Influence of supporting electrolyte on the electrochemical oxidation of formic acid on boron-doped diamond electrodes

Article abstract of DOI:10.1246/bcsj.20130008

The influence of support electrolyte on the electrochemical degradation of formic acid was studied on boron-doped diamond (BDD) electrodes. Experimental data are compared with a theoretical model, which is based on organic degradation via hydroxyl radical

Full text of DOI:10.1246/bcsj.20130008

© 2013 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 86, No. 6, 749-754 (2013)
749
Influence of Supporting Electrolyte on the Electrochemical Oxidation
of Formic Acid on Boron-Doped Diamond Electrodes
Stéphane Fierro,1 Yuki Honda,1 and Yasuaki Einaga*1,2
1Department of Chemistry, Faculty of Science and Technology, Keio University,
3-14-1 Hiyoshi, Yokohama, Kanagawa 223-8522
2JST, CREST, 3-14-1 Hiyoshi, Yokohama, Kanagawa 223-8522
Received January 9, 2013; E-mail: einaga@chem.keio.ac.jp
The influence of support electrolyte on the electrochemical degradation of formic acid was studied on boron-doped
diamond (BDD) electrodes. Experimental data are compared with a theoretical model, which is based on organic
degradation via hydroxyl radicals under current or mass transport controlled regimes depending on the difference between
the applied and limiting current. The kinetics of the different reactions involved in the process is also briefly studied. It is
shown that when sulfuric acid is used as support electrolyte, the reactions involving electro-generated peroxodisulfate
anions during the degradation of formic acid are highly dependent on temperature. For this reason, the influence on the
overall process of the chemical reaction between these anions and formic acid tends to increase with increasing
temperatures and when the electrolysis becomes controlled by mass transport. However, it is proven that degradation of
formic acid in the bulk occurs under current controlled regime as well. Furthermore, it is shown that the use of sulfuric
acid as support electrolyte enhances the degradation of formic acid at high temperatures and when the process is
controlled by mass transfer compared to other inert support electrolytes.
Electrochemical degradation of organic compounds has
•
BDDð OHÞ þ R ! BDD þ mCO2 þ nH2O þ Hþ þ eꢀ ð2Þ
attracted much attention recently and especially on boron-
doped diamond (BDD) electrodes due to the high efficiency
and environmental compatibility of this process.1-3 Moreover,
BDD has remarkable properties compared to other electrode
materials such as physical durability and electrochemical
stability4 as well as a large overpotential for oxygen evolution.5
In fact, it was reported that the mineralization of organic com-
pounds occurs with a high efficiency due to the electrogenera-
tion of highly reactive hydroxyl radicals at the surface of a
BDD electrode.6
Recently, a comprehensive model has been proposed to
describe the mechanisms involved during the electrochemical
oxidation of organic compounds on BDD electrodes.7-10 In this
model, BDD is classified as “non-active type” anode, which
mainly oxidize organic compounds via the electrogeneration
of quasi-free hydroxyl radicals.11 In fact, the non-active anode
is defined as an electrode that does not participate during
any anodic reactions and does not provide any active site for
adsorption of reactants and/or products in aqueous medium.
Outer sphere reactions are in principle the only possible anodic
reactions occurring on non-active type anodes.
•
BDDð OHÞ ! BDD þ 1=2O2 þ Hþ þ eꢀ
ð3Þ
The main assumption12 of the model is that the global rate
of the electrochemical oxidation of organics is fast. The process
is controlled by current or mass transport of the organic
compounds to the anode surface depending on the difference
between the applied and limiting current, which is determined
using the following relation under given hydrodynamic con-
ditions (eq 4):
jlimðtÞ ¼ nFkmCODðtÞ
ð4Þ
where jlim is the limiting current density for organic oxidation
(A m¹2), F is the faraday constant (C mol¹1), km is the mass
transport coefficient (m s¹1), COD(t) is the chemical oxygen
demand at time t (molO2 m¹3).
