C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6

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Superior capacitive and electrocatalytic propertiesof carbonized nanostructured polyaniline upona low-temperature hydrothermal treatment

0008-6223/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.carbon.2013.07.100

* Corresponding author: Fax: +381 11 2187 133.E-mail address: slavko@ffh.bg.ac.rs (S. Mentus).

Milica Vujkovic a, Nemanja Gavrilov a, Igor Pasti a, Jugoslav Krstic b,Jadranka Travas-Sejdic c,d, Gordana Ciric-Marjanovic a, Slavko Mentus a,e,*

a University of Belgrade, Faculty of Physical Chemistry, Studentski trg 12–16, 11158 Belgrade, Serbiab University of Belgrade, Institute of Chemistry, Technology and Metallurgy, Department of Catalysis and Chemical Engineering, Njegoseva

12, 11000 Belgrade, Serbiac Polymer Electronics Research Centre, School of Chemical Sciences, University of Auckland, 23 Symonds Street, Auckland, New Zealandd MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New

13 Zealande Serbian Academy of Science and Arts, Knez Mihajlova 35, 11000 Belgrade, Serbia

A R T I C L E I N F O A B S T R A C T

Article history:

Received 20 April 2013

Accepted 30 July 2013

Available online 6 August 2013

Carbonized nanostructured polyaniline (C.PANI) was hydrothermally treated in 1 mol dm�3

KOH at 200 �C. The treatment caused significant reduction of micropore volume but negli-

gible changes in mesoporous domain, as evidenced by nitrogen adsorption measurements,

as well as significant increase of surface N/C and O/C ratios, as evidenced by XPS method.

In comparison to the C.PANI precursor, the new material, denoted as C.PANI.HAT200, deliv-

ered twice as high gravimetric capacitances, amounting to 363, 220 and 432 F g�1, in

6 mol dm�3 KOH, 2 mol dm�3 KNO3 and 1 mol dm�3 H2SO4, respectively, when measured

potentiodynamically at a scan rate of 5 mV s�1. Moreover, its exceptionally high electrocat-

alytic activity towards the oxygen reduction reaction (ORR), almost one order of magnitude

higher than that of C.PANI was evidenced in alkaline media, approaching closely a four-

electron pathway. The onset potential for ORR matched the one of platinum-based electro-

catalyst. Significant improvements of both capacitive and electrocatalytic properties of

C.PANI.HAT200 were discussed in correlation to the modification of surface functional

groups. These findings affirm the low temperature hydrothermal post-synthetic modifica-

tion of N-doped nanocarbons as a route of production of advanced multifunctional carbon

materials with exceptional characteristics.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Different carbon materials such as activated carbon, carbon

nanotubes, graphene, carbon aerogels, carbon nanofibers

and so on, have been extensively studied from the viewpoint

of both capacitive [1–10] and electrocatalytic properties

[11–14]. Numerous literature reports in this field indicated

that both charge storage capability and electrocatalytic activ-

ity of these materials depend primarily on their textural

behavior and surface chemistry. Many authors correlated

the charge storage to textural characteristics (specific surface

area and pore size distribution) of carbon materials in order to

C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6 473

unveil whether the microporosity [15,16], mesoporosity

[17,18] or their particular balance [19] direct the double-layer

behavior. Chmiola et al. [15] found anomalous increase in

the double layer capacitance by reduction of the pore diame-

ter below the diameter of solvated electrolyte ions. Their

statement on the role of micropores in effective utilization

of surface area for double layer formation is supported by

other research groups [20,21]. Apart from the textural effects,

it is widely accepted that the presence and distribution of N-

and O- containing groups on carbon surface also influences

the capacitor performance [2,10,21–23]. The oxygen func-

tional groups usually present on the carbon surface can con-

tribute to the carbon capacitance either through the faradaic

reactions [24,25] or through the improvement of wettability of

microporous region [24–26]. Moreno-Castilla et al. [10] have

recently reported a linear relationship between the double

layer capacitance of carbon aerogels and areal oxygen con-

centration. However, as pointed out by Hulicova-Jurcakova

et al. [2], a high-performance carbon-based supercapacitor

may be realized by the combined effects of pore structure

and carbon surface functionalities. Electrocatalytic properties

of carbon materials are also largely influenced by the hetero-

atoms (N and O) fixed at the surface, particularly towards oxy-

gen reduction reaction (ORR) [11–14,27–29]. High activity of N-

containing carbons toward ORR is commonly attributed to the

pyridinic nitrogen [13]. We discussed recently how diverse

pore structures and nitrogen surface functional groups of

similar N-containing carbon nanomaterials influence ORR

electrocatalytic activity [11]. It was also shown that a fine tun-

ing of the charge state of carbon/electrolyte interface, realized

by potentiodynamic polarization, can affect the ORR activity,

thus leading to the conclusion that high capacities and elec-

trocatalytic activities of heteroatom-doped nanocarbons are

closely correlated [11].

Even if the key factors of high capacitance and electrocat-

alytic activities were known, the synthesis of materials with

the desired properties may present a problem. The carboniza-

tion of nitrogen containing nanostructured polymers such as

polyaniline (PANI) and its derivatives [7,11,12,30–32] presents

a relatively new and easy way to obtain N-containing carbon

nanostructures (N-CNS) with tailored morphology and high

nitrogen content (up to �10%). In our previous works [7,11]

it was shown that the type of nanostructured PANI salt pre-

cursor (especially the type of the dopant, i.e., 5-sulfosalicy-

late, 3,5-dinitrosalicylate, hydrogen sulfate ions) used for

the preparation of N-CNS strongly affected properties of pro-

duced C.PANIs and consequently their capacitive [7] and elec-

trocatalytic [11] properties. The C.PANI obtained by

carbonization of PANI synthesized in the presence of in situ

formed hydrogen sulfate counter-ions [30], was subjected also

to another type of surface modification, namely to a hydro-

thermal treatment in 1 M KOH solution at 150 �C [12]. Such a

modification caused a negligible decrease in the specific sur-

face area, but a significant enhancement in ORR activity in

alkaline solution, however, its capacitive properties were not

studied. Proceeding from this experience, in this study, we

subjected the same C.PANI material to a somewhat stronger

alkali treatment at 200 �C, and examined simultaneously the

capacitive properties (in alkaline, neutral and acidic media),

and the electrocatalytic (in alkaline medium) behavior toward

ORR of the modified carbonaceous product. In spite of a small

increase in HAT temperature in comparison to that used in

Ref. [12], the material modified at 200 �C displayed a drastic

decrease in specific surface area and significant improvement

in ORR activity compared to that modified at 150 �C [12], as

well as a twice as high gravimetric capacitance compared to

that of unmodified C.PANI [7]. The explanations of these

somewhat contradictory results were sought in the changes

of surface chemical composition. The attention was particu-

larly paid to the question how the modification of N- and

O-containing surface functional groups caused by HAT, com-

bined with the changes in textural characteristics, influences

the electrochemical properties of the investigated material.

