Nitrogen-enriched bituminous coal-based active carbons as materials for supercapacitors

Fuel 89 (2010) 3457–3467

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Nitrogen-enriched bituminous coal-based active carbons as materials for supercapacitors R. Pietrzak a,*, K. Jurewicz b, P. Nowicki a, K. Babeł c, H. Wachowska a a

´ , Poland Laboratory of Coal Chemistry and Technology, Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan ´ University of Technology, Piotrowo 3, 60-965 Poznan ´ , Poland Institute of Chemistry and Technical Electrochemistry, Poznan c ´ , Wojska Polskiego 38/42, 60-635 Poznan ´ , Poland Institute of Chemical Wood Technology, University of Life Sciences in Poznan b

a r t i c l e

i n f o

Article history: Received 17 November 2009 Received in revised form 11 June 2010 Accepted 16 June 2010 Available online 29 June 2010 Keywords: Activated carbon Ammoxidation Activation XPS Supercapacitors

a b s t r a c t The paper presents the results of a study on obtaining N-enriched active carbons from bituminous coal and on testing its use as an electrode material in supercapacitors. The coal was carbonised, activated with KOH and ammoxidised by a mixture of ammonia and air at the ratio 1:3 at 300 °C or 350 °C, at different stages of the production, that is, at those of precursor, carbonisate, and active carbon. The products were microporous N-enriched active carbon samples of well-developed surface area reaching from 1577 to 2510 m2/g and containing 1.0 to 8.5 wt% of nitrogen. The XPS measurements have shown that in the active carbons enriched in nitrogen at the stage of precursor and at the stage of carbonisate, the dominant nitrogen species are the N-5 groups, while in the samples ammoxidised at the last stage of the treatment the dominant nitrogen species are the surface groups of imines and/or nitriles, probably accompanied by amines and amides. The paper reports the results of a comprehensive study of the effect of the structure and chemical composition of a series of active carbon samples of different properties on their capacity performance in water solutions of H2SO4 or KOH, with the behaviour of positive and negative electrodes analysed separately. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Electrochemical capacitors have recently become a subject of intense research at a number of research centres in the world. The main reason for this interest is the possibility of their use in the storage of electric energy. The critical factor for obtaining a capacitor of high power is the use of stable electrode materials characterised not only by high capacity but also first of all by high charge exchange. The possible materials meeting these demands are active carbons. Recently, a considerable progress was made in the field of application of active carbons in electrochemical capacitors, as it results from constantly growing number of reports on this subject [1–10]. The interest in this field results both from a relatively low manufacturing cost of active carbons, their good electric and thermal conductance, high resistance to corrosion and from well-developed porous structure and high surface area as well as a possibility of working in different media (from strongly acidic to strongly basic ones) and in a wide temperature range. Typical conditions of carbonisation and activation permit the production of active carbons of acidic character of the surface, which enhances the carbon capacity as a result of the increased * Corresponding author. Tel.: +48 618291476; fax: +48 618291505. E-mail address: [email protected] (R. Pietrzak). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.06.023

contribution of the pseudocapacitance effects [11]. Unfortunately, the electron acceptor character of the materials of this type favours the work of one electrode, so to improve the capacitance of the whole capacitor the improvement of the performance of the counter electrode is needed. This effect can be obtained by the use of the nitrogen-enriched active carbons showing electron donor properties following their basic character [12–14]. The nitrogen-enriched carbons can be obtained in many ways. The nitrogen-introducing agents are ammonia, amines, urea and its derivatives [15–17]. A much promising process is that of ammoxidation, so are simultaneous oxidation and nitrogenation of the carbon material [18,19]. The enriched carbons obtained in this process contain significant amounts of nitrogen in the form of surface species such as amines, amides and lactams [20,21]. The processes of pyrolysis and activation involve a significant decrease in the nitrogen content accompanied by the conversion of the surface species to pyridine nitrogen (N-6), pyrrole nitrogen (N-5), quaternary nitrogen (N-Q) and N-pyridine oxide (N-X), directly incorporated into the graphene planes [13]. The temperature of the thermal treatment determines the rate of conversion and the site of the nitrogen atoms built into the graphene structure [22]. The incorporation of nitrogen into the carbon structure has a profound effect on its electrochemical properties. Particularly desired is the nitrogen positioned in the ‘‘valley” or at the ‘‘centre” in the