Under galvanostatic conditions, two different regimes are
defined: the current control regime ( japp < jlim) and the mass
transport control regime ( japp > jlim).
In the case of current control ( japp < jlim), the rate of organics
oxidation is constant and the chemical oxygen demand (COD)
decreases linearly with time (eq 5). Moreover, the instanta-
neous current efficiency (ICE) is 100%.
Three reactions are involved in the oxidation of organics on
BDD: first, the water discharge, which leads to the formation of
hydroxyl radicals (eq 1). These radicals are physisorbed at the
electrode surface and hence are highly reactive. The second
reaction is the electrochemical oxidation of organic compounds
via hydroxyl radicals (eq 2). The last reaction is the competing
side reaction of oxygen evolution (eq 3).
ꢀ
ꢁ
Akm
VR
CODðtÞ ¼ COD0 1 ꢀ -
t
ð5Þ
Where - is the dimensionless current density (¹), A is the
geometric anode surface area (m2), COD0 is the initial chemical
oxygen demand (molO2 m¹3), t is the time (s), COD(t) is the
chemical oxygen demand at time t (molO2 m¹3), and VR is the
reaction volume (m3).
•
H2O þ BDD ! BDDð OHÞ þ Hþ þ eꢀ
ð1Þ
Published on the web May 18, 2013; doi:10.1246/bcsj.20130008

750
Bull. Chem. Soc. Jpn. Vol. 86, No. 6 (2013)
Electrochemical Oxidation on Boron-Doped Diamond Electrodes
In case of mass transport control ( japp > jlim), the COD
decreases exponentially with time (eq 6). In this regime, due
to the competing side reaction of oxygen evolution the ICE
for organics oxidation decreases exponentially with time, as
described by eq 7:
ꢀ
ꢁ
Akm
VR
1 ꢀ -
COD ¼ -COD0 exp ꢀ
t þ
ð6Þ
ð7Þ
-
ꢀ
ꢁ
Akm
1 ꢀ -
ICE ¼ exp ꢀ
t þ
VR
-
Therefore, according to this model, which was verified for
a variety of organic compounds, the mineralization efficiency
tends to decrease drastically when the process is controlled
by mass transfer because the electrochemical degradation via
hydroxyl radicals occurs at the vicinity of the electrode material
surface.
In this work, the influence of support electrolyte on the
electrochemical degradation of formic acid was studied on
boron-doped diamond (BDD) electrodes. Two electrolytic
media have been selected: perchloric acid and sulfuric acid.
The choice is based on the fact that perchloric acid is
electrochemically inactive on BDD electrode contrarily to
sulfuric acid which can be oxidized to peroxodisulfate (eqs 8
and 9).13
Figure 1. Experimental set-up of the electrolysis using
the two compartment cell: C1 Nafionμ separated two com-
partments cell; R1 anodic electrolyte tank; R2 cathodic
electrolyte tank; P1, P2 pumps.
gas. The BDD film thickness was about 40 ¯m after 8 h
deposition. The film quality was confirmed by Raman spec-
troscopy (not shown). The BDD electrodes were pretreated by
ultrasonication in 2-propanol for about 10 min followed by
rinsing with high purity water in order to remove any organic
impurities that may have remained within the BDD film after
deposition in the MP-CVD chamber.
Kinetics of Formic Acid Oxidation. To investigate the
kinetics of formic acid oxidation by peroxodisulfate, 1000 mL
of water was heated until the desired temperature was reached,
then, 60 mM of sodium peroxodisulfate and 40 mM of formic
acid were added and samples were taken at regular intervals (30
min) and the chemical oxygen demand (COD) was monitored
by spectrophotometry (Hach method15) using a WTW photolab
6100 Vis device. The concentration of formic acid can be
determined from the COD value assuming that formic acid is
mineralized without any formation of intermediates.