2. Experimental

2.1. Samples synthesis and hydrothermal treatment

The nanostructured PANI was prepared by the following pro-

cedure [30]: equal volumes (0.5 dm3) of the aqueous solutions

of aniline (0.4 M) and oxidant ammonium peroxodisulfate

(APS) (0.5 M) were mixed to initiate the oxidation. The reac-

tion mixture was stirred for 2 h, after which the precipitated

nanonostructured PANI was collected on a filter, rinsed with

5 · 10�3 mol dm�3 H2SO4 and dried in vacuum. C.PANI was ob-

tained by the carbonization of nanostructured PANI by means

of gradual heating in N2 atmosphere up to 800 �C at a heating

rate of 10 �C min�1. For carbonization a Carbolite CTF 12/75/

700 tube furnace with temperature regulation by Eurotherm

815P Prog/Controller was used. To execute modification by

HAT, total amount of 50 mg of C.PANI was dispersed in

25 mL of 1 M KOH solution and loaded into a stainless steel

autoclave (32 mL) with polytetrfluoroethylene liner. It was

then sealed and heated at a rate of 10 �C min�1 up to 200 �C,

and then kept at this temperature for 6 h. Upon cooling to

room temperature, the obtained slurry was centrifuged and

thoroughly washed with diluted hydrochloric acid and, con-

secutively, water. Finally, it was dried at 60 �C overnight. The

obtained sample is denoted hereafter as C.PANI.HAT200.

2.2. Samples characterization

The elemental microanalysis (C, H, N and O) was carried out

using the Elemental Analyzer Vario EL III (Elementar).

The electrical conductivity was measured at room temper-

ature by means of Waynne Kerr Universal Bridge B 224, oper-

ating at a fixed frequency of 1.0 kHz. During the

measurement, the powdery sample was placed between

stainless steel pistons within an insulating hard-plastic tube

and subjected to a constant pressure of 80 MPa.

XPS spectra were recorded on a Kratos Axis Ultra DLD (Kra-

tos Analytical, Manchester UK), using monochromatic Al Ka

line (1486.69 eV) with X-ray power of 150 W. Survey spectra

were collected with 160 eV pass energy, whilst core-level

scans were collected with pass energy of 20 eV. The pressure

in the system was 2 · 10�9 Torr. The analysis area for the data

collection using the hybrid electrostatic and magnetic lens

system and the slot aperture was approximately

300 · 700 mm. Data analysis was performed using CasaXPS

474 C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6

using Kratos’ relative sensitivity factors. Core level scans were

calibrated based on a peak fit to the C 1s scan, with the com-

ponent due to aromatic carbon set to 284.7 eV. Shirley back-

grounds were used throughout. Gaussian–Lorentzian

product lineshapes were used with 30% Lorentzian weighting.

Adsorption–desorption isotherms of C.PANI-HAT200 and

C.PANI reference samples were obtained by nitrogen adsorp-

tion at 77 K using a Sorptomatic 1990 Thermo Finnigan de-

vice. Prior to adsorption, the samples were degassed for 1 h

at room temperature under vacuum and a further 16 h at

383 K at the same residual pressure. Software ADP Version

5.13 CE Instruments was used to analyze the resulting N2

adsorption isotherms. Micropore volume (Vmic) was estimated

using Dubinin–Radushkevich (DR) equation [33], the aS-meth-

od [34], using standard isotherm given by Lecloux and Pirard

[35], and Horvath–Kawazoe (HK) method [36]. Specific surface

area of micropores (Smic) was evaluated using Kaganer modi-

fication of Dubinin’s method (DK method) [34] and HK meth-

od. The later approach was also used to determine the pore-

size distribution in the microporous region [36]. Mesopore vol-

ume (Vmeso), mesopore surface area (Smeso) and mesopore size

distributions were determined by the Dollimore and Heal (DH)

method [37]. Smeso was also estimated using aS-method [34].

Specific surface areas (Stot) were estimated using BET method

and aS-method [34]. Stot was also estimated as the sum of

Smic-DK and Smeso-DH [7].

2.3. Electrochemical measurements

To prepare electrodes, the required amount of carbon sample

was dispersed in ethanol/water mixture (40 v/v%). After

30 min of ultrasonication, 10 ll of the homogenized ink was

transferred by a micropipette onto the glassy carbon (GC) disk

surface (0.196 cm2), polished previously to a mirror finish by a

diamond paste. Upon drying in an argon stream, in order to

fix the remaining carbon film, it was covered with a droplet

of 10 ll of 0.05 wt% Nafion solution in ethanol, and subjected

to solvent evaporation. Having in mind that the active mass

loading caused the measured capacity to increase up to satu-

ration [7], we paid attention to use an optimal loading provid-

ing the accurate measurement. Thus, the anticipated loading

of investigated N-containing carbon material was

250 lg cm�2. Then the GC supported film electrode was con-

nected as a working electrode in a conventional one-compart-

ment cell with a wide Pt foil and a calomel electrode (SCE)

serving as a counter and reference electrode, respectively.

The electrolyte solutions of 6 mol dm�3 KOH, 2 mol dm�3

Table 1 – The elemental composition of C.PANI and C.PANI.HAT

Element C.PANI

XPS at.% Elemental analysis wt

C 87.9 [7] 74.8 [7]N 5.80 [7] 8.9 [7]O 6.30 [7] 14.2 [7]H – 2.1N/C 0.066O/C 0.072

KNO3 and 1 mol dm�3 H2SO4 were used for the capacitance

measurement by means of cyclic voltammetry. The kinetics

of ORR was investigated in 0.1 mol dm�3 KOH, using a rotating

disk electrode (RDE) voltammetry in the potential window be-

tween +0.27 to –0.97 V vs. SCE at 20 mVs�1.

