Preparation of a tetrahydroxyphenazine-modified carbon as cathode material for supercapacitor in aqueous acid electrolyte

Article abstract of DOI:10.1016/j.elecom.2016.06.018

A procedure for the grafting of oxocarbon compounds is proposed by condensation reaction with a benzenediamine to obtain an attached-phenazine moieties. A technical proof of concept is given by the covalent capture of rhodizonic acid on the Norit activated carbon and potentiality for supercapacitors is evidenced. The composite material obtained was tested as positive electrode for aqueous supercapacitors in 1?M H₂SO₄. The redox activity covering a wide range of potential gives an unprecedented increase in specific charge of 350% and a specific energy at the discharge 3.4 times higher than the unmodified carbon.

Full text of DOI:10.1016/j.elecom.2016.06.018

Electrochemistry Communications 70 (2016) 47–50
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Preparation of a tetrahydroxyphenazine-modified carbon as cathode
material for supercapacitor in aqueous acid electrolyte
Stéphanie Legoupy a, Estelle Lebègue b, Charles Cougnon a,
⁎
a
Université d'Angers, CNRS UMR 6200, Laboratoire MOLTECH-Anjou, 2 bd Lavoisier, 49045 Angers cedex, France
Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2 rue de la Houssinière, BP32229, 44322 Nantes Cedex 3, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 June 2016
Received in revised form 29 June 2016
Accepted 30 June 2016
Available online 04 July 2016
A procedure for the grafting of oxocarbon compounds is proposed by condensation reaction with a benzenediamine
to obtain an attached-phenazine moieties. A technical proof of concept is given by the covalent capture of rhodizonic
acid on the Norit activated carbon and potentiality for supercapacitors is evidenced. The composite material obtain-
ed was tested as positive electrode for aqueous supercapacitors in 1 M H2SO4. The redox activity covering a wide
range of potential gives an unprecedented increase in specific charge of 350% and a specific energy at the discharge
3.4 times higher than the unmodified carbon.
Keywords:
© 2016 Elsevier B.V. All rights reserved.
Supercapacitor
Activated carbon
Diazonium salt
Oxocarbons
Grafting
1. Introduction
2. Experimental section
Grafting of electroactive molecules on the high specific surface
of a porous carbon has found a widespread use for improving the
performances of energy storage devices by adding a Faradaic contri-
bution into the electrical double layer of a polarizable electrode [1].
Nevertheless, the major drawback of the grafting is the weak con-
centration of molecules attached to the surface. Here, we propose
to focus our attention on a specific redox group exchanging several
electrons per molecule. While redox-active molecules with a reversible
one or two-electron process are widely illustrated in literature [2–4],
mention of three or four-electron systems is rare [5,6]. In the case of
oxocarbon derivatives, such as rhodizonic acid, the main difficulty
consists in attaching the molecule to a surface without sacrificing the
carbonyl groups which are essential in the redox activity. For addressing
this complex issue, we propose to use the facile condensation reaction
of primary aromatic amines with oxocarbon compounds for the covalent
capture of rhodizonic acid at carbon surface. A phenazine compound
designed for grafting was obtained from rhodizonic acid and 4-nitro-
o-phenylenediamine.
2.1. General methods
1H NMR and 13C NMR spectra were recorded on Bruker Avance III
300 (1H: 300 MHz; 13C: 75 MHz) Spectrometer using DMSO-d6 as
solvent. The chemical shift of DMSO-d6 was calibrated at 2.50 ppm in
1H NMR spectra and 39.52 ppm in 13C NMR. High resolution mass spec-
troscopy was obtained by using a Jeol-JMS-700 double-focusing mass
spectrometer using the electron impact (EI) method. X-ray photoelec-
tron spectroscopy (XPS) data have been collected in the CEA (constant
analyser energy) mode with analyser pass energy of 20 eV, using a
Kratos Nova spectrometer with a monochromatized Al anode X-ray
source (hν = 1486.6 eV). Data treatment was performed with CasaXPS
software and all spectra were calibrated taking 284.4 eV (graphite like
carbon) as a reference binding energy. Electrochemical measurements
were performed in a three-electrode configuration with an aqueous
1 M sulfuric acid electrolyte and a potentiostat/galvanostat model
VMP3 (from Bio-Logic) monitored by ECLab software was used. Working
electrodes were glassy carbon (from Bioanalytical Systems, model
MF-2012) or modified and unmodified activated Norit carbon electrodes
prepared according to a previously published procedure [7]. All the poten-
tial values were reported versus an Ag|AgCl reference electrode, except
for galvanostatic charge/discharge experiments performed in a three
electrode Swagelok cell with a silver wire as a quasi-reference electrode.
⁎
Corresponding author.
E-mail address: charles.cougnon@univ-angers.fr (C. Cougnon).
http://dx.doi.org/10.1016/j.elecom.2016.06.018
1388-2481/© 2016 Elsevier B.V. All rights reserved.