2HSO4 ! S2O82ꢀ þ 2Hþ þ 2eꢀ E0 ¼ 2:123 V ð8Þ
ꢀ
2SO4 ! S2O82ꢀ þ 2eꢀ E0 ¼ 2:010 V
ð9Þ
2ꢀ
Peroxodisulfate anions are strong oxidants and are more stable
than quasi-free hydroxyl radicals so the aim is to investigate if
it is possible to assist the electrochemical oxidation of organics
at the electrode surface via hydroxyl radicals with bulk
degradation via peroxodisulfate anions when the efficiency of
the electrochemical process decreases due to mass transport
control.1,10,13,14 Therefore, in this work, the electrochemical
oxidation of formic acid using sulfuric acid as support elec-
trolyte is investigated. The kinetics of formic acid degradation
is also studied. Later, experimental data are compared with
the predictions of the aforementioned model in order to
evaluate the influence of peroxodisulfate electrogeneration on
the mineralization of formic acid on boron-doped diamond
electrode under current controlled and mass transport con-
trolled regimes.
Bulk Electrolysis Set-Up.
A divided electrolytic flow
cell was used for the measurements (Figure 1). Anodic and
cathodic compartments are separated by a Nafionμ N117/H+
(Du Pont Polymers, Fayetteville, North Carolina) membrane.
The Nafion membrane was pretreated for 2 h in a 2 M nitric
acid bath at 80 °C and washed with Millipore water before use.
The anode was BDD and the cathode was zirconium, both
having an effective surface area of 12.57 cm2. The interelec-
trode gap is 20 mm. The anodic and cathodic compartments
of the electrolytic cell are both thermostated tanks, each with
a volume of 500 mL. Using two pumps, the catholyte/anolyte
solutions circulate inside the cell through the electrode
chambers.
Determination of the Mass Transfer Coefficient. The
mass transfer coefficient of [Fe(CN)6]3¹ (eq 10) can be calcu-
lated (eq 11) from the anodic limiting current density ( jlim
corresponding to various concentrations (20, 40, 60, 80, and
100 mM) of [Fe(CN)6]3¹ in 1 M NaOH, which can be measured
by cyclic voltammetry measurements.16 The limiting current
density, jlim is plotted against [Fe(CN)6]3¹ concentration, then
the average mass transfer coefficient was determined using
eq 11:
Experimental
Chemicals and Materials.
All chemicals including
acetone, B(OCH3)3, Na2S2O8, NaOH, KMnO4, [Fe(CN)6]3¹
,
nitric acid, perchloric acid, sulfuric acid, and formic acid were
purchased from Wako. All chemicals were used without further
purification.
Preparation of BDD Electrodes. Boron-doped diamond
thin films were deposited on conducting p-Si substrate by
microwave plasma-assisted chemical vapor deposition (MP-
CVD). Acetone was used as carbon source and B(OCH3)3 as
boron source. The concentration of the latter was 0.1% (w/w).
The surface morphology and crystalline structures were char-
acterized using scanning electron microscopy (not shown in
this work). The typical size of the diamond crystals was about
10 ¯m. BDD films were deposited on Si(100) wafers in the MP-
CVD chamber at 5 kW using high purity hydrogen as carrier
)
4ꢀ
½FeðCNÞ6ꢁ3ꢀ þ eꢀ ! ½FeðCNÞ6ꢁ
ð10Þ

S. Fierro et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 6 (2013)
751
jlim ¼ kmFC
ð11Þ
where F is Faraday’s constant and C is the concentration
of [Fe(CN)6]3¹. From the results obtained in the electrolytic
(i)
cell, the mass transfer coefficient was estimated at 2.718 ©
(ii)
¹1
10¹5 m s
.
It is worthwhile to mention that the mass transfer coefficient
measured above, which corresponds to reaction (eq 10), is most
likely different from the mass transfer coefficient associated
with the oxidation of formic acid (km,FA).