Gamry PCI4/750 Potentiostat/Galvanostat equipped with a

Pine rotator was used for voltammetric investigations. Before

and during the measurements, a gentle gas flow of N2 or O2

(purity 99.9995 vol.%) was introduced just beneath the elec-

trolyte surface. All the measurements were performed at

room temperature (25.0 ± 0.5 �C). Reported current densities

are evaluated with respect to the geometrical cross section

area of supporting GC disk.

3. Results and discussion

3.1. Elemental composition and electrical conductivity

In Table 1, the bulk and surface elemental compositions of

C.PANI.HAT200, determined by elemental microanalysis and

XPS, respectively, were compared to the corresponding data

of the C.PANI precursor reported previously [7]. While, upon

HAT, the total C and N contents increased slightly from 74.8

to 77.1 and from 8.9 to 9.1 wt.%, respectively, the total O con-

tent decreased from 14.2 to 11.9 wt.% (Table 1). In addition,

the comparison of the elemental composition of C.PANI.-

HAT200 to the elemental composition of C.PANI treated at

150 �C [12], indicated that the increase of the temperature of

the HAT progressively increased C content, and decreased

the O and H content. Significant changes of the surface ele-

mental content caused by the applied HAT were evidenced

by XPS analysis. The surface C content decreased from 87.9

to 84.0 at.%, whereas a small increase in the surface N con-

tent (5.80! 6.25 at.%) and significant increase in the surface

O content (6.30! 9.75 at.%) were observed. Consequently,

surface N/C and O/C ratios also increased (Table 1).

Significant increase in surface oxygen concentration,

which is known to have a beneficial effect on the electro-

chemical properties of carbon materials, could be explained

by the combined effects of partial homogenization and sur-

face oxidation of C.PANI under applied HAT conditions

[2,9,21,38]. Before HAT, C.PANI had a heterogeneous core/shell

composition with the shell richer in C (surface C content of

87.9 at.%, as determined by XPS which excludes surface H

content, Table 1) than the core (total C content of 74.8 wt.%,

as determined by elemental analysis, which corresponds to

80.3 at.% if H is excluded from calculation, Table 1). After

200 determined by XPS and elemental microanalysis.

C.PANI.HAT200

.% XPS at.% Elemental analysis wt.%

84.0 77.16.25 9.19.75 11.9– 1.90.0740.116

C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6 475

HAT, C.PANI obtained a more homogenous composition with

the surface C content (84.0 at.%, as determined by XPS which

excludes surface H content) more close to the total C content

(77.1 wt.%, or 82.2 at.% if H is excluded from the calculation),

Table 1. Partial homogenization relates not only to the signif-

icantly decreased difference between surface and total con-

tents of C, but also to the markedly decreased difference

between surface and total contents of O upon HAT (Table 1).

Obviously, some O-containing structural segments in the core

became exposed at the surface upon the HAT. No appreciable

change of electrical conductivity (r), from 0.35 S cm�1 [7] to

0.38 S cm�1, was observed upon HAT, diminishing the role of

conductivity in further explanation of electrochemical perfor-

mance of investigated carbons in Section 3.4.

Fig. 1 – (A) and (B): Fitted high-resolution XPS N1s (A) and O1s (

data for C.PANI are adapted from [7] with the permission of Else

spectra at binding energies around 403.6 eV. (D): Form of quatern

N associated with nearby and adjacently located hydroxyl moie

3.2. XPS characterization of C.PANI surface modificationby the HAT

The changes of the nature and distribution of surface N and O

functionalities of C.PANI caused by HAT, examined by the

deconvolution of corresponding N1s and O1s high-resolution

XPS signals, are illustrated in Fig 1(A) and (B), and Table 2. The

chemical states of nitrogen atoms (Table 2), with XPS peaks at

binding energies of 398.3 eV, 399.8 eV, 400.8 eV, and 402.5 eV

for C.PANI and 398.2 eV, 399.6 eV, 400.7 eV and 402.0 eV for

C.PANI.HAT200, are identified as pyridinic (N-6), pyrrolic/pyri-

done (N-5), quaternary (N-Q) nitrogen and pyridine-N-oxide

(N+-O–), respectively [39–41]. The other N-containing groups

assigned as N-X and N-X 0 with high binding energies of

B) spectra of C-PANI.HAT200 (top) and C-PANI (bottom). The

vier. (C): Phenazine-di-N-oxide-like structures appear in XPS

ary nitrogen (N-Q) according to Kelemen et al. [45]: pyridinic-

ty from phenol or carboxyl group.

Table 2 – XPS peak position and relative content of nitrogen and oxygen species in C.PANI and C.PANI.HAT200 samples. Thedata for C.PANI are taken from [7].

C.PANI [7] C.PANI.HAT200

Nitrogen species Peak position (eV) % N atoms Peak position (eV) % N atoms

N-1 396.5 4.40 – –N-6 398.3 35.0 398.2 29.38N-5 399.8 13.0 399.6 15.04N-Q 400.8 40.9 400.7 37.15N+-O� 402.5 6.70 402.0 8.29N-X 403.6 6.50N-X 0 406.0 3.64

Oxygen species Peak position (eV) % O atoms Peak position (eV) % O atoms

O–I 530.7 35.3 530.4 24.14O–II 532.3 42.1 531.9 38.20O–III 533.6 22.6 533.1 29.80O2,ads, H2Oads – – 534.4 7.87