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S. Legoupy et al. / Electrochemistry Communications 70 (2016) 47–50
Thermogravimetric analyses (TGA) were performed with a TA Instru-
ments by heating carbon products in N2 atmosphere to 1000 °C at a rate
consumed corresponded to 6 F·mol−1, the electrolysis was stopped
and NaNO2 (20 mg; 3 equiv.) was added under stirring at room temper-
ature for producing in situ the 2,3-dihydroxy-7-diazoniumphenazine-
1,4-quinone salt and then the carbon powder (64 mg) was added.
After stirring at room temperature for 24 h, the carbon product was
vacuum filtered and thoroughly washed before to be dried over night
at 80 °C. The carbon product containing the molecule just adsorbed
was prepared by dispersing the carbon powder in acetonitrile with
0.02 equiv. of nitro compound for 24 h.
of 10 °C·min−1
.
2.2. Synthesis of the 2,3-dihydroxy-7-nitrophenazine-1,4-quinone
Synthesis of 2,3-dihydroxy-7-nitrophenazine-1,4-quinone was
performed by condensation between rhodizonic acid and 4-nitro-
o-phenylenediamine, according to a previously published procedure
[8]. 4-nitrobenzene-1,2-diamine (367.5 mg, 2.42 mmol) dissolved in
warm H2SO4 (10 mL, 25% w/v) was added at 100 °C to a rhodizonic acid
dihydrate (500 mg, 2.42 mmol) solution in water (10 mL). After 10 min
stirring at 100 °C, the heating was stopped and the mixture returns at
room temperature for 2 h. The precipitate was recovered by filtration
and washed with water, ethanol, acetone and ether to give the 2,3-
dihydroxy-7-nitrophenazine-1,4-quinone as a red solid (416 mg, 60%).
1H NMR (DMSO-d6) δ: 9.06 (d, 1H, Hc, J = 2.2 Hz), 8.66 (dd, 1H, Hb,
J = 9.2, 2.2 Hz), 8.52 (d, 1H, Ha, J = 9.2 Hz). 13C NMR (DMSO-d6) δ:
177.11 (C_O), 148.98, 145.56, 145.20, 145.10, 144.46, 141.06, 132.06,
125.86, 125.64. HRMS for C12H5N3O6: calcd 287.0.178; found 287.0174.
3. Results and discussion
Condensation of rhodizonic acid with 4-nitrobenzene-1,2-diamine
yields the 2,3-dihydroxy-7-nitrophenazine-1,4-quinone (7-nitro
DHPhQ) in situ diazotized through a sequential approach already
published by our group [9]. In this procedure, the nitro group was first
completely reduced into the amine under controlled potentiostatic con-
ditions at 0 V vs. Ag/AgCl in 0.5 M HCl on a glassy carbon plate, which is
subsequently in situ diazotized (Fig. 1a). Fig. 1b shows two cyclic volt-
ammograms (CVs) recorded on a glassy carbon electrode (GCE) before
(solid line) and after electrolysis (dotted line). At the end of the electrol-
ysis, CV shows two reversible systems centered at 0.005 and 0.300 V.
Following the nitro consumption, an excess of NaNO2 was added and
after 15 min stirring, a series of CVs was recorded showing an irrevers-
ible cathodic wave at 0.1 V assigned to the reduction of the diazonium
salt in situ produced, which decreases until complete passivation of
the GCE (inset in Fig. 1b). CV recorded in 1 M H2SO4 with the modified
2.3. Chemical modification of the activated carbon
2,3-Dihydroxy-7-nitrophenazine-1,4-quinone (28 mg) was first
sonicated for 30 min in 80 mL of 0.5 M HCl before to be electrochemically
converted into the amino compound under controlled potentiostatic
conditions at 0 V on a glassy carbon plate. When the cathodic charge
Fig. 1. (a) Schematic illustration for the grafting of an oxocarbon compound. (b) CVs on GCE at 100 mV·s−1 of 1 mM 7-nitro derivative in 0.5 M HCl before (solid line) and after electrolysis
(dotted line) at 0 V. Inset in Fig. 2b shows CVs on GCE at 50 mV·s−1 of the 7-diazonium derivative generated in situ. (c) CVs in 1 M H2SO4 at 100 mV·s−1 of the THPh-modified GCE, 2 mM
rhodizonic acid and 0.3 mM phenazine.