(a)
Despite this difference, the mass transfer coefficient for
formic acid oxidation (km,FA) under the same hydrodynamic
conditions can be estimated using the following relation
(eq 12):
Q/AhL−1
DFA
3ꢀ
km,FA
¼
km,½FeðCNÞ ꢁ
ð12Þ
6
3ꢀ
D
½FeðCNÞ6ꢁ
where D (m2 s¹1) is the diffusion coefficient.
(i)
(ii)
Electrochemical Oxidation of Formic Acid. The electro-
lytic flow cell described in Figure 1 was used for the experi-
ments. As anolyte, a solution containing formic acid in either
1 M sulfuric acid or 1 M perchloric acid was used whereas
the catholyte was the support electrolyte solution (1 M HClO4
or 1 M H2SO4). The experiments were performed at two
different temperatures (323 or 353 K) and under galvanostatic
conditions. Samples were withdrawn at regular intervals and
the chemical oxygen demand (COD) was monitored by spec-
trophotometry (Hach method15) using a WTW photolab 6100
Vis device.
(b)
Q/AhL−1
Figure 2. Evolution of formic acid concentration with
passed charge during the degradation of formic acid at
(a) 50 and (b) 80 °C on BDD electrode. Initial con-
centration of formic acid: 100 mM. Current density: 32
mA cm¹2. Comparison between the experimental results
(dots) and the proposed model (continuous line). (i): under
current control regime, (ii) under mass transport control
regime. Support electrolyte: H2SO4.
Results and Discussion
Figure 2 shows the evolution of the concentration of formic
acid as a function of passed charge during its electrochemical
oxidation on BDD using sulfuric acid as support electrolyte
at (a) 50 and (b) 80 °C. The monitored values of COD could
be converted to formic acid concentration for all electrolysis
experiments knowing that the later is oxidized on BDD without
any formation of intermediates through the following reaction:
HCOOH ! CO2 þ 2Hþ þ 2eꢀ
ð13Þ
Both figures show that there is a good agreement between the
predictions model and the experimental results, which proves
that the assumptions of the model are accurate i.e., formic acid is
degraded mainly by hydroxyl radicals through a fast reaction
and the process is controlled by current (i) or mass transport (ii)
depending on the difference between the applied current and
the limiting current. However, these figures show also that the
theoretical model is more accurate for the experiment performed
at 50 °C (a) whereas the depletion of formic acid at 80 °C seems
to proceed faster than the predictions of the model. This is
probably due to the fact that the electrochemical oxidation
of formic acid on BDD proceeds not only through hydroxyl
radicals but is also assisted by the electrogeneration of
peroxodisulfate anions at higher temperatures (reaction 14).
Figure 3. Evolutions of formic acid concentration during
its chemical oxidation by an excess of peroxodisulfate
anions carried out at different temperatures: ( : 50, : 55,
: 60, : 65, : 70, and ©: 80 °C).
ꢀ
S2O82ꢀ þ HCOOH ! CO2 þ 2HSO4
ð14Þ
different temperatures and the concentration of formic acid
was measured at regular time intervals. Figure 3 shows the
evolution of formic acid concentration as a function of time and
for different temperatures. This figure shows that the reaction is
In fact, in order to evaluate the influence of temperature on
the participation of peroxodisulfate anions during the oxidation
of formic acid, the reaction was carried out in a beaker for

752
Bull. Chem. Soc. Jpn. Vol. 86, No. 6 (2013)
Electrochemical Oxidation on Boron-Doped Diamond Electrodes
(a)
Q/AhL−1
Q/AhL−1
Figure 5. Evolution of peroxodisulfate concentration with
passed charge electrogenerated from 1 M H2SO4 on BDD
¹2
electrode at 80 °C. Current density: 32 mA cm
.
the ICE decreases rapidly and reaches a value of almost zero
¹1
after 6 A h L
.
Figure 5 shows the evolution of peroxodisulfate concen-
tration (determined by titration using Fe(II) and KMnO417) as
a function of passed charge on BDD electrode. This figure
highlights the influence of the presence of formic acid during
electrolysis experiments performed under galvanostatic con-
ditions at 80 °C and using sulfuric acid as support electrolyte.
This figure shows that the measured concentration of
electrogenerated peroxodisulfate is much higher in the absence
of formic acid. This proves that in the presence of formic acid,
peroxodisulfate anions participate during the degradation of
the organic compound. Furthermore, this figure suggests that
the chemical reaction between formic acid and peroxodi-
sulfate occurs during the entire electrolysis experiment i.e.,
under current and mass transport controlled regimes. However,
as the discrepancies between the theoretical model predicting
the evolution of formic acid with passed charge and the
experimental results increased under mass transport controlled
regime (Figure 2b), it suggests that the influence of peroxodi-
sulfate anions on the overall process is more pronounced when
the efficiency of the electrochemical process tends to decrease
exponentially (under mass transport control).
(b)
Q/AhL−1
Figure 4. Evolution of instantaneous current efficiency
(ICE) with passed charge during the degradation of formic
acid at (a) 50 and (b) 80 °C on BDD electrode. Initial
concentration of formic acid: 100 mM. Current density:
32 mA cm¹2. Comparison between the experimental results
(dots) and the proposed model (continuous line). (i): under
current control regime, (ii) under mass transport control
regime. Support electrolyte: H2SO4.
very slow at temperatures below 60 °C. For this reason, the
electrochemical degradation of formic acid using sulfuric acid
as support electrolyte proceeds faster than the predictions of the
model only at high temperatures (Figure 2).
Furthermore, assuming that the oxidation of formic acid by
peroxodisulfate anions is pseudo first order and according to
Figure 6 displays the evolution of formic acid concen-
tration as a function of passed charge during its electro-
chemical oxidation on BDD electrode at 80 °C and using (a)
perchloric acid and (b) sulfuric acid as support electrolyte,
respectively.
It is worthwhile to notice that the parameters of both experi-
ments were set so that the electrolysis proceeded solely under
mass transport controlled regime.
Arrhenius equation, the activation energy (EA) for the afore-
¹1
mentioned reaction was estimated around 101.7 kJ mol
.
Figures 4a and 4b show the evolution of instantaneous
current efficiency (ICE) corresponding to the experiments
presented in Figures 2a and 2b, respectively. The experimental
ICE was determined using the following equation (eq 15):
This figure shows that the experimental results of the elec-
trolysis performed using perchloric acid as support electrolyte
match with a high accuracy the predictions of the theoretical
model. However, when sulfuric acid was used as support
electrolyte, the degradation of formic acid proceeded faster
than the predictions of the model. This is probably due to the
participation of electrogenerated peroxodisulfate anions during
the process whereas the oxidation of formic acid is performed
exclusively by hydroxyl radicals when perchloric acid is
used as support electrolyte. Furthermore, this shows that at high
n ꢂ F ꢂ VR ꢂ ꢀc
ICE ¼
ð15Þ
I ꢂ ꢀt
where n is the number of electrons involved during formic acid
oxidation (n = 2), ¦c is the difference in formic acid con-
centration (mol m¹3) observed during the time interval ¦t (s)
and I is the current (A). Again, both figures show that there is
a good agreement between the experimental data and the
predictions of the theoretical model. Furthermore, Figure 4b
shows that when the process is controlled by mass transport,

S. Fierro et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 6 (2013)
753
(a)
(a)
Q/AhL−1
Q/AhL−1
(b)
(b)
Q/AhL−1
Q/AhL−1
Figure 6. Evolution of formic acid concentration with
passed charge during the degradation of formic acid at
Figure 7. Evolution of instantaneous current efficiency
(ICE) with passed charge during the degradation of formic
acid at 80 °C on BDD electrode. Initial concentration
of formic acid: (a) 65 and (b) 58 mM. Current density:
80 mA cm¹2. Comparison between the experimental results
(dots) and the proposed model (continuous line). Support
electrolyte: (a) HClO4 and (b) H2SO4.
