Thermo Chemical Treatments Based on NH3 O2 for Improved Graphite Based Fiber Electrodes in VRFB 2013
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Transcript of Thermo Chemical Treatments Based on NH3 O2 for Improved Graphite Based Fiber Electrodes in VRFB 2013
C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8
.sc iencedi rect .com
Avai lab le at wwwjournal homepage: www.elsev ier .com/ locate /carbon
Thermo–chemical treatments based on NH3/O2 for improvedgraphite-based fiber electrodes in vanadium redox flowbatteries
Cristina Flox a,*, Javier Rubio-Garcıa a, Marcel Skoumal a, Teresa Andreu a, Juan RamonMorante a,b
a Catalonia Institute for Energy Research, IREC, Jardins de les Dones de Negre 1, 08930 Sant Adria de Besos, Barcelona, Spainb Departament d’Electronica, Facultat de Fısica Universitat de Barcelona, Martı i Franques 1, 08028 Barcelona, Spain
A R T I C L E I N F O
Article history:
Received 4 December 2012
Accepted 9 April 2013
Available online 18 April 2013
Keywords:
Polyacrylonitrile fiber
Electrochemical properties
Electrochemical surface area
Energy storage
Vanadium redox flow battery
0008-6223/$ - see front matter � 2013 Elsevihttp://dx.doi.org/10.1016/j.carbon.2013.04.038
* Corresponding author: Fax: +34 93 3563802.E-mail address: [email protected] (C. Flox).
A B S T R A C T
Electrochemical behavior of the polyacrylonitrile (PAN)-based graphite as a low cost elec-
trode material for vanadium based redox batteries (VFB) in sulfuric acid medium has been
improved by means of the successful introduction of nitrogen and oxygen-containing
groups at the graphite surface by thermal activation under NH3/O2 (1:1) atmosphere. Influ-
ence of the temperature and treatment duration times have been studied towards the posi-
tive reaction of VFB. The structure, composition, and electrochemical properties of the
treated samples have been characterized with field emission scanning electron microscopy,
X-ray photoelectron spectroscopy, cyclic voltammetry and electrochemical impedance
spectroscopy. The estimation of electrochemical surface area has also been evaluated.
The treatment of PAN graphite material at 773 K for 24-h leads to electrode materials with
the best electrochemical activity towards the VOþ2 /VO2+ redox couple. This method pro-
duces an increase of the nitrogen and oxygen content at the surface up to 8% and 32%,
respectively, and is proved to be a straightforward and cost-effective methodology. This
improvement of the electrochemical properties is attributed to the incorporation of the
nitrogen and oxygen-containing groups that facilitate the electron transfer through the
electrode/electrolyte interface for both oxidation and reduction processes.
� 2013 Elsevier Ltd. All rights reserved.
1. Introduction
The redox flow energy storage systems have recently received
considerable attention as they possess promising characteris-
tics such as long life, flexible design and high reliability, as
well as a low operation and maintenance cost [1,2]. The all-
vanadium redox flow battery (VFB) employs solutions of
VOþ2 /VO2+ as the catholyte and V3+/V2+ as the anolyte, both
of them in sulfuric acid medium. Their main advantage over
other available redox flow batteries lies in the fact that ions of
er Ltd. All rights reserved
the same chemical element are used in both half cells, thus
preventing cross-contamination by metal cation crossover
through the ion exchange membrane.
Although unlike other energy storage systems, in VFB, the
energy and power capabilities are independently scalable, the
major drawback of this technology is its low energy-to-weight
ratio (i.e., about 25–35 Whkg�1 of electrolyte) [3]. The general
improvement of these batteries requires a higher energy den-
sity electrolyte as well as higher efficiency electrodes offering
a better chemical reaction performance. Aiming to overcome
.
C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8 281
these inconveniences, the development of newelectrode mate-
rials plays a key initial role without whom it becomes even
more difficult to take advantage of the improved electrolyte.
