Superior capacitive and electrocatalytic properties of carbonized nanostructured polyaniline upon a low-temperature hydrothermal treatment

CARBON

6 4 ( 2 0 1 3 ) 4 7 2 –4 8 6

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Superior capacitive and electrocatalytic properties of carbonized nanostructured polyaniline upon a low-temperature hydrothermal treatment Milica Vujkovic´ a, Nemanja Gavrilov a, Igor Pasˇti a, Jugoslav Krstic´ b, Jadranka Travas-Sejdic c,d, Gordana C´iric´-Marjanovic´ a, Slavko Mentus

a,e,*

a

University of Belgrade, Faculty of Physical Chemistry, Studentski trg 12–16, 11158 Belgrade, Serbia University of Belgrade, Institute of Chemistry, Technology and Metallurgy, Department of Catalysis and Chemical Engineering, Njegosˇeva 12, 11000 Belgrade, Serbia c Polymer Electronics Research Centre, School of Chemical Sciences, University of Auckland, 23 Symonds Street, Auckland, New Zealand d MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New 13 Zealand e Serbian Academy of Science and Arts, Knez Mihajlova 35, 11000 Belgrade, Serbia b

A R T I C L E I N F O

A B S T R A C T

Article history:

Carbonized nanostructured polyaniline (C.PANI) was hydrothermally treated in 1 mol dm3

Received 20 April 2013

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

Accepted 30 July 2013

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

Available online 6 August 2013

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, delivered twice as high gravimetric capacitances, amounting to 363, 220 and 432 F g1, in 6 mol dm3 KOH, 2 mol dm3 KNO3 and 1 mol dm3 H2SO4, respectively, when measured potentiodynamically at a scan rate of 5 mV s1. Moreover, its exceptionally high electrocatalytic 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 fourelectron pathway. The onset potential for ORR matched the one of platinum-based electrocatalyst. 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 modification 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 activity 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

* Corresponding author: Fax: +381 11 2187 133. E-mail address: [email protected] (S. Mentus). 0008-6223/$ - see front matter  2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.07.100

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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 diameter 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 Nand O- containing groups on carbon surface also influences the capacitor performance [2,10,21–23]. The oxygen functional groups usually present on the carbon surface can contribute 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 concentration. 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 heteroatoms (N and O) fixed at the surface, particularly towards oxygen reduction reaction (ORR) [11–14,27–29]. High activity of Ncontaining 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 tuning 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 electrocatalytic activities of heteroatom-doped nanocarbons are closely correlated [11]. Even if the key factors of high capacitance and electrocatalytic activities were known, the synthesis of materials with the desired properties may present a problem. The carbonization 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 precursor (especially the type of the dopant, i.e., 5-sulfosalicylate, 3,5-dinitrosalicylate, hydrogen sulfate ions) used for the preparation of N-CNS strongly affected properties of produced C.PANIs and consequently their capacitive [7] and electrocatalytic [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 hydrothermal treatment in 1 M KOH solution at 150 C [12]. Such a modification caused a negligible decrease in the specific surface 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

473

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 particularly paid to the question how the modification of N- and O-containing surface functional groups caused by HAT, combined 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 procedure [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 reaction mixture was stirred for 2 h, after which the precipitated nanonostructured PANI was collected on a filter, rinsed with 5 · 103 mol dm3 H2SO4 and dried in vacuum. C.PANI was obtained 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 min1. 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 min1 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, consecutively, 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 temperature by means of Waynne Kerr Universal Bridge B 224, operating 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 (Kratos 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 · 109 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

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using Kratos’ relative sensitivity factors. Core level scans were calibrated based on a peak fit to the C 1s scan, with the component due to aromatic carbon set to 284.7 eV. Shirley backgrounds 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 adsorption at 77 K using a Sorptomatic 1990 Thermo Finnigan device. 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-method [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 modification of Dubinin’s method (DK method) [34] and HK method. The later approach was also used to determine the poresize distribution in the microporous region [36]. Mesopore volume (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 saturation [7], we paid attention to use an optimal loading providing the accurate measurement. Thus, the anticipated loading of investigated N-containing carbon material was 250 lg cm2. Then the GC supported film electrode was connected as a working electrode in a conventional one-compartment 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 dm3 KOH, 2 mol dm3

KNO3 and 1 mol dm3 H2SO4 were used for the capacitance measurement by means of cyclic voltammetry. The kinetics of ORR was investigated in 0.1 mol dm3 KOH, using a rotating disk electrode (RDE) voltammetry in the potential window between +0.27 to –0.97 V vs. SCE at 20 mVs1. 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 electrolyte 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 content 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 elemental 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 content (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 electrochemical properties of carbon materials, could be explained by the combined effects of partial homogenization and surface 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

Table 1 – The elemental composition of C.PANI and C.PANI.HAT200 determined by XPS and elemental microanalysis. Element

C N O H N/C O/C

C.PANI

C.PANI.HAT200

XPS at.%

Elemental analysis wt.%

XPS at.%

Elemental analysis wt.%

87.9 [7] 5.80 [7] 6.30 [7] – 0.066 0.072

74.8 [7] 8.9 [7] 14.2 [7] 2.1

84.0 6.25 9.75 – 0.074 0.116

77.1 9.1 11.9 1.9

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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 significantly decreased difference between surface and total contents 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 cm1 [7] to 0.38 S cm1, was observed upon HAT, diminishing the role of conductivity in further explanation of electrochemical performance of investigated carbons in Section 3.4.

475

3.2. XPS characterization of C.PANI surface modification by 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/pyridone (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

Fig. 1 – (A) and (B): Fitted high-resolution XPS N1s (A) and O1s (B) spectra of C-PANI.HAT200 (top) and C-PANI (bottom). The data for C.PANI are adapted from [7] with the permission of Elsevier. (C): Phenazine-di-N-oxide-like structures appear in XPS spectra at binding energies around 403.6 eV. (D): Form of quaternary nitrogen (N-Q) according to Kelemen et al. [45]: pyridinicN associated with nearby and adjacently located hydroxyl moiety from phenol or carboxyl group.

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Table 2 – XPS peak position and relative content of nitrogen and oxygen species in C.PANI and C.PANI.HAT200 samples. The data 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 N-6 N-5 N-Q N+-O N-X N-X 0

396.5 398.3 399.8 400.8 402.5

4.40 35.0 13.0 40.9 6.70

– 398.2 399.6 400.7 402.0 403.6 406.0

– 29.38 15.04 37.15 8.29 6.50 3.64

Oxygen species

Peak position (eV)

% O atoms

Peak position (eV)

% O atoms

O–I O–II O–III O2,ads, H2Oads

530.7 532.3 533.6 –

35.3 42.1 22.6 –

530.4 531.9 533.1 534.4

24.14 38.20 29.80 7.87

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 neglected. 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 nitrogen 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 (Table 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 fibres in nitric acid [38], can be excluded since the binding energies 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 distinguished 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 postheating 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 during 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] correlated 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 quaternary nitrogen (N-Q) in coals is actually pyridinic-N associated 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 NQ 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 assignment of this signal to phenol group is a hard task [39–41], taking 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% relative 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 electrode preparation.

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3.3. area

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Micro/mesopore size distribution and specific surface

The adsorption–desorption isotherms obtained by N2 physisorption 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 order to provide reliable conclusions about the evolution of textural 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 applied treatment for 42%, 8.5% and 41%, respectively. The disappearance of micropores was much more pronounced than the disappearance of mesopores (Table 3, Fig. 3), but nevertheless, C.PANI.HAT200 remained intrinsically microporous material. In addition to the progressive change of elemental composition of C.PANI, the increment in the temperature of HAT from 150 C [12] to 200 C also resulted in a significant decrement of Vmic, and consequently, Smic. This influenced also the estimated Stot-BET value. All these findings indicate that C.PANI.HAT200 is essentially new material, and, as it will be demonstrated 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 oxygencontaining 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-containing 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 treated 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.

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which could also be responsible for the reduction of micropore 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-Zie˛vska 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 decrement 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, obtained from mustard and almond oil, and commercial highsurface microporous carbon samples. The surfaces of MWCNTs were also functionalized by a large amount of phenolic OH surface groups using an alkaline (2 M NaOH)-mediated hydrothermal treatment under autogenous pressure at 180 C [49], and it was observed that the morphology of original MWCNTs was not affected by the treatment, whereas the solubility of MWCNTs in water, methanol, butanone and tetrahydrofurane 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 functional 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 reaction can be foreseen by XPS characterization, especially for N surface functionalities (Fig. 4(A)). In comparison to the carbonaceous materials free of covalently bonded N atoms, for C.PANI, both the carbon shell layers peeling-off and the collapse of micropores, should be expected to be more pronounced. 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 carbon 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 adsorbed water and hydrophilic surface (Fig. 4(B)) introduced by low-temperature hydrothermal treatment of C.PANI, contribute 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 results on the functionalization of MWCNTs by similar treatment, 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 described HAT is more effective for nitrogen rich carbon, but, at this point, it is still elusive how a similar HAT would reflect on the properties of different nitrogen-rich carbons.

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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 determination of these properties are denoted as: DR – Dubinin-Radushkevich, as–as method, HK – Horwath–Kawazoe, DK – Dubinin– Kaganer, DH – Dollimore–Heal. Characteristics 3

1

C.PANI

C.PANI.HAT200

Micropores

Vmic-DR (cm g ) Vmic-as (cm3 g1) Vmic-HK (cm3 g1) Smic-DK (m2 g1) Smic-HK (m2 g1)

0.147 0.133 [7] 0.125 414 329

0.081 0.081 0.076 228 180

Mesopores

Vmeso-DH (cm3 g1) Smeso-DH (m2 g1) Smeso-as (m2 g1)

0.071 [7] 47.7 [7] 20.0

0.065 41.3 17.6

Specific surface area

Stot-BET (m2 g1) Stot-as (m2 g1) Smic-DK + Smeso-DH (m2 g1)

335 316 462

195 190 270

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.

3.5. The electrochemical C.PANI.HAT200 3.5.1. The enhancement hydrothermal treatment

of

performances:

C.PANI

C.PANI

capacitance

vs.

upon

Recently, high gravimetric capacitances ranging 200–400 F g1 for N-containing carbonized polyanilines were evidenced in 6 mol dm3 KOH solution by both potentiodynamic and galvanostatic measuremens [7]. In the present work, capacitive properties of C.PANI and C.PANI.HAT200 were comparatively

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).

investigated in three different aqueous electrolytes, 6 mol dm3 KOH, 1 mol dm3 H2SO4 and 2 mol dm3 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 respect to potential axis was attributed elsewhere to the pseudo-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 regardless of the nature of the electrolyte solution, this treatment caused a pronounced increase in the capacitance of

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479

Fig. 5 – Cyclic voltammograms of C.PANI (thin line) and C.PANI.HAT200 (thick line) in 6 mol dm3 KOH (left), 2 mol dm3 KNO3 (middle) and 1 mol dm3 H2SO4 (right) at the common scan rate of 20 mV s1.

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 dm3 H2SO4  6 mol dm3 KOH > 2 mol dm3 KNO3. Fig. 7 shows the cyclic voltammograms of C.PANI.HAT200 in 6 mol dm3 KOH, 2 mol dm3 KNO3 and 1 mol dm3 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 s1, C.PANI.HAT200 displayed a very high capacitance amounting to 433 F g1 in H2SO4 solution, and 363 F g1 in KOH solution. Corresponding value found in KNO3 solution was markedly smaller, and amounted to 220 F g1. Significantly higher capacitance in H2SO4

solution compared to the ones observed in other solutions (KOH and KNO3 ones), at the scan rates in the range 5– 20 mV s1 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 decrease 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 frequency. This effect is operative in both ion-adsorptive and pseudo-capacitive control of double layer capacitance. At high sweep rates (50 mV s1 and 100 mV s1) specific deviation of the CV shape, accompanied by a considerable

Fig. 6 – Measured gravimetric capacitances of C.PANI and C.PANI.HAT200 at 5 mV s1 in 6 mol dm3 KOH, 2 mol dm3 KNO3 and 1 mol dm3 H2SO4 and the corresponding factors of capacitance enhancement.

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Fig. 7 – The cyclic voltammograms of C.PANI.HAT200 at different scan rates in 6 mol dm3 KOH (left), 2 mol dm3 KNO3 (middle) and 1 mol dm3 H2SO4 (right) aqueous electrolytes, normalized by scan rate.

Table 4 – Gravimetric capacitance (in F g1) of C.PANI.HAT200 and C.PANI calculated from the cyclic voltammetry measurements in aqueous electrolytes at different potential scan rates. Scan rate (mV s1)

Gravimetric capacitance (F g1) 6 mol dm3 KOH

2 mol dm3 KNO3

1 mol dm3 H2SO4

C.PANI.HAT200 5 10 20 50 100

363 338 300 270 203

220 210 183 150 116

433 389 325 237 170

C.PANI 5 10 20 50 100

217 203 193 157 88

151 138 115 84 64

240 204 160 114 89

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 capacitance of majority of carbonaceous materials in aqueous electrolytic solutions falls within the range 50–350 F g1 [53]. For PANI-derived carbon materials these values are typically between 150 and 200 F g1 [54–56]. Nevertheless, in some cases, typically for the activated PANI-derived carbons, significantly higher capacitances were reported. For submicron-sized rodshaped carbonized PANI activated in KOH solution (time of activation 1.5 h, 850 C), Yan et al. [57] reported specific capacitance of 455 F g1 in 6 mol dm3 KOH at a very low sweep rate of 1 mV s1. For carbonized PANI activated with K2CO3 specific capacitance of 210 F g1 was measured in 6 mol dm3 KOH at sweep rate 2 mV s1 [58]. In addition, Yuan et al. [59] reported the specific capacitance of 327 F g1 for carbonized PANI nanowires. For a complete overview in this field the reader is referred to the review by C´iric´-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 capacitors.

materials

for

electrochemical

3.5.2. The effects of hydrothermal treatment of C.PANI on the electrocatalytic performance toward ORR Similarly 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), recorded in nitrogen purged 0.1 mol dm3 KOH, displayed similar ‘‘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 functionalities incorporated into the material surface, further affected 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 polarization 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

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6 4 (2 0 1 3) 4 7 2–48 6

481

Fig. 8 – Blank cyclic voltammograms of C.PANI (thin line) and C.PANI.HAT200 (thick line) in 0.1 mol dm3 KOH (left, top) along with corresponding background-corrected ORR polarization curves (left, bottom; sweep rate 20 mV s1, common electrode rotation rate 300 rpm, catalysts loading 250 lg cm2). 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.

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 determine 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: 1 1 1 1 1 ¼ þ ¼  j jk jd jk 0:62  n  F  DO2=3  m1=6  x1=2  cO2

ð1Þ

2

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 solution (0.01 cm2 s1 [63]), DO2 is the diffusion coefficient of O2 (1.9 · 105 cm2 s1 [64]) and cO2 is the concentration of dissolved O2 (1.2 · 106 mol cm3 [64]). This analysis revealed that C.PANI.HAT200 displayed superior characteristics compared 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 Supplementary Information, Fig. S2). In the investigated potential window (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 densities (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 comparison to the one of C.PANI. Actually, C.PANI.HAT200, by its ORR performance, surpasses majority of carbon-based materials reported so far in the literature [11,66–68]. It also supersedes 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 previously 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

482

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6 4 ( 2 0 1 3 ) 4 7 2 –4 8 6

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).

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

3.5.3. The influence of porosity and surface chemical composition on the enhancement of both capacitance and ORR electrocatalytic activity As described above for the studied carbon samples, the decrease 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 accompany its high electrocatalytic activity toward ORR [7,11]. Similar 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 pronounced, 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. Moreno-Castilla et al. [10] demonstrated that the double layer capacitance of N-doped carbon xerogels decreased with the rise in fraction of micropore surface area. Considering the doublelayer capacitance, they suggested that the decrease of Smic increases 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 treatment 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 electrochemical 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-electron 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, positively charged N-X functional groups also have enhancing effects on the capacitance due to improved electron transfer through the carbon matrix [2,21]. Furthermore, the preferential exposure of the edge plane sites as well as the enhancement 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 N6 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 effects of (i) redistribution of the surface nitrogen-containing groups, (ii) the increase of the surface nitrogen content, and

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6 4 (2 0 1 3) 4 7 2–48 6

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 capacitance of C.PANI.HAT200 through the redistribution of N-containing 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 surface 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 capacitance. 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 surface [24–26], which facilitates the access of ions into micropores. 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 rearrangement of surface functional groups of C.PANI by HAT displayed 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 between 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. Interestingly, Luo et al. [77] reported the synthesis of pyridinic Ndoped 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 functionalities. 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-containing PANI-derived carbons the highest ORR activity was achieved using the one with no N-5 functionalities (containing 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 provide suitable active sites enabling the charge transfer, while oxygen surface functionalities can augment catalytic regenerative 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

483

not contain any metallic component i.e., they can be considered as a completely metal-free catalysts. In addition, oxygen surface functionalities enable wettability 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 Nand 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 functionalities. 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 conductivity of the obtained material, C.PANI.HAT200, was not markedly changed after the treatment, its elemental composition and textural properties were significantly modified. Introduction 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 enrichment of the surface by nitrogen and, especially, oxygen functionalities. Measured gravimetric capacitances of C.PANI.HAT200 were up to 2 times higher than corresponding ones of starting C.PANI material, while the capacitance retention of C.PANI.HAT200, upon increasing sweep rate, was improved. The highest capacitance was measured in 1 mol dm3 H2SO4, amounting to 433 F g1, at 5 mV s1. The activity of new material toward ORR, investigated in 0.1 mol dm3 KOH solution, was one order of magnitude higher 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 evidenced 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 behavior 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 contributes 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.

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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.

[12]

[13]

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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