S. Legoupy et al. / Electrochemistry Communications 70 (2016) 47–50
49
GCE shows two reversible redox systems located at 0.03 V and 0.281 V
vs. Ag/AgCl (Fig. 1c, bottom). Due to the variety of electroactive func-
tions, it is difficult to finely attribute all the electrochemical systems.
Indeed, both the phenazine unit and the tetrahydroxybenzene group
are expected to be electroactive at a carbon electrode in aqueous acid
solutions. For addressing this point, CVs presented in Fig. 1c shows the
electrochemistry of phenazine and rhodizonic acid at GCE in 1 M H2SO4
and provide guidance for better understanding the electrochemical
behavior of the tetrahydroxyphenazine-modified carbon electrode. In
acidic media, rhodizonic acid exhibits a reversible electrochemical system
ascribed to the hexahydroxybenzene/tetrahydroxybenzoquinone (HHB/
THQ) redox couple, while the oxidation of THQ has no corresponding
cathodic peak at 100 mV·s−1 due to rapid hydration reaction of
rhodizonic acid [10]. On the other hand, the phenazine, which is soluble
under its protonated form, exhibits two successive one-electron reduc-
tion steps, and under certain experimental conditions, such as the phena-
zine concentration and the acidity of the electrolyte, new voltammetric
waves attributed to the redox activity of adsorbed products can be
observed [11]. When comparing the electrochemistry of the modified
carbon electrode with the voltammetric responses of these two model
compounds, care is needed to clearly identify which function is active.
Accordingly, the first system has a large peak width at mid-height and
can be suspected to be a multi-peak system. If both the phenazine and
the tetrahydroxybenzene group are electroactive, a global exchange of 6
electrons is expected, making this molecule a promising candidate for
supercapacitors in aqueous electrolytes.
In order to evaluate the potentiality of this approach for
supercapacitors, a redox active surface was designed on the activated
Norit-S50 carbon (NS) by the same procedure. The formation of the
composite material was confirmed by XPS with the appearance of a
broad N1s XPS peak which correlates well with two photoelectron
components at binding energies of 398.9 eV and 400.2 eV attributed to
phenazine type of nitrogen and azo bonds, as it was already postulated
for the surface modification based on diazonium salts (Fig. 2a) [12].
Additionally, TGA experiments were carried out for estimating the mass
fraction of molecules in the chemically modified carbon product by
heating the composite material up to 1000 °C under N2 atmosphere
(Fig. 2b). A global weight loss of 10.5% at 700 °C is determined. This result
agrees well with a weight percentage of nitrogen of 1.7 0.04 deter-
mined by elemental analysis, which gives a mass fraction of molecules
equal to 11.6%. Fig. 2c shows typical CVs recorded at 2 mV·s−1 in 1 M
H2SO4 for unmodified Norit carbon electrode and for modified electrodes
prepared either by reacting carbon with the diazonium salt or just by
impregnation with the 7-nitro DHPhQ. Compared to the nearly perfect
capacitive behavior of the pristine carbon, the chemically modified
electrode obtained from diazonium chemistry shows two reversible
systems centered at −0.010 V and 0.271 V vs. Ag/AgCl that cannot be
attributed just to an adsorption product.
Fig. 2. (a) XPS spectra of N1s core level for unmodified and modified Norit carbons. (b) TGA decomposition profiles of unmodified and modified Norit carbons. (c) CVs in 1 M H2SO4 at
2 mV·s−1 of unmodified (curve 1), adsorbed (curve 2) and chemically modified (curve 3) carbon electrodes. (c) Galvanostatic charge/discharge curves of modified (solid line) and
unmodified (dotted line) Norit carbon electrodes at a specific current of 1 A·g−1. Inset shows the effect of the current rate on the specific charge delivered during discharge.
(d) Specific energy delivered at 1 A·g−1 and derivative of the energy with respect to the cell voltage for unmodified (curves 1 and 1′), catechol-modified (curves 2 and 2′) and THPh-
modified (curves 3 and 3′) Norit carbon electrodes. The specific current, charge and energy are reported with respect to the mass of activated carbon (without conducting agent and PTFE).

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S. Legoupy et al. / Electrochemistry Communications 70 (2016) 47–50
In order to assess the cyclability of the modified carbon product,
number of electron globally exchanged per molecule decreases from
3.6 to 2 after 7000 cycles and both the peak position and the shape of
the CV curve change, indicating that molecules undergo irreversible
damage under oxidation. Nucleophilic addition on activated carbonyl
functions is well-known and can be expected to occur, but this issue
requires further investigations.
galvanostatic charge/discharge experiments were performed from
−0.2 to 0.4 V at different current densities ranging from 1 A·g−1 to
10 A·g−1 (Fig. 2d). As I × Δt corresponds to the average specific charge,
the increase in charge/discharge time for the modified carbon is indica-
tive of an increase in the global charge. During discharge at 1 A·g−1, the
specific charge is found to be 180 C·g−1 and 51 C·g−1 for the modified
and unmodified carbon electrodes, yielding an unprecedented charge
gain of 350%. Inset in Fig. 2d shows that the modified carbon electrode
retains its high charge gain when the current density increases, with a
specific charge retention of 89% at 10 A·g−1. The faradaic charge stored
at the level of molecules can be deduced from the total specific charge
by subtracting the double-layer contribution and the number of elec-
trons globally exchanged per molecule results immediately by taking
into account the mass fraction of molecule. 3.6 electrons were found
to be exchanged per molecule in average, making the THPh-modified
carbon a good candidate for charge storage applications.
4. Conclusion
Here, we propose to use the facile condensation reaction of 4-
nitrobenzene-1,2-diamine with oxocarbon compounds for the covalent
capture of rhodizonic acid at carbon surface through a phenazine-type
linkage. A 350% increase in specific charge and a specific energy at the
discharge 3.4 times higher than the unmodified carbon provide potenti-
ality for charge storage applications.
Acknowledgment
Combined, the reversible systems cover a wide range of potential
that impacts the way in which the energy ξ is redistributed at the dis-
charge (Fig. 2e). This situation can be evidenced by deriving the energy
with respect to the cell voltage Ucell (Fig. 2e, dotted lines). With the
unmodified carbon electrode (curves 1 and 1'), the derivative of the
energy continuously decreases as the cell voltage decreases, because
the electrostatic charge inside the electrochemical double-layer linearly
decreases with the potential. With modified electrodes, an extra amount
of energy, marked by a peak in the derivative graph of the function ξ =
f(U), is delivered at each reversible electron transfer process. For the
well-known catechol-modified carbon electrodes (curves 2 and 2'),
which present only one reversible system [13], a single maximum in the
derivative of the energy appears and an extra amount of energy corre-
sponds, implying that the total specific energy delivered is doubled. For
the THPh-modified electrode (curves 3 and 3'), the total specific energy
increases by 3.4 compared to the pristine carbon, due to the presence of
a multi-peak system covering a wide range of potential.
The stability of the modified carbon powder was evaluated by
repetitive charge/discharge curves at 1 A·g−1, revealing that the
chemical modification stabilizes the faradaic contribution over a
long-time period (Fig. 3).
After a 31% decrease in specific charge over the first 5000 cycles, the
total charge stabilizes. The loss in specific charge is often attributed to
the release of physisorbed molecules, but when comparing the two
CVs recorded at the beginning and at the end of the series of charge/
discharge cycles, it appears that molecules suffer from incomplete
redox activity after cycling during a long-time period. The average
We thank the Pr. T. Brousse of Université de Nantes for experimental
help. This work was supported by the Centre National de la Recherche
Scientifique (CNRS-France) and the Université d'Angers through the
frameworks of the ICROSS project (ANR-13-PRGE-0011-02) and the
ACS project.
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Fig. 3. Evolution of the total specific charge with the number of charge-discharge cycles at
1 A·g−1 for the modified (filled circle) and unmodified (empty circle) Norit carbon
electrodes. CVs showed in inset refer to the first and the last cycle.