80 °C on BDD electrode. Initial concentration of formic
¹2
acid: (a) 65 and (b) 58 mM. Current density: 80 mA cm
.
Comparison between the experimental results (dots) and
the proposed model (continuous line). Support electrolyte:
(a) HClO4 and (b) H2SO4.
temperatures (80 °C), it becomes advantageous to use sulfuric
acid as support electrolyte and especially when the overall
process is controlled by mass transport i.e., when the efficiency
of the electrochemical process decreases exponentially.
applied and limiting current. The kinetics of formic acid degra-
dation by peroxodisulfate anions was studied as well. The
principal conclusions of this work are the following:
The chemical reaction between peroxodisulfate anions and
formic acid is highly dependent on temperature as suggested
by the activation energy reported (in the order of 100 kJ mol¹1).
Electrolysis of formic acid on BDD performed at different
temperatures and using sulfuric acid as support electrolyte have
shown that discrepancies between the model and the exper-
imental results increase with increasing operating temper-
ature due to the higher influence of the chemical degradation
of formic acid by peroxodisulfate anions. Furthermore, this
influence tends to increase when the instantaneous current
efficiency (ICE) decreases exponentially i.e., under mass trans-
port controlled regime. However, the evolutions of peroxodi-
sulfate concentration during these experiments suggest that the
chemical oxidation of formic acid occurs also under current
controlled regime.
Figures 7a and 7b show the evolution of instantaneous
current efficiency (ICE) as a function of passed charge for the
experiments presented in Figures 5a and 5b, respectively.
These figures prove clearly that the discrepancies between
the experimental data and the theoretical model increase when
sulfuric acid is used as support electrolyte compared to per-
chloric acid for electrolysis performed under the same exper-
imental conditions. This is the case because the theoretical
model does not include the participation of electrogenerated
peroxodisulfate anions during the degradation of formic acid.
Conclusion
In this paper, the influence of support electrolyte on the
electrochemical oxidation of formic acid on boron-doped
diamond (BDD) electrode was investigated. Experimental data
have been compared with a theoretical model, which considers
that the degradation of formic acid occurs via highly reactive
hydroxyl radicals and that the process is controlled by current
or mass transport depending on the difference between the
For high operating temperatures and especially when the
electrolysis is controlled by mass transport, it becomes advan-
tageous to use sulfuric acid as support electrolyte during the
degradation of formic acid compared to other support electro-
lytes such as perchloric acid.

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Bull. Chem. Soc. Jpn. Vol. 86, No. 6 (2013)
Electrochemical Oxidation on Boron-Doped Diamond Electrodes
8
D. Gandini, E. Mahé, P. A. Michaud, W. Haenni, A. Perret,
Ch. Comninellis, J. Appl. Electrochem. 2000, 30, 1345.
J. Iniesta, P. A. Michaud, M. Panizza, G. Cerisola, A.
Stéphane Fierro gratefully thanks the Fond National Suisse
pour la Recherche Scientifique (FNSRS) for the financial
support. The authors want to thank Pr. Christos Comninellis for
fruitful discussions.
9
Aldaz, Ch. Comninellis, Electrochim. Acta 2001, 46, 3573.
10 M. Panizza, P. A. Michaud, G. Cerisola, Ch. Comninellis,
J. Electroanal. Chem. 2001, 507, 206.
11 B. Marselli, J. Garcia-Gomez, P.-A. Michaud, M. A.
Rodrigo, Ch. Comninellis, J. Electrochem. Soc. 2003, 150, D79.
12 A. Kapałka, G. Fóti, Ch. Comninellis, J. Appl. Electro-
chem. 2008, 38, 7.
13 P.-A. Michaud, E. Mahé, W. Haenni, A. Perret, C.
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14 K. Serrano, P. A. Michaud, C. Comninellis, A. Savall,
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