2.5 mg cm , with
dissolved O2 in 0.5 M H SO4 at 21 C, produced H O at an initial
rate of about 21 ␮mol cmgeo
◦
2
2
2
2
−1
−2 −1
Vulcan XC72 powder substrate is 200 m g from N2 adsorption
isotherms [37], and that the RF–AQ molecules are rigid spheres
with a volume estimated by the computational chemistry method,
the complete RF–AQ monolayer coverage of Vulcan XC72 is esti-
mated to occur at an RF–AQ loading of 17.7 wt%. This assumption
however does not consider the molecular shape and orientation
of RF–AQ over the carbon substrate. During CV experiments in the
de-aerated electrolyte, decrease in redox waves of the second scan
cycle from the first was observed for RF–AQ loadings over 15 wt%,
h
with current efficiencies up to
70% at a constant cathode potential of 0.1 V vs. RHE. Electrolysis
◦
using the same composite catalyst loading, at 50 C promoted the
thermochemical step of the mediated O2 reduction reaction and
−
produced H O at an initial rate of 46 ␮mol cm h
geo
2
−1
with about
2
2
75% current efficiency under the same potentiostatic conditions.
◦
However, at 50 C the apparent current efficiency dropped below
70% when the H O concentration exceeded 200 ppm. This loss in
2
2
◦
◦
apparent current efficiency, observed at 50 C but not at 21 C, may

A. Wang et al. / Electrochimica Acta 66 (2012) 222–229
229
be primarily due to H O disproportionation more pronounced at
higher temperatures.
[15] W.F. Schumb, C.N. Satterfield, R.L. Wentworth, J. Am. Pharm. Assoc. 45 (1955)
28.
2
2
1
[
16] E.L. Gyenge, C.W. Oloman, J. Appl. Electrochem. 33 (2003) 665.
17] E.L. Gyenge, W. Oloman, J. Appl. Electrochem. 33 (2003) 655.
[
Acknowledgment
[18] J.C. Forti, R.S. Rocha, M.R.V. Lanza, R. Bertazzoli, J. Electroanal. Chem. 601 (2007)
3.
6
[
[
19] M.S. Hossain, D. Tryk, E. Yeager, Electrochim. Acta 34 (1989) 1733.
20] T. Wilson, J. Zhang, C.O. Oloman, D.M. Wayner, Int. J. Electrochem. Sci. 1 (2006)
99.
The authors would like to acknowledge the financial support of
NSERC through the Res’eau WaterNET program.
[
[
[
[
21] M. Gao, F. Yang, X. Wang, G. Zhang, L. Liu, J. Phys. Chem. C 111 (2007) 17268.
22] M. Gao, F. Yang, G. Zhang, L. Liu, X. Wang, Electrochim. Acta 54 (2009) 2224.
23] S.Y. Yeh, C.M. Wang, J. Electroanal. Chem. 592 (2006) 131.
24] S. Berchmans, R. Vijayavalli, Langmuir 11 (1995) 286.
References
[1] J.M. Campos-Martin, G. Blanco-Brieva, J.L.G. Fierro, Angew. Chem. Int. Ed. 45
[25] P. Manisankar, A. Gomathi, J. Power Sources 150 (2005) 240.
[26] P. Manisankar, A.M. Pushpalatha, S. Vasanthkumar, A. Gomathi, S. Viswanathan,
J. Electroanal. Chem. 571 (2004) 43.
[27] N.V. Klassen, D. Marchington, H.C.E. McGowan, Anal. Chem. 66 (1994) 2921.
[28] T.C.J. Ovenston, W.T. Rees, Analyst 75 (1950) 204.
[29] T.J. Schmidt, H.A. Gasteiger, Rotating thin-film method for supported cata-
lysts, in: W. Vielstich, A. Lamm, H.A. Gasteiger (Eds.), Handbook of Fuel Cells:
Fundamentals, Technology and Applications, Wiley, New York, 2003, p. 328.
[30] P. Vanysek, Electrochemical series, in: CRC Handbook of Chemistry and Physics,
CRC Press, Boca Raton, 1998, pp. 8, 28.
[31] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applica-
tions, 2nd ed., John Wiley & Sons, New York, 2001.
[32] D.R. Lide, CRC Handbook of Chemistry and Physics, 1996.
[33] M. Castelain, H.J. Salavagione, R. Gomez, J.L. Segura, Chem. Commun. 47 (2011)
7677.
[34] D. Young, Computational Chemistry: A Practical Guide for Applying Techniques
to Real World Problems, John Wiley & Sons, Inc., New York, 2001.
[35] C.J. Cramer, Essentials of Computational Chemistry: Theories and Models, 2nd
ed., Wiley, 2004.
[36] Y.H. Zhao, M.H. Abraham, A.M. Zissimos, J. Org. Chem. 68 (2003) 7368.
[37] P. Ferreira-Aparicio, M.A. Folgado, L. Daza, J. Power Sources 192 (2009) 57.
(
2006) 6962.
[
[
[
[
[
2] P.C. Foller, R.T. Bombard, J. Appl. Electrochem. 25 (1995) 613.
3] C. Oloman, A.P. Watkinson, J. Appl. Electrochem. 9 (1979) 117.
4] F. Alcaide, E. Brillas, P-L. Cabot, J. Casado, J. Electrochem. Soc. 145 (1998) 3444.
5] K. Otsuka, I. Yamanaka, Electrochim. Acta 35 (1990) 319.
6] I. Yamanaka, T. Onizawa, S. Takenaka, K. Otsuka, Angew. Chem. 115 (2003)
3
781.
[7] A. Bonakdarpour, D. Esau, H. Cheng, A. Wang, E. Gyenge, D.P. Wilkinson, Elec-
trochim. Acta 56 (2011) 9074.
[
[
8] G. Jürmann, D.J. Schiffrin, K. Tammeveski, Electrochim. Acta 53 (2007) 390.
9] M. Kullapere, G. Jurmann, T.T. Tenno, J.J. Paprotny, F. Mirkhalaf, K. Tammeveski,
J. Electroanal. Chem. 599 (2007) 183.
[
[
[
[
[
10] E. Lobyntseva, T. Kallio, N. Alexeyeva, K. Tammeveski, K. Kontturi, Electrochim.
Acta 52 (2007) 7262.
11] A. Sarapuu, K. Helstein, D.J. Schiffrin, K. Tammeveski, Electrochem. Solid State
Lett. 8 (2005) E30.
12] K. Vaik, U. Maeorg, F.C. Maschion, G. Maia, D.J. Schiffrin, K. Tammeveski, Elec-
trochim. Acta 50 (2005) 5126.
13] K. Vaik, A. Sarapuu, K. Tammeveski, F. Mirkhalaf, D.J. Schiffrin, J. Electroanal.
Chem. 564 (2004) 159.
14] K. Vaik, D.J. Schiffrin, K. Tammeveski, Electrochem. Commun. 6 (2004) 1.