In this framework, polyacrylonitrile (PAN)-based graphite
fiber felt is one of the more suitable electrode materials for
the VFB because of its wide operation potential range, stabil-
ity as either anode or cathode and availability of high surface
area at an affordable cost [4] taking into account the strong
acidity of the supporting electrolyte.
However, graphite-felt electrodes usually lead to a slow
charge transfer, a low reversibility and a low current density
which must be optimized [5]. Consequently, much attention
has been paid to the modification of this material [6], trying
to enhance its electrochemical properties in order to improve
its charge transfer capability as it is a low cost material with a
tridimensional structure which allows for a higher specific
area. The main objective of such modifications is the introduc-
tion of functional groups onto the surface in order to increase
its hydrophilicity, wetting properties and to improve the
charge-exchange ability of electron [7,8]. Particularly, oxygen
and nitrogen functionalization of carbonaceous materials have
been proven to yield materials exhibiting a higher electrocata-
lytic activity [9,10], as demonstrated in many electrochemical
devices such as fuel cells and biosensors [11,12].
In recent years, several processes have been developed for
synthesizing functionalized carbon electrodes [13]. These
methods usually involve inconveniences such as dangerous
and complicated operation, C–N and C–O functionalities (e.g.,
amine) that are unstable under strong catalysis conditions
(e.g., acid electrolyte) and leads to low concentration of oxygen-
and nitrogen-containing groups and short-term stability elec-
trodes. Thus, there is still a high demandfor exploring new con-
venient and inexpensive methods to fabricate functionalized
PAN-based electrodes to increase the interfacial wettability
and the concentration of N- and O-functional groups without
sacrificing the mechanical properties of graphite fibers.
It has been shown that the redox reactions involving the
V3+/V2+ couple are highly reversible and exhibit very fast reac-
tion kinetics. In contrast, the VOþ2 /VO2+ reaction kinetics are
much slower and more complex, since they involve at least
three elementary steps (i.e., one electron and oxygen transfer
and two proton exchanges) and several complex intermedi-
ates depending on the electrolyte pH and electrode potentials
[13]. Therefore, the electrochemical kinetics limitation of the
VFB arises from the positive side and thus, the development
of novel electrodes with a higher catalytic activity toward
the VOþ2 /VO2+ system is mandatory.
This paper focuses on the synthesis of PAN-derived (d-
PAN) electrodes using a simple and viable thermal-chemical
post-synthesized method to functionalize with active nitro-
gen and oxygen-like groups identified from NH3/O2 (1:1)
atmosphere treatments, aimed at yielding a higher cathode
performance compared to that for the unmodified PAN elec-
trode. Unlike other previous studies, special emphasis has
been addressed to the influence of the time and temperature
of the treatment. These parameters have been evaluated in
order to find the optimal experimental conditions to ensure
the best electrochemical, morphological and stability elec-
trode properties. Different electrochemical techniques have
been employed to study the behavior of the synthesized
electrodes regarding the VOþ2 /VO2+ couple, in order to assess
the estimation of electrochemical surface area and their elec-
trochemical activity.
2. Experimental
2.1. Electrode preparation
Samples of PAN-based graphite felt (Beijing Great Wall) with
1.5 · 10�4 m2 geometric area were treated thermally under
NH3/O2 atmosphere (1:1) at 673 K, 773 K and 883 K and for dif-
ferent treatment times, i.e., 6-h, 12-h, 24-h and 36-h, using a
tubular furnace. Hereafter, d-PAN (T, t) is used as the notation
for the modified electrodes, with T and t being the tempera-
ture and duration of the treatment, respectively. After the
treatment, the samples were cooled down under vacuum be-
fore carrying out the measurements.
2.2. PAN-derived electrodes characterization
The morphology of the PAN electrodes was examined using a
Hitachi H-4100FE field emission scanning electron micro-
scope (FESEM). The chemical composition changes of the sur-
face of the d-PAN electrodes were analyzed by X-ray
photoelectron spectroscopy (XPS) using a PHI instrument
model 5773 Multitechnique with A1 Ka radiation (1486.6 eV).
2.3. Electrochemical measurements
In order to determine the electrochemical properties of the
modified electrodes, a standard three-electrode glass cell
was employed and nitrogen gas was used to deaerate all the
solutions. The Hg/Hg2SO4/K2SO4 (sat.) electrode was used as
the reference electrode, being placed into a Luggin capillary,
and a platinum wire was employed as the counter electrode.
Each of the thin layer PAN-based electrodes was used as the
working electrode, being attached to a platinum plate that
acted as the current collector. A 500 mol m�3 VOSO4 (Alfa Ae-
sar) solution in 3000 mol m�3 H2SO4 (Sigma–Aldrich) [14] was
used as the aqueous electrolyte, being all the electrodes
soaked into it for 9 h before use. In order to evaluate the elec-
trochemical activity and estimate the electrochemical surface
area (ECSA) [15–19], several cyclic voltammetry (CV) experi-
ments were carried out between �0.4 V and 1.2 V at several
scan rates ranging from 0.001 to 0.02 V s�1.
Likewise, electrochemical impedance spectroscopy (EIS) at
open circuit voltage from 105 to 10�2 Hz, was employed to
confirm the electrocatalytic effect of the d-PAN electrodes.
The solutions were prepared with ultra-pure water and poten-
tial are reported with respect to Hg/Hg2SO4/K2SO4 (sat.).
All the electrochemical measurements were carried out
with a Biologic VMP-3 multipotentiostat controlled by EC-lab
software.
3. Results and discussion
3.1. Morphology of PAN electrodes
Fig. 1 shows the surface morphology observed by FESEM for
the unmodified PAN (Fig. 1a), as well as that of the d-PAN
(a)
(d)
(f)
(c)
e)
(b)
(g) (h)
Fig. 1 – Surface morphology of several PAN-based electrodes: (a) raw PAN felt as received, (b) d-PAN (673, 24), (c) d-PAN (773, 6),
(d) d-PAN (773, 12) , (e) d-PAN (773, 24), (f) d-PAN (773, 36), (g) d-PAN (873, 6), (h) d-PAN (873, 12).
282 C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8
(Fig. 1b–h) electrodes obtained upon different treatments. By
comparing Fig. 1a and b, it can be seen that the d-PAN (673,
24) maintains the initial fiber morphology and, apparently,
the surface roughness remains unaltered. However, a higher
Fig. 2 – Effect of temperature treatment (a) cyclic
voltammograms of untreated PAN electrodes (1); d-PAN (673,
12) (2); d-PAN (773, 12) (3) and d-PAN (873, 12) (4). Effect of
time of treatment (b) cyclic voltammograms of untreated
PAN electrodes (1), d-PAN (773, 6) (2), d-PAN (773, 12) (3),
d-PAN (773, 24) (4) and d-PAN (773, 36) (5). (c) Influence of
thermal and chemical effect upon electrocatalytic activity (a)
cyclic voltammograms of untreated PAN electrodes (1);
d-PAN (773, 24) in inert atmosphere (2); d-PAN (773, 24) in
NH3/O2 atmosphere (3). Measurements were performed
using 30 cm3 of a 500 mol m�3 VOSO4 in 3000 mol m�3
H2SO4 solution. Scan rate: 0.001 V s�1.