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N-Q group (showing pyrolic character), as this positioning enhances the electron donor properties of the material. Much less advantageous is the nitrogen incorporation into top typical position of the pyridine group, which does not improve the electron donor properties. The aim of the study was to obtain such nitrogen-enriched active carbons from bituminous coal that would show beneficial capacitance performance of both electrodes of the electrochemical capacitor. To obtain active carbons with optimum properties, the effect of the stage (from precursor to activate) at which the ammoxidation was applied and the effect of the thermal treatment in different regimes was tested. 2. Experimental 2.1. Materials The starting material was a bituminous coal from the Sos´nica colliery (Poland), and are characterised in Table 1. The initial raw sample (S) of grain size 6 0.2 mm was demineralised by concentrated hydrochloric and hydrofluoric acids according to the Radmacher and Mohrhauer method [23]. The demineralised coal (SD) was ammoxidised, carbonized and activated in various sequence. Ammoxidation (N) was applied to demineralised precursor, carbonisation product or active carbon. This process was performed with the mixture of ammonia and air at the ratio 1:3 at the temperature of 300 °C (N1) or 350 °C (N2) for 5 h. Carbonisation process (C) was carried out in argon atmosphere (flow rate 170 mL/min) at the temperature increase rate of 5 °C/min to the final temperature of 700 °C, maintained for 1 h. Carbonisation was applied to demineralised and ammoxidised precursors. Activation (A) was applied to carbonisation products (SDC and SDNC) or nitrogen-enriched carbonisates (SDCN). KOH was directly mixed at room temperature with samples at the weight ratio of 4:1. After the physical mixing, the samples were activated at 700 °C for 45 min in argon atmosphere (flow rate 330 mL/min). The obtained activated carbons (SDCA, SDNCA and SDCNA) were washed first with 5% HCl solution and then with distilled water until free of chloride ions. The washed activated carbons were dried at 110 °C for 24 h. 2.2. Methods Proximate analysis of the initial coal and elemental analysis (C,H,N,S) of the products obtained at each stage of the processing were carried out on an elemental analyser CHNS Vario EL III (Elementar Analysensysteme, GmbH, Germany). The surface oxide functional groups were determined by the Boehm method [24]. The chemical state of selected elements and surface composition of the samples were determined by X-ray photoelectron spectroscopy using VSW spectrometer (Vacuum Systems Workshop Ltd., England) equipped with Al Ka source and 18-channel 2-plate analyzer. The spectra were taken in a FAT mode (DE = const) with pass energy of 22 eV. They were smoothed and the Shirley background was subtracted. The calibration was carried out to the main C1s peak at 284.6 eV. The concentration of the elements was calcu-

lated using the intensity of an appropriate line and XPS cross-sections (as given by Scofield [25]). The fitting of the N1s peaks gave the following binding energies: 398.1 ± 0.1 eV – aromatic imines and/or nitriles, 398.7 ± 0.3 eV – N-6 (pyridinic), 400.3 ± 0.1 eV – N-5 (pyrrolic and pyridonic), 401.4 ± 0.1 eV – N-Q (nitrogen substituents in aromatic graphene structures – quaternary nitrogen), 402.5 ± 0.1 eV – (pyridine-N-oxide or ammonia) and 404.5 ± 0.1 eV – N–Ox (chemisorbed nitrogen oxides) [13,21,22,26–28]. It must be regarded that the intensity of the peak at 398.7 ± 0.3eV assigned to the pyridinic groups can be influenced by the presence of amines and amides, which have similar binding energy. Characterisation of the pore structure of activated carbons was performed on the basis of low temperature nitrogen adsorption– desorption isotherms measured on a sorptometer ASAP 2010 manufactured by Micromeritics Instrument Corp. (USA). Surface area and pore size distribution were calculated by BET and BJH methods, respectively. Total pore volume and average pore diameter were determined as well. Micropore volume and micropore area were calculated using t-plot method. The capacity parameters of carbon materials were measured in a three-electrode Swagelok type capacitor in acidic (4 M H2SO4) or alkaline (7 M KOH) conditions versus Hg/Hg2SO4 and Hg/HgO reference electrodes, respectively, using galvanostatic GC (0.2 A/g), potentiodynamic CV (1, 5 and 25 mV/s) and impedance spectroscopy EIS methods (10 3–105 Hz, amplitude of signal 10 mV). Samples were examined in the form of pellets obtained by pressing a mixture of activated carbon (85 wt%), acetylene black (5 wt%) and polyvinylidene fluoride PVDF (10 wt%). The study was made at ambient temperature using computer-controlled electrochemical equipment (potentiostat/galvanostat Arbin-BT2000 and Autolab-PGSTAT 30/FRA).

3. Results and discussion Ammoxidation of the precursor (SD) leads to the grafting of considerable amount of nitrogen into the carbon structure (Table 2), however, the nitrogen introduced in this way is thermally unstable as indicated by a decrease in its content as a result of pyrolysis or activation. Ammoxidation of the carbonisate (DC) is of different character. According to the data presented in Table 2, at this stage of coal processing the process of ammonolysis is less effective than the ammoxidation of the precursor, as the amount of nitrogen introduced into the coal structure is lower by 50%. This much lower efficiency of ammoxidation at the stage the carbonisate is most probably due to the result of changes in the coal structure taking place on pyrolysis. The amount of nitrogen introduced into sample

Table 2 Elemental analysis of the all samples obtained (wt%).