476 C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6

403.6 eV and 406.0 eV, respectively, appeared on the surface as

a result of applied HAT. We suppose that the peak at 403.6 eV

could be attributed to the oxidized forms of nitrogen like

those in phenazine-di-N-oxide (Fig. 1(C)), which is structurally

similar to the benzo[c]cinnoline-5,6-dioxide known to have

the XPS N1s peak at 403.6 eV [42], rather than to the existence

of shake-up satellite peaks [39,43]. However, the contribution

of shake-up effects (on-site excitation of an electron from the

HOMO to the LUMO) due to the p–p* transitions cannot be ne-

glected. The peak at 406.0 eV is most probably due to the nitro

type complexes -NO2 [40]. HAT also resulted in the complete

disappearance of N-1 peak, assigned to the tetrahedral nitro-

gen bonded to the sp3-carbon [44], as well as in the decreased

contents of N-6 and N-Q and the increased content of N-5 (Ta-

ble 2). The decrease of contents of the N-6 and N-Q by their

transformation into N-containing aliphatic functional groups

(amide, amine, nitroso and lactam), similar to that observed

upon the oxidation of polyacrylonitrile-based carbonized fi-

bres in nitric acid [38], can be excluded since the binding ener-

gies of these aliphatic structures were not observed in the XPS

spectrum of C.PANI.HAT200. Pels et al. [40] interpreted the

appearance of N-5 at the expense of N-6 as a conversion of

the pyridinic-N to the pyridone N, which cannot be distin-

guished from pyrrolic nitrogen by XPS analysis [40], arguing

that it is rather hard to envisage a mechanism that converts

6-membered rings into 5-membered rings (N-pyrrole) under

low temperature condition. On the other hand, it is known

that pyridine-N-oxide species can be formed by the post-

heating oxidation of the pyridinic functional group during

storage in air [40,45,46]. Taking into account all these facts,

it could be concluded that the oxidation C.PANI surface upon

the low-temperature HAT causes conversion of the pyridinic

N to the pyridone and pyridine-N-oxide moieties. Besides

the possible formation of these oxidized N-containing species

at the C.PANI surfaces as a result of oxidation processes dur-

ing the HAT, partial homogenization of C.PANI upon HAT,

indicated by the combined elemental/XPS analysis results,

may also contribute to the observed increase of the pyridone

and pyridine-N-oxide concentrations at C.PANI surfaces. It

means that the pyridone and pyridine-N-oxide structural

segments present in the core of core/shell structured C.PANI

before the HAT could become exposed at C.PANI surface upon

the HAT by peeling-off-shell, deaglomeration and other

homogenization processes.

The origin of the observed decrease of N-Q content upon

the HAT is also open to discussion. Kelemen et al. [45] corre-

lated the decrease of quaternary nitrogen content with the

loss of oxygen during pyrolysis and hydro-pyrolysis of coal

at low temperature of about 400 �C. They suggested that qua-

ternary nitrogen (N-Q) in coals is actually pyridinic-N associ-

ated with nearby and adjacently located hydroxyl moiety

from phenol or carboxyl group, which results in +1 charge

on N atom (Fig. 1(D)). According to Kelemen et al. [45] this

association breaks during (hydro)pyrolisis of coals, so the N-

Q remains in the coal char as its original pyridinic nitrogen

form. In our case, the disappearance of quaternary nitrogen

caused by HAT is accompanied by the corresponding decrease

of XPS signal to which C–OH phenol group might be ascribed

(see below, Table 2, O–II species). Although definite assign-

ment of this signal to phenol group is a hard task [39–41], tak-

ing into account the evolution of independently analyzed N1s

and O1s XPS signals upon HAT, and previous work of Kelemen

et al. [45], it can be reasonably expected that N-Q observed in

C.PANI is pyridine-type nitrogen associated with phenol

group, protonated through the formation of H-bridge [40,45].

Its disappearance in C.PANI.HAT200 can be understood as

its alkaline-promoted conversion to N-6 moiety and further

transformation into the pyridone and pyridine-N-oxide

moieties.

In the region of O1s XPS response (Table 2), the peaks at

binding energies around 530.5 eV, 532 eV and 533 eV are

attributed to C@O quinone-type groups (O–I), C–OH phenol

groups (O–II) or ether C–O–C groups, and –COOH carboxyl

groups (O–III), respectively [39–41].The COOH contribution

to the O1s profile after HAT increases from 22.6% to 29.8%.

The peak with binding energy at 534.42 eV, with 7.87% rela-

tive contribution to oxygen species, appeared at the carbon

surface after HAT and corresponds to chemisorbed oxygen

and/or water [40]. A high fraction of chemisorbed oxygen/

water makes a carbon surface hydrophilic and improves its

dispersibility in water, which was observed during the elec-

trode preparation.

C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6 477

3.3. Micro/mesopore size distribution and specific surfacearea

The adsorption–desorption isotherms obtained by N2 physi-

sorption measurements at 77 K of both C.PANI and C.PANI.-

HAT200 samples have the same shapes, corresponding to a

microporous material with type Ib isotherm in low pressure

region, and to a macroporous material with type II isotherm,

in high pressure region (Fig. 2).

The isotherms were analyzed using several methods, in or-

der to provide reliable conclusions about the evolution of tex-

tural properties of C.PANI during HAT (Table 3). Regardless of

the specific approach used, one can conclude with significant

confidence that Vmic, Vmeso and Stot decreased upon the ap-

plied treatment for 42%, 8.5% and 41%, respectively. The dis-

appearance of micropores was much more pronounced than

the disappearance of mesopores (Table 3, Fig. 3), but neverthe-

less, C.PANI.HAT200 remained intrinsically microporous

material.

In addition to the progressive change of elemental compo-

sition of C.PANI, the increment in the temperature of HAT

from 150 �C [12] to 200 �C also resulted in a significant decre-

ment of Vmic, and consequently, Smic. This influenced also the

estimated Stot-BET value. All these findings indicate that C.PA-

NI.HAT200 is essentially new material, and, as it will be dem-

onstrated later, its electrochemical properties are significantly

changed in comparison to C.PANI. Hulicova et al. [2,21] used

the functionalization of carbons with nitrogen and oxygen-

containing groups, which caused also a significant decrease

of both the surface area and micropore volume. They claimed

that the decrease of porosity was due to both the destruction

of pore walls and the partial pore blocking by N- and O-con-

taining groups. The hydrothermal treatment of C.PANI that

we applied introduced the oxygen-containing surface groups

causing the more hydrophilic character of micropores. We

suggest that the more pronounced hydrophilicity of the trea-

ted sample might cause the capillary forces upon drying,

Fig. 2 – Nitrogen adsorption–desorption isotherms of C.PANI

and C.PANI.HAT200. Inset gives high relative pressures

region. The data for C.PANI are adapted from [12] with the

permission from Elsevier.

which could also be responsible for the reduction of micro-

pore volume.

3.4. On the mechanism of C.PANI modification by HAT

A hydrothermal treatment with ammonia or alkalies such as

KOH and NaOH at temperatures 150–250 �C, without addition

of oxidants (e.g., H2O2 and K2S2O8), has been, although rather

rarely, used to modify pore structure of the carbonaceous

materials. Skubiszewska-Zievska et al. [47] observed that a

hydrothermal treatment in 10% ammonia at 250 �C of both

high and low surface area activated carbons caused the dec-

rement of their surface areas SBET and Smic. However, the

authors did not examine the surface chemistry. Similarly,

the decrease of SBET and Smic accompanied with the increase

of the average pore diameter was observed by Akolekar

et al. [48] upon a hydrothermal treatment in 4 M NaOH at

200 �C, of both low-surface macroporous carbon samples, ob-

tained from mustard and almond oil, and commercial high-

surface microporous carbon samples. The surfaces of

MWCNTs were also functionalized by a large amount of phe-

nolic OH surface groups using an alkaline (2 M NaOH)-medi-

ated hydrothermal treatment under autogenous pressure at

180 �C [49], and it was observed that the morphology of origi-

nal MWCNTs was not affected by the treatment, whereas the

solubility of MWCNTs in water, methanol, butanone and tet-

rahydrofurane was significantly enhanced.

Based on the physico-chemical characterization of C.PANI

and C.PANI.HAT200, we suggest that the HAT produces the

following effects: (i) the chemical reactions of surface func-

tional groups with KOH at elevated temperature and pressure,

(ii) the peeling-off carbonaceous shell of C.PANI and (iii) the

collapse of micropores upon drying. In the case (i), the reac-

tion can be foreseen by XPS characterization, especially for

N surface functionalities (Fig. 4(A)). In comparison to the car-

bonaceous materials free of covalently bonded N atoms, for

C.PANI, both the carbon shell layers peeling-off and the col-

lapse of micropores, should be expected to be more pro-

nounced. As previously evidenced in Ref. [50], nitrogen

within carbon network reduced chemical stability of C.PANI

under oxidizing conditions, compared to nitrogen-free Vulcan

XC-72. Hence, one may expect that, due to the presence of

nitrogen, C.PANI might be more susceptible to structural

and chemical modifications relative to the nitrogen-free car-

bon materials. In C.PANI.HAT200, compared to C.PANI, higher

content of hydrophilic surface functional groups was detected

by XPS analysis. In our opinion, attractive forces between ad-

sorbed water and hydrophilic surface (Fig. 4(B)) introduced by

low-temperature hydrothermal treatment of C.PANI, contrib-

ute to the collapse of the microporous structure. Namely, the

surface tension of water exerts capillary forces strong enough

to break the pores during drying, while a weaker C–N bond,

compared to a C–C bond, may facilitate this process. The re-

sults on the functionalization of MWCNTs by similar treat-

ment, which resulted in no changes of tube length and

diameter and preserved end-cap structure [49], support such

a statement. Hence, one may reasonably expect that the de-

scribed HAT is more effective for nitrogen rich carbon, but,

at this point, it is still elusive how a similar HATwould reflect

on the properties of different nitrogen-rich carbons.

Table 3 – Textural properties of C.PANI and C.PANI.HAT200: Vmic (micropore volume), Smic (micropore surface area), Vmeso

(mesopore volume), Smeso (mesopore surface area) and Stot (total specific surface area). Particular methods for determinationof these properties are denoted as: DR – Dubinin-Radushkevich, as–as method, HK – Horwath–Kawazoe, DK – Dubinin–Kaganer, DH – Dollimore–Heal.

Characteristics C.PANI C.PANI.HAT200

Micropores Vmic-DR (cm3 g�1) 0.147 0.081Vmic-as (cm3 g�1) 0.133 [7] 0.081Vmic-HK (cm3 g�1) 0.125 0.076Smic-DK (m2 g�1) 414 228Smic-HK (m2 g�1) 329 180

Mesopores Vmeso-DH (cm3 g�1) 0.071 [7] 0.065Smeso-DH (m2 g�1) 47.7 [7] 41.3Smeso-as (m2 g�1) 20.0 17.6

Specific surface area Stot-BET (m2 g�1) 335 195Stot-as (m2 g�1) 316 190Smic-DK + Smeso-DH (m2 g�1) 462 270

Fig. 3 – Micropore (top) and mesopore (bottom) size

distribution for C.PANI and C.PANI.HAT200 obtained using

N2 physisorption data and Horwath–Kawazoe and

Dollimore–Heal methods, respectively. The data for C.PANI

are adapted from [7] with the permission from Elsevier.

Fig. 4 – Chemical transformations of N-surface

functionalities of C.PANI during HAT (A) and (B) schematic

depiction of micropore collapse due to increased

hydrophilicity of C.PANI.HAT upon the incorporation of

hydrophilic groups (HG).

478 C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6

3.5. The electrochemical performances: C.PANI vs.C.PANI.HAT200

3.5.1. The enhancement of C.PANI capacitance uponhydrothermal treatmentRecently, high gravimetric capacitances ranging 200–400 F g�1

for N-containing carbonized polyanilines were evidenced in

6 mol dm�3 KOH solution by both potentiodynamic and galva-

nostatic measuremens [7]. In the present work, capacitive

properties of C.PANI and C.PANI.HAT200 were comparatively

investigated in three different aqueous electrolytes,

6 mol dm�3 KOH, 1 mol dm�3 H2SO4 and 2 mol dm�3 KNO3,

under potentiodynamic conditions (i.e. by cyclic voltammetry

(CV) technique). For both C.PANI and C.PANI.HAT200 (i) CV

curves are very similar in shape, slightly dependent on pH,

and (ii) with somewhat larger slope registered at the negative

sweep direction (Fig. 5). The asymmetry of CV curves with re-

spect to potential axis was attributed elsewhere to the pseu-

do-faradaic processes on the surface involving surface

nitrogen and oxygen functional groups [10,51]. Although the

applied HAT did not affect the shape of the CV curve regard-

less of the nature of the electrolyte solution, this treatment

caused a pronounced increase in the capacitance of

Fig. 5 – Cyclic voltammograms of C.PANI (thin line) and C.PANI.HAT200 (thick line) in 6 mol dm�3 KOH (left), 2 mol dm�3 KNO3

(middle) and 1 mol dm�3 H2SO4 (right) at the common scan rate of 20 mV s�1.

C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6 479

C.PANI.HAT200 compared to the one of C.PANI (Figs. 5 and 6).

With the variation of the electrolyte solution, the capacitance

increased in the following order: 1 mol dm�3 H2SO4 -

� 6 mol dm�3 KOH > 2 mol dm�3 KNO3.

Fig. 7 shows the cyclic voltammograms of C.PANI.HAT200

in 6 mol dm�3 KOH, 2 mol dm�3 KNO3 and 1 mol dm�3

H2SO4, recorded at different scan rates. The gravimetric

capacitances calculated from these CV curves are presented

in Table 4.

At the scan rate of 5 mV s�1, C.PANI.HAT200 displayed a

very high capacitance amounting to 433 F g�1 in H2SO4 solu-

tion, and 363 F g�1 in KOH solution. Corresponding value

found in KNO3 solution was markedly smaller, and amounted

to 220 F g�1. Significantly higher capacitance in H2SO4

Fig. 6 – Measured gravimetric capacitances of C.PANI and C.PAN

and 1 mol dm�3 H2SO4 and the corresponding factors of capacit

solution compared to the ones observed in other solutions

(KOH and KNO3 ones), at the scan rates in the range 5–

20 mV s�1 can be attributed to stronger interaction between

H3O+ ions and surface heteroatoms compared to analogous

interactions of K+ ion.

The increase in the sweep rate caused generally the de-

crease of capacitance, which is a common behavior of real

supercapacitors. It is a widely accepted explanation of this

behavior that the ohmic resistance of micropores limits the

mass transfer to the surface commensurably to the fre-

quency. This effect is operative in both ion-adsorptive and

pseudo-capacitive control of double layer capacitance. At

high sweep rates (50 mV s�1 and 100 mV s�1) specific

deviation of the CV shape, accompanied by a considerable

I.HAT200 at 5 mV s�1 in 6 mol dm�3 KOH, 2 mol dm�3 KNO3

ance enhancement.

Table 4 – Gravimetric capacitance (in F g�1) of C.PANI.HAT200 and C.PANI calculated from the cyclic voltammetrymeasurements in aqueous electrolytes at different potential scan rates.

Scan rate (mV s�1) Gravimetric capacitance (F g�1)

6 mol dm�3 KOH 2 mol dm�3 KNO3 1 mol dm�3 H2SO4

C.PANI.HAT2005 363 220 433

10 338 210 38920 300 183 32550 270 150 237

100 203 116 170

C.PANI5 217 151 240

10 203 138 20420 193 115 16050 157 84 114

100 88 64 89

Fig. 7 – The cyclic voltammograms of C.PANI.HAT200 at different scan rates in 6 mol dm�3 KOH (left), 2 mol dm�3 KNO3

(middle) and 1 mol dm�3 H2SO4 (right) aqueous electrolytes, normalized by scan rate.

480 C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6

capacitance loss, was observed in H2SO4 solution. It may be

attributed to the reduction of ability of SO42� ions, relative

to the ability of other ions, to penetrate into micropores at

high sweep rates, taking into account that the size of actual

hydrated ions follows the order OH� < K+ � H3O+ < SO42� [52].

According to a number of literature reports, specific capac-

itance of majority of carbonaceous materials in aqueous elec-

trolytic solutions falls within the range 50–350 F g�1 [53]. For

PANI-derived carbon materials these values are typically be-

tween 150 and 200 F g�1 [54–56]. Nevertheless, in some cases,

typically for the activated PANI-derived carbons, significantly

higher capacitances were reported. For submicron-sized rod-

shaped carbonized PANI activated in KOH solution (time of

activation 1.5 h, 850 �C), Yan et al. [57] reported specific capac-

itance of 455 F g�1 in 6 mol dm�3 KOH at a very low sweep rate

of 1 mV s�1. For carbonized PANI activated with K2CO3 specific

capacitance of 210 F g�1 was measured in 6 mol dm�3 KOH at

sweep rate 2 mV s�1 [58]. In addition, Yuan et al. [59] reported

the specific capacitance of 327 F g�1 for carbonized PANI

nanowires. For a complete overview in this field the reader

is referred to the review by Ciric-Marjanovic et al. [60]. Hence,

it can be concluded that C.PANI.HAT200 by its performance in

acidic and alkaline solutions falls within the group of most

promising carbonaceous materials for electrochemical

capacitors.

3.5.2. The effects of hydrothermal treatment of C.PANI on theelectrocatalytic performance toward ORRSimilarly to the capacitive properties, the electrocatalytic

activity of C.PANI was also found to be affected by HAT. The

CV curves of C.PANI and C.PANI.HAT200 (Fig. 8, top left), re-

corded in nitrogen purged 0.1 mol dm�3 KOH, displayed simi-

lar ‘‘pear’’ shape, however, a cathodic shoulder, as indicated

by horizontal arrow, is shifted toward higher potentials for

C.PANI.HAT200. This shoulder-making hump can be ascribed

to pseudo-capacitive interactions of different surface func-

tionalities incorporated into the material surface, further af-

fected by pore structure, resulting in various accessibility of

surface reactive sites under electrochemical conditions [11].

Considering now the voltammetric curves in O2 saturated

solution (Fig. 8, bottom left; for the entire set of ORR polariza-

tion curves recorded at different electrode rotation rates the

reader is referred to Supplementary Information, Fig. S1),

one may note that the onset potential for ORR was also

shifted toward higher potentials as a consequence of HAT.

The onset potential for C.PANI.HAT200 was close to �20 mV

Fig. 8 – Blank cyclic voltammograms of C.PANI (thin line) and C.PANI.HAT200 (thick line) in 0.1 mol dm�3 KOH (left, top) along

with corresponding background-corrected ORR polarization curves (left, bottom; sweep rate 20 mV s�1, common electrode

rotation rate 300 rpm, catalysts loading 250 lg cm�2). Koutecky-Levich plots evaluated at �0.6 V vs. SCE for C.PANI (h) and

C.PANI.HAT200 (s) are enclosed (right, top; straight lines indicate theoretical Koutecky–Levich plots for 2e- and 4e-pathways

for ORR in 0.1 M KOH) alongside with the apparent number of electrons (n) found in wide potential window (right, bottom).

ORR-RDE curves, Koutecky–Levich plot and apparent number of electrons for C.PANI are adapted from reference [12] with the

permission from Elsevier.

C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6 481

vs. SCE (around 0.95 V vs. RHE), which matches the ORR onset

potential of Pt-based catalysts in the same solution [61]. The

RDE curves were further processed by the Koutecky–Levich

(K–L) analysis [62] (Fig. 8, top right), which allowed to deter-

mine the apparent number of electrons consumed per O2

molecule (n).The number of electrons is determined from

the slope of K–L lines defined by:

1j¼ 1

jkþ 1

jd¼ 1

jk� 1

0:62 � n � F � D2=3O2� m�1=6 � x1=2 � cO2

ð1Þ

In Eq. (1) j, jk and jd are the measured current density, kinetic

current density and the limiting diffusion current density.

Furthermore, m presents the kinematic viscosity of the solu-

tion (0.01 cm2 s�1 [63]), DO2 is the diffusion coefficient of O2

(1.9 · 10�5 cm2 s�1 [64]) and cO2 is the concentration of dis-

solved O2 (1.2 · 10�6 mol cm�3 [64]). This analysis revealed

that C.PANI.HAT200 displayed superior characteristics com-

pared to C.PANI (Fig. 8, bottom right, complete set of K-L plots

evaluated at different electrode potentials in the region of

preferably diffusion control of ORR is provided in Supplemen-

tary Information, Fig. S2). In the investigated potential win-

dow (�0.4 to �0.8 V vs. SCE), in comparison to starting

material whose n value was between 2.4 and 2.8, C.PANI.-

HAT200 displayed n between 3.4 and 3.9, again, approaching

n value for Pt-based catalysts. Namely, it is known that

Pt-based catalysts display n close to 4 in alkaline solutions

in the whole region of diffusion control of ORR [65].

Using K–L analysis we extracted the kinetic currents den-

sities (jk) for C.PANI and C.PANI.HAT200, which were used

for more proper comparison of catalytic performances of

these two materials. In Fig. 9, left, kinetic current densities

measured at �0.2 and �0.4 V vs. SCE were presented as a

bar diagrams, confirming that ORR activity of C.PANI.HAT200

is approximately one order of magnitude higher in compari-

son to the one of C.PANI. Actually, C.PANI.HAT200, by its

ORR performance, surpasses majority of carbon-based mate-

rials reported so far in the literature [11,66–68]. It also super-

sedes the C.PANI HAT treated at 150 �C [12]. Namely, the

onset potential of ORR is more positive for C.PANI.HAT200,

while also attaining higher values of n. For C.PANI treated at

150 �C, n was found to be between 2.8 and 3.5 [12]

A comparison of catalytic activity of C.PANI.HAT200 to the

one of platinum requires attention with respect to the strong

influence of catalyst loading if one deals with nanodispersed

carbon supported platinum catalysts. Therefore, to simplify

the comparison, we have chosen high surface area Pt-poly

disk (roughness factor � 20) as a reference material. As previ-

ously stated, the onset potential of ORR on C.PANI.HAT200

and on Pt-poly is quite similar (Fig. 9, right), while kinetic

current densities for Pt-poly (evaluated with respect to

Fig. 9 – Comparison of kinetic current densities (evaluated using geometrical cross section area of supporting GC disk) for

C.PANI and C.PANI.HAT200 at �0.2 and �0.4 V vs. SCE (left) and Tafel plots for ORR on C.PANI.HAT200-modified GC disk (s)

and high surface area Pt disk (h) (right).

482 C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6

geometrical cross section area of RDE) are only slightly higher

than the ones of C.PANI.HAT200. The values of Tafel slope for

ORR we found in this case are within the range of the ones of

Pt-based catalysts, i.e. 60 to �120 mV, while the mechanism of

ORR on C.PANI and C.PANI.HAT200 might be so called ‘‘pseu-

do’’ 4-electron pathway, as previously discussed in detail [11].

3.5.3. The influence of porosity and surface chemicalcomposition on the enhancement of both capacitance and ORRelectrocatalytic activityAs described above for the studied carbon samples, the de-

crease of specific surface area (i) and the rearrangement of

surface N- and O-functionalities (ii), were accompanied by

(i) an increase of the gravimetric capacitances for a factor

approaching two, and (ii) an increase of the catalytic activity

toward ORR for a factor ten. We suggest to note that good

capacitive behavior of N-containing carbon material accom-

pany its high electrocatalytic activity toward ORR [7,11]. Sim-

ilar conclusion one may derive considering the other studies

[69,70], although such an aspect was not considered there.

As reported by Pandolfo et al. [71], since the edge sites of

graphite are often associated with unpaired electrons, their

contribution to the double layer capacitance is very pro-

nounced, namely, ten times that of the sites in basal layer.

Therefore, the fraction of edge orientation is expected to be

commensurate to the capacitance of carbon material. Moren-

o-Castilla et al. [10] demonstrated that the double layer capac-

itance of N-doped carbon xerogels decreased with the rise in

fraction of micropore surface area. Considering the double-

layer capacitance, they suggested that the decrease of Smic in-

creases the ratio of edge-to-basal sites because the basal

planes are dominant in the formation of micropore walls.

Hulicova-Jurcakova et al. [2] showed that the microporous

coconut-shell-based activated carbon obtained lower specific

surface area, but higher capacitance, upon oxidation treat-

ment in HNO3, and the explanation was found in the increase

of surface oxygen content and its positive effect to the

pseudocapacitance.

The presented literature survey suggests that surface

chemical composition plays a dominant role in the electro-

chemical behavior of here studied carbon samples. Many

other literature reports support this statement

[10,21,39,66,72–74]. It is known that the nitrogen content in

the pyridone-N moiety, as the form of pyrollic-N, improves

the electron mobility providing two p-electrons to the p-elec-

tron system [40]. Moreno-Castilla et al. [10] correlated the

double-layer capacitance of N-doped carbon xerogels with

the areal concentration of N-5, N-6 and N-Q functionalities

and showed the best correlation with the N-5 functionalities,

assigned as pyrollic or pyridonic nitrogen. In addition, posi-

tively charged N-X functional groups also have enhancing ef-

fects on the capacitance due to improved electron transfer

through the carbon matrix [2,21]. Furthermore, the preferen-

tial exposure of the edge plane sites as well as the enhance-

ment of N-6 content have been commonly outlined as a

reason of improved activity of N-containing materials toward

ORR [66,72,73]. Considering the effects of surface oxygen

groups on ORR activity, Subramanian et al. [74] demonstrated

the increase of capacitive current and ORR activity of oxidized

carbon but ascribed improved ORR activity to the enlarged N-

6 content. Biddinger et al. [75] showed that the activity of

nitrogen-containing carbon nanofibers toward ORR and the

selectivity to water formation experienced slight and high

improvement, respectively, upon an oxidative treatment in

HNO3.

In this study, XPS analysis revealed that HAT of C.PANI

caused the rearrangement of the surface composition, while

N2 physisorption measurements evidenced the reduction of

micro- and mesopore volumes of C.PANI, as well as of total

specific surface area, Stot. Hence, it seems that the coupled ef-

fects of (i) redistribution of the surface nitrogen-containing

groups, (ii) the increase of the surface nitrogen content, and

C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6 483

especially (iii) the increase of the surface oxygen content

caused by HAT, should be considered as beneficial in charge

storage ability, providing an easier access of the electrolyte

species responsible to both the double layer formation and

the pseudo-faradaic reactions. The increase of the capaci-

tance of C.PANI.HAT200 through the redistribution of N-con-

taining groups can be explained by the conversion of N-6

and N-Q into N-5 and N-X functional groups [10]. In addition,

the role of oxygen-containing surface functional groups is to

be acknowledged [5,25], although particular redox processes

associated with pseudocapacitive contribution of oxygen sur-

face functionalities are not clearly discernible. The dominant

role of surface functionalities is also supported by the less

pronounced capacitance fade at lower sweep rates, which

indicates that surface functionalities determine the capaci-

tance. In the case of C.PANI.HAT200 chemisorbed oxygen

and/or water is present within the microporous region, as

confirmed by the appearance of O1s peak at 534.4 eV. Such

oxygen-rich surface (�9.7%) enhances wettability of the sur-

face [24–26], which facilitates the access of ions into microp-

ores. This enhances the surface utilization of micropores

interior, i.e., the capacitance of C.PANI.HAT200. Although

the introduction of new surface functionalities and the rear-

rangement of surface functional groups of C.PANI by HAT dis-

played positive effect to the measured gravimetric

capacitances, on the basis of available literature [5,71,76] it

is reasonably to expect that the presence of surface functional

groups, especially oxygen-containing groups, can also cause

the capacitance fading upon prolonged cycling. This may be

expectedfor C.PANI.HAT too, however, having in mind high

values of gravimetric capacitance achieved, low price of such

obtained carbonaceous material and simplicity of the HAT,

there is enough space to search for an optimum balance be-

tween the gravimetric capacitance and cycling stability for a

specific purpose.It is much more challenging to explain

extraordinary ORR activity of C.PANI.HAT200, having in mind

that pyridinic-N content, usually considered as responsible

for high ORR activity, actually decreased in this study. Inter-

estingly, Luo et al. [77] reported the synthesis of pyridinic N-

doped graphene, demonstrating 2-electron pathway in ORR.

This result led the authors to stress some doubt regarding

the supporting role of pyridinic N-functionalities in catalytic

activity toward ORR. Previously [78], catalytic activity was

attributed to N-5 and N-X, e.g., atoms in pyrrolic N-type func-

tionalities. However, this disagrees with higher ORR activity of

C.PANI.HAT200 in comparison to C.PANI. Moreover, our recent

study [11] demonstrated that among three different N-con-

taining PANI-derived carbons the highest ORR activity was

achieved using the one with no N-5 functionalities (contain-

ing the highest amount of pyridinic nitrogen). To avoid these

apparent discrepancies, one should assume that certain type

of synergy exists between N and O surface functionalities.

Assuming a case of ‘‘pseudo’’ 4-electron pathway for this type

of carbon materials [11], we suggest that N-dopant may pro-

vide suitable active sites enabling the charge transfer, while

oxygen surface functionalities can augment catalytic regener-

ative cycle in which formed HO2� is disproportionated to OH�

and O2. Such kind of action attributed is so far to the metallic

impurities [79], however, both C.PANI and C.PANI.HAT200 do

not contain any metallic component i.e., they can be consid-

ered as a completely metal-free catalysts.

In addition, oxygen surface functionalities enable wetta-

bility of the surface, as already documented [24–26]. It is

important to stress out that the prerequisite for this kind of

synergic action might be the surface density (SD) of nitrogen

and oxygen groups. Brief comparison points that SDs of N-

and O-groups are 1.8 and 2.6 times higher in C.PANI.HAT200,

respectively, than in C.PANI. High SDs of N- and O- surface

groups could also modify the surface electronic structure in

a way that C atoms in the bond network get activated towards

ORR. Furthermore, oxygen sites itself might serve as active

sites for the charge transfer, as suggested by Subramanian

et al. [74]. These assumptions explain not only increased

ORR activity of C.PANI.HAT200, but also increased selectivity

of HNO3-treated CNx catalysts as previously documented by

Biddinger et al. [75] and low ORR performance of pyridine

N-doped graphene [77] which missed O-surface functional-

ities. The nature of O-surface moieties responsible for such

action is elusive at this point.

4. Conclusion

Carbonized nanostructured polyaniline was subjected to low

temperature hydrothermal alkali treatment. While conductiv-

ity of the obtained material, C.PANI.HAT200, was not mark-

edly changed after the treatment, its elemental composition

and textural properties were significantly modified. Introduc-

tion of new surface functional groups, evidenced by XPS,

boosted both capacitive and electrocatalytic properties of

C.PANI and provided an excellent bifunctional material. The

HAT resulted in reduction of pores system and the enrich-

ment of the surface by nitrogen and, especially, oxygen func-

tionalities. Measured gravimetric capacitances of

C.PANI.HAT200 were up to 2 times higher than corresponding

ones of starting C.PANI material, while the capacitance reten-

tion of C.PANI.HAT200, upon increasing sweep rate, was im-

proved. The highest capacitance was measured in

1 mol dm�3 H2SO4, amounting to 433 F g�1, at 5 mV s�1. The

activity of new material toward ORR, investigated in

0.1 mol dm�3 KOH solution, was one order of magnitude high-

er compared to C.PANI, while the onset potential matched the

one of Pt-based catalysts (��20 mV vs. SCE). In addition, ORR

selectivity was markedly improved upon the HAT, as evi-

denced through the increased number of electrons consumed

per O2 molecule. While previous literature reports served as a

solid basis for the explanation of improved capacitive behav-

ior of C.PANI.HAT200, enhanced ORR activity, in conjunction

with materials’ surface chemistry and textural properties,

pointed to a new view of the role of the surface functionalities

when it comes to ORR activity. It was proposed that synergy

exists between N- and O-surface functionalities which con-

tributes to high ORR activity and improved selectivity to water

formation. While N-surface functionalities provide active

sites enabling the charge transfer in ORR, O-surface groups

can contribute catalytic regenerative cycle during which

HO2� decomposes to OH� and O2, which enter another charge

transfer process, improving selectivity for O2 reduction to

water.

484 C A R B O N 6 4 ( 2 0 1 3 ) 4 7 2 – 4 8 6

The here demonstrated low temperature hydrothermal

treatment might be suggested as a general approach for

post-synthetic modification of N-doped nanocarbons for the

production of new advanced multifunctional materials with

exceptional performances.

Acknowledgments

This work was supported by the Serbian Ministry of Education

and Science (Contracts III45014, IO172043 and III45001). S.V.M.

acknowledges the support provided by the Serbian Academy

of Science and Arts through the project ‘‘Electrocatalysis in

the contemporary processes of energy conversion’’.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/j.carbon.

2013.07.100.

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