C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8 283
temperature leads to a decrease of the roughness, as can be
observed in the image of the d-PAN (773, 6) in Fig. 1c because
of binder removing. Fig. 1d and e shows that this roughness
increases with longer treatment times (i.e., from 12-h to 24-
h). These features corroborate the surface degradation as a
consequence of high density of defects in the fibers which
is enhanced by the treatment conditions. It was found that
when the raw PAN was treated at such high temperature over
a prolonged time period, i.e., 773 K for 36-h, the initial small
pores scattered on the surface identified at 773 K 6-h
(Fig. 1c) became larger, as shown in Fig. 1f. The presence of
these large pores is caused by the over effect of the pressure
of the NH3/O2 gas mixture on the fiber surface taking place
during longer treatments. Finally, when the d-PAN material
was obtained at 873 K for 6-h, the PAN fibers underwent a
clear deformation, as shown in Fig. 1g. The bending observed
due to the lower stiffness at high temperature was even more
pronounced as the duration of the treatment increased,
which also led to a larger consumption of the graphitic car-
bon, as can be seen in the worn-out fibers of Fig. 1h.
As a general observation, the prolonged treatment at a gi-
ven temperature led to both the increase of the surface rough-
ness and the appearance of a larger number of pores. The best
treatment conditions are those generating the highest num-
ber of active sites favouring the VOþ2 /VO2+ process. In order
to identify the best material treatment, the PAN-derived elec-
trodes will be electrochemically characterized.
3.2. Electrochemical behavior of the d-PAN electrodes
The influence of temperature and time of treatment of PAN
electrodes were evaluated by cyclic voltammetry using solu-
tions of supporting electrolyte, yielding the voltammograms
of Fig. 2a and b, respectively. Well-defined anodic and catho-
dic peaks were recorded for the VOþ2 /VO2+ redox couple. The
redox peaks are attributed to the following reversible
reaction:
VO2þ þH2O ¡ VOþ2 þ 2Hþ þ e� ð1Þ
The electrochemical activity (basically, peak current den-
sity, Ia and peak potential values, Ep-ox) and the reversibility
(DEp) of reaction (1) (summarized in Table 1) are the key crite-
ria for evaluating the performance of a given electrode when
considering the VOþ2 /VO2+ redox couple in a VFB.
Regarding the temperature dependence, as it can be de-
duced from these values, the electrochemical kinetics of the
oxidation process in reaction (1) on the electrodes decrease
in the order: d-PAN (773, 12) > d-PAN (673,12) > d-PAN
(873,12) � untreated PAN. On the other hand, regarding the
influence of treatment duration for all d-PAN electrodes trea-
ted at 773 K (values are given in the Table 1 and shown in
Fig. 2b), the electrochemical ability follows the order as d-
PAN (773, 24) > d-PAN (773, 12) � d-PAN (773, 6) > d-PAN (773,
36) � untreated PAN electrode.
In comparison with the untreated PAN electrode, we find
that the anodic current density is increased from 254 to
371 A m�2, and the anodic peak potential values is decreased
from 0.58 to 0.52 V at the electrode d-PAN (773, 24). It should
be noticed that treatments times over 6-h at a temperature
of 873 K leads to reduce considerably the electric conductivity,
and consequently these electrodes presented a poor electro-
chemical activity towards reaction (1). For this reason,
Table 1 – Electrochemical parameters for all PAN-derived electrodes studied obtained from CVs at 0.001 V s�1 in the Fig. 2.Anodic peak potential values (Ep-ox), anodic peak current density (Ia), and the reversibility (DEp).
Electrode Ep-ox (V) Ia (A m�2) DEp (V)
Untreated 0.576 254.1 0.24d-PAN (673, 6) 0.539 271.6 0.18d-PAN (673, 12) 0.539 270.9 0.17d-PAN (673, 24) 0.525 279.7 0.17d-PAN (673, 36) 0.525 273.5 0.16d-PAN (773, 6) 0.532 288.3 0.18d-PAN (773, 12) 0.525 306.1 0.17d-PAN (773, 24) 0.527 370.3 0.16d-PAN (773, 36) 0.543 229.0 0.19d-PAN (873, 6) 0.528 307.7 0.16d-PAN (873, 12) 0.565 228.9 0.19
284 C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8
d-PAN (873, 24) and d-PAN (873, 36) electrodes have not been
taken into account.
Note that the difference between anodic and cathodic
peak potential values decreases strongly from 0.24 to 0.16 V
for untreated PAN electrode and d-PAN (773, 24) electrode,
respectively. This value remains larger than the ideal value
of 0.057 V for fast electrode kinetics. The values of Ipa/Ipc were
close to unity for both redox couples for the electrode d-PAN
(773, 24). Therefore, the d-PAN (773, 24) electrode not only
exhibits high stability and morphology after the treatment,
but also a higher electrochemical activity towards the reac-
tion (1). It is noteworthy that the important role of the chem-
ical treatment upon the electrocatalytic activity towards the
positive reaction. As it is showed in the Fig. 2c, the thermal
treatment in inert atmosphere leads to decrease the current
density collected in comparison with chemical and thermal
treatment.
These results suggest that the best treatment conditions to
improve the PAN electrode electrochemical activity consist of
thermal treatment of the d-PAN at 773 K for 24-h in NH3/O2
atmosphere. Such conditions lead to a higher reversibility, a
lower anodic potential peak value and a higher current den-
sity for the reaction (1).
3.3. Surface characterization of PAN electrodes
Based on the above analysis, we can conclude that
PAN-derived electrodes treated at 773 K show the best electro-
chemical activity. For the sake of a better understanding of
the enhancement of the electrochemical properties, the nat-
ure of the chemical functionalities onto surface created was
investigated by XPS for d-PAN electrodes treated at 773 K for
Table 2 – Relative contents of functional groups in O1s and N1s
PAN electrode Atomic concentrationa O1s-w
C O N O1
Untreated 77.0 22.3 0.7 9.526-h 68.8 29.9 1.3 18.4512-h 64.6 31.7 3.7 10.6824-h 60.8 31.5 7.7 4.95
a C + N + O = 100%.
6-h, 12-h, 24-h. Note that the d-PAN (773, 36) electrode was ex-
cluded due to the loss of the electrochemical properties to-
ward the reaction (1).
Table 2 shows a XPS comparison of the atomic content and
weighted concentration of N- and O-containing groups for
each electrode treated at 773 K. The weighted concentration
is calculated by multiplying relative concentrations in atom-
ic% by the total atomic content of either nitrogen or oxygen.
Based on the signal intensity of oxygen in these spectra, the
oxygen content in the samples treated at 773 K is higher than
that in the raw PAN, and then it slightly increases in the order:
PAN < d-PAN (773, 6) < d-PAN (773, 12) � d-PAN (773, 24), as
summarized in Table 2. Fig. 3 shows the high-resolution
XPS O1s and N1s spectra of the d-PAN electrodes prepared
at 773 K during 24-h of treatment. The deconvolution of
the O1s spectra yielded the following peaks: peak O1
(531.5 ± 0.2 eV), carbonyl oxygen atoms in esters, anhydrides
and oxygen atoms in hydroxyl groups; peak O2 (533.8 ±
0.2 eV), non-carbonyl (ether type) oxygen atoms in esters
and anhydrides; peak O3 (535.5 ± 0.2 eV), adsorbed molecular
water and oxygen [20]. On the other hand, the nitrogen con-
tent in the samples treated at 773 K increases in a similar
trend. The high resolution XPS N1s spectrum given in
Fig. 3b reveals the presence of N-functional groups in the
PAN structure associated with peaks: peak N1 (398.7 ±
0.2 eV), pyridine-N; peak N2 (400.3 ± 0.2 eV) pyrrole-N; peak
N3 (401.4 ± 0.2 eV) Quaternary-N, that is, graphite-like nitro-
gen incorporated into the structure of extended aromatic sys-
tem of the PAN-graphite fibers and peak N4 (404.1 ± 0.2 eV)
[21], that has been proposed to be pyridine-N-O groups [22].
It is noteworthy that the d-PAN (773, 24) electrode presents
the highest functionalization compared to other treatment
from XPS spectra for untreated and d-PAN (773, t) electrodes.
% N1s-w%
O2 O3 N1 N2 N3 N4
11.82 0.96 0.14 0.56 n.d. n.d.10.02 1.44 0.28 0.89 0.13 n.d.18.13 2.88 0.96 2.56 0.18 n.d.22.52 4.03 1.71 3.97 1.65 0.37
Fig. 4 – (a) Plot of the anodic peak current from cyclic
voltammograms of PAN-derived electrodes treated at 773 K
during 6, 24 and 36 h vs. the square root of the scan rate. (b)
log (Ip: anodic peak current) vs. log (v: scan rate).Fig. 3 – High-resolution of O1s (a) and N1s (b) spectrum for d-
PAN (773, 24) electrode.
C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8 285
conditions, which is correlated and explains its higher elec-
trochemical activity.
3.4. Estimation of electrochemical surface area of PAN-derived electrodes
The estimation of electrochemical surface area (ECSA) that is
available for the electron transfer to/from the species con-
tained in solution can be estimated from the Randles–Sevcik
equation (2) for VOþ2 /VO2+ redox system [23]. The values of
the diffusion coefficients were obtained from bibliography
(1.0 · 10�10 m2 s�1) [23].
Ip ¼ 0:4961 nFAC0/ nFvD
RT
� �1=2
ð2Þ
where Ip is the anodic peak current of oxidation peaks of VOþ2(A), n is the number of exchanged electrons, F the Faraday
constant, A is the ECSA (cm2), C0 is the initial bulk concentra-
tion of the electroactive species (mol cm�3), a is the transfer
coefficient (0.5), v is the scan rate (V s�1) and D is its diffusion
coefficient of the molecule in solution (cm2 s�1), R the univer-
sal gas constant and T the absolute temperature.
The slopes of the Ip vs. v1/2 plots shown in Fig. 4a allowed
the estimation of the ECSA for each PAN electrode presented
in Table 3. The dependence of the anodic peak current on
scan rate can be described using the following relationship:
Ip ¼ Kvx ð3Þ
logðIpÞ ¼ logðKÞ þ xlogðvÞ ð4Þ
where Ip is the peak oxidation current, v is the scan rate and x is
the exponent of scan rate and K is the proportionality constant.
The log of peak oxidation current was plotted against the log of
scan rate for the data shown in Fig. 4b. A linear relationship was
obtained with slope values of 0.58, 0.56, 0.58 and 0.57 for un-
treated PAN, d-PAN (773, 6), d-PAN (773, 24) and d-PAN (773,
36) electrode, respectively. Ideally, the slope should approach
0.50 under semi-infinite diffusion conditions where the diffu-
sion of vanadium ions from bulk solution to the electrode sur-
face is rate limiting. This small deviation from the theoretical
0.5 value can be explained by the nature of the electrodes ana-
lyzed which is porous and could introduce deviations due to dif-
ferent diffusion gradients on the surface of the electrode. Also,
the Randles–Sevcik equation is adequate to be used for diluted
solutions. Nevertheless, a very good linearity was observed for
all studied system [24,25].
Table 3 – Estimated ECSA values and atomic concentration ratios obtained from XPS analysis for untreated electrode and PAN– derived electrodes prepared at 773 K.
PAN electrode ECSA (m2 kg�1) DECSA (%) N/C O/C
Untreated 78 0.0090 0.28966-h 98 25 0.0188 0.434524-h 135 73 0.0572 0.490736-h 91 16 – –
286 C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8
Actually, the increase estimated ECSA from 25% to 73% can
only be interpreted in terms of the higher catalytic activity
associated with a higher density of O- and N-containing
groups onto the surface of electrode d-PAN (773,6) and d-
PAN (773, 24) which can form good electron pathways be-
tween electrode and electrolyte. The better performances of
the N- and O-contained PAN electrodes may be ascribed to
the higher wettability and the change of the density of elec-
tronic states (DOEs) around the Fermi level of PAN electrode
[26]. The mechanism of the VOþ2 /VO2+ redox reaction is still
controversial. Some theories of catalysis have hypothesized
that the aforementioned O- and N-functional groups at the
PAN electrodes play an important role in this reaction because
they favor the interaction of a larger amount of vanadium
ions and oxygen onto the electrode surface. As a result, the
electron and oxygen transfer processes involved in reaction
(1) become accelerated at these derived electrodes.
3.5. Electrochemical impedance spectra
In order to ensure the faster capability of electron and oxygen
transfer processes of the d-PAN (773, 24) electrode in compari-
son with untreated electrode, electrochemical impedance spec-
tra (EIS) are recorded in the same experimental conditions at
open-circuit potential. The Nyquist plot is shown in Fig. 5. A
semicircle and a linear part are observed in the frequency from
105 to 10�2 Hz, indicating that the VOþ2 /VO2+ redox reaction is
controlled by both charge transfer and diffusion process. The
semicircle part at high frequencies reflects the charge-transfer
process and the linear part at low frequencies reflects the diffu-
sion processes in graphite felt. The smaller arc radius implies a
faster reaction, so it can be seen, the resistance related to the
charge transfer reaction increases in the following order: raw
Fig. 5 – Nyquist plot obtained for PAN-derived electrode
treated at 773 K and 24-h and untreated electrode.
PAN electrodes <<< d-PAN (773, 24). Therefore, both EIS and
CV results show that both the reduction and oxidation pro-
cesses of the VOþ2 /VO2+ redox couple are best enhanced at the
surface of the d-PAN (773, 24) electrode.
3.6. Role of O- and N-containing groups
Fig. 6a shows the variation of anodic current density obtained
from CVs of the Fig. 2 as a function of the total amount of
nitrogen and oxygen for each electrode shown in Table 3.
These results suggest that the increment of the electrochem-
ical activity is proportional to the total amount of N- and O-
containing groups introduced onto surface of the PAN-derived
electrodes at the first stages of treatment. The decrease in
current density after 12-h of treatment is evident.
A detailed analysis (Fig. 6b) of the individual influence of
the amount of the nitrogen and oxygen content onto the var-
iation of anodic current density points out the increment of
the anodic current density caused by the incorporation, first,
of a high amount of O-containing groups onto the surface
electrode. After 6-h of treatment, the role of the N-containing
groups becomes much more significant. They are incorpo-
rated onto the electrode surface and they become the princi-
pal responsible of variation of current density. In this stage,
the concentration of oxygen onto surface is constant, proba-
bly because of saturation of its chemisorption process. It is
noteworthy that the incorporation of N-containing groups is
a key step due to substantial improvement of the electro-
chemical activity (i.e., higher current density, low oxidation
potential and higher reversibility). This enhancement of the
electrocatalytic activity is attributed to the N atoms that
would decorate the PAN electrode and introduce changes on
the electro conductivity properties.
In order to study the effect of nitrogen on the electrochem-
ical activity of the electrodes towards the positive reaction of
the VFB, the weighted nitrogen concentration of the species de-
tected from XPS analysis are plotted against the variation of
anodic current density and shown in Fig. 6c. It is observed that
all types of N-containing groups have a positive effect on the
anodic current density due to the enhancement of the electron
transfer process. The most influential species on the enhance-
ment of electron conductivity can be five-member pyrrole rings
which clearly act as electron-donors (N2), or more stable six-
member rings where nitrogen contributes to the p-system with
one p-electron, if present in pyridine (N1). However, surpris-
ingly, the d-PAN (773, 24) electrode contains the highest
amount of quaternary groups (i.e., nitrogen in aromatic elec-
tron-donor groups, (N3) and it is possible that these groups
may play also an important role for the enhancement of the
electrochemical activity [22]. Therefore, N-containing groups
might play an important role in regulating the electronic
Fig. 6 – (a) Relationship between the total amount of the
weighted nitrogen and oxygen concentrations of species
detected from XPS analysis and variation of anodic
current density obtained from de CVs in the Fig. 2 for
the untreated PAN, d-PAN (773, 6), d-PAN (773, 12) and
d-PAN (773, 24) electrode. (b) Relationship between the
individual amount of the nitrogen and oxygen atomic
concentration of species detected from XPS analysis and
variation of anodic current density obtained from de CVs
in the Fig. 2 for the untreated PAN, d-PAN (773, 6), d-
PAN (773, 12) and d-PAN (773, 24) electrodes. (c)
Relationship between the weighted N-containing groups
concentrations of species detected from XPS analysis and
variation of anodic current density obtained from de CVs
in the Fig. 2.
C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8 287
properties and enhancing the electrochemical activity of the
PAN-derived electrodes in electrochemical systems.
Therefore, due to these O- and N-containing groups onto
surfaces of the d-PAN (773, 24) electrode, more reactive ions
can be interact on the electrode surface [27] and the electron
and oxygen transfer process can also be catalyzed thus mak-
ing the d-PAN (773, 24) electrode the most effective one. The
abundant surface area as well as surface sites introduced by
the thermal treatment onto d-PAN (773, 24) surface electrode
can promote the electrochemical reactions.
3.7. Long-term assessment of the d-PAN(773, 24)electrode
In order to assess the long-term stability of the d-PAN (773, 24)
electrode, up to 50th repetitive cyclic voltammetric were per-
formed and showed in the Fig. 7a. Fig. 7b shows the FE-SEM
image of this electrode after this measurement. No significant
change in electrocatalytic activity (i.e., peak current density
and reversibility), neither the morphology. Consequently, this
functionalization treatment supposes a step forward for VFB
system due to their outstanding properties. A detail study of
atmosphere composition applied in the thermal treatment
with lower quantities of oxygen is required and will be pub-
lished in elsewhere.
Fig. 7 – (a) Stability test of the d-PAN (773, 24) electrode in
30 cm3 of a 500 mol m�3 VOSO4 in 3000 mol m�3 H2SO4
solution. Scan rate: 0.001 V s�1. (b) FESEM image of the
d-PAN (773, 24) electrode after 50th cycle.
288 C A R B O N 6 0 ( 2 0 1 3 ) 2 8 0 – 2 8 8
4. Conclusions
In order to improve the cathode at the VFB in sulfuric acid
medium, the surface of a PAN electrode was functionalized
with N- and O-containing groups as a result of a thermal
treatment under NH3/O2 (1:1) atmosphere within different
temperatures and residence times. The highest electrochem-
ical activity towards the VOþ2 /VO2+ redox couple was found for
PAN electrode treated at 773 K for 24 h. Experimental results
reveal that the d-PAN (773, 24) electrodes possess advantages
such as a high N and O content up to 8% and 32%, respec-
tively, and morphological stability. These results suggest that
the amount of ‘‘graphite-like’’ C–N bonds is increased with the
time of treatment being mainly responsible of the enhancing
of electrochemical activity. When comparing the PAN-derived
electrodes, the d-PAN (773, 24) electrode appears to be the
most promising with 73% increase in its electroactive surface
compared to the untreated electrode given rise to an effective
increase of the electrode current of almost 50%. Due to the
enhancement of the electrochemical activity of the long-term
d-PAN (773, 24) electrode is expected to be a potential applica-
tion of electrode materials in VFB.
Acknowledgements
The research was supported by European Regional Develop-
ment Funds (ERDF, FEDER Programa Competitivitat de Catalu-
nya 2007–2013), MINECO-INNPACTO, project REDOX 2015
(IPT-2011-1690-920000), by MINECO, project NANO-EN-ESTO
(ref. MAT2010-21510), and by Ministerio de Economıa y Com-
petitividad-CONSOLIDER Ingenio 2010, project MULTICAT
(CSD2009-00050). The research was supported by EIT and
KIC-InnoEnergy under the project KIC-EES (33_2011_IP29_
Electric Energy Storage).
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