Table 1 Characteristics of raw and demineralised coal (wt%).

a

a

Sample

Moisture

Ashd

VMdaf

Cdaf

Hdaf

Ndaf

Sdaf

Odaf

S SD

2.4 0.0

5.2 2.3

32.0 32.1

81.7 82.3

5.2 5.2

1.4 1.3

0.9 1.1

10.8 10.1

By difference.

a

Sample

Ad

Cdaf

Hdaf

Ndaf

Sdaf

Odafa

SD SDN1 SDN2 SDN1C SDN2C SDN1CA SDN2CA SDC SDCN1 SDCN2 SDCN1A SDCN2A SDCA SDCAN1 SDCAN2

2.3 1.2 1.4 1.8 1.4 0.5 0.2 1.9 2.1 1.8 0.7 0.3 0.3 0.2 0.3

82.3 72.3 72.6 84.4 85.8 89.7 91.5 93.4 84.9 81.8 92.5 92.3 93.5 84.6 83.3

5.2 2.9 2.8 1.8 1.7 0.8 0.6 2.0 1.6 1.6 0.7 0.6 0.9 0.9 0.9

1.3 13.9 12.7 6.5 7.3 2.1 1.2 1.2 6.7 8.4 1.4 1.0 0.4 7.7 8.5

1.1 0.5 0.6 0.3 0.2 0.2 0.2 0.5 0.4 0.4 0.2 0.1 0.1 0.1 0.1

10.1 10.4 11.3 7.0 5.0 7.2 6.5 2.9 6.4 7.8 5.2 5.9 4.7 6.7 7.2

By difference.

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SDC significantly depends on temperature; it is much lower in sample SDCN1 modified at 300 °C than in sample SDCN2 ammoxidised at 350 °C (Table 2). The nitrogen species introduced into the carbonisate (similarly as in samples SDN) are characterised by low thermal stability and low chemical stability towards alkali metals, which is manifested by a considerable decrease in their amount on activation with KOH. The amount of nitrogen introduced on ammoxidation of active carbon SDCA (Table 2) is somewhat higher than that introduced on ammoxidation of the carbonisate, which indicates that active carbon shows greater reactivity towards the air–ammonia mixture. Most probably it is a result of the presence of oxygen functional groups (formed, e.g. on activation) at the active carbon surface as their presence significantly facilitates the nitrogen incorporation into the coal structure [17]. The effect of temperature is similar as for the carbonisate greater amount of nitrogen is introduced upon ammoxidation at 350 °C (SDCAN2) than at 300 °C (SDCAN1). It is common knowledge that nitrogen groups can be introduced into the carbon structure either via the oxygen groups present on their surface (formed upon activation or oxidation) or via oxygen complexes formed in situ in the reaction with the air–ammonia mixture (ammoxidation). According to the literature data, the presence of e.g. carboxyl groups favours the introduction of nitrogen in the structure of coals [17]. However, the performance of oxidation as an additional step followed by the reaction with ammonia for activated carbons (having many oxygen groups on the surface) seems economically unjustified. The process of ammoxidation also leads to changes in the content of the other elements. The amount of carbon decreases by about 10 wt%, irrespective of the stage at which the ammonolysis is performed, also the amount of hydrogen decreases and is the most pronounced upon ammoxidation of the precursor. Samples SDC and SDCA (Table 2) also undergo oxidation in the reaction with the air–ammonia mixture, which is evidenced by a significant increase in the content of oxygen, particularly notable for samples SDCN (Table 2), in which a twofold increase in the content of oxygen is observed relative to that in the initial carbonisate. To characterise the surface properties of the active carbons obtained, the content of the surface oxygen functional groups was estimated (Table 3). The active carbon samples were found to have considerable amount of oxide functional groups besides the nitrogen groups. The surface of the carbon not subjected to ammoxidation – sample SDCA – has decidedly acidic character in contrast to the surfaces of the nitrogen-enriched materials whose surfaces have rather basic character. The acid–base properties of the active carbons depend on the stage at which ammoxidation was performed. The carbon subjected to ammoxidation at the stage of precursor (SDNCA) had the highest content of basic groups on the surface from among all samples studied, which was almost three times higher than that of the unmodified carbon (SDCA). The carbons also contain considerable amount of acidic groups, therefore their surface has intermediate acid–base character. A similar situation is observed for the carbons ammoxidised at the stage of the carboni-

sate (SDCNA), however, on their surface the content of acidic and basic groups is lower than on the surface of SDNCA. The surface properties of the carbon modified after activation (SDCAN) are close to those of sample SDCNA. Interestingly, sample SDCAN1, ammoxidised at 300 °C, is characterised by the lowest content of acidic groups from among all active carbon samples studied. As follows from these data, there is some correlation between the content of the oxygen functional groups of acidic character and the ammoxidation temperature. The samples ammoxidised at 300 °C have a lower content of surface acidic groups than those modified at 350 °C. The difference is most pronounced for the samples modified at the stage of active carbon (SDCAN). The situation is different as far as the content of basic groups is concerned; lower temperature of ammoxidation favours the formation of greater number of basic groups, except for the samples ammoxidised at the stage of the carbonisate (SDCNA). 3.1. XPS study To characterise the changes taking place on the coal surface and to establish the types of nitrogen species introduced upon the processes applied, the active carbon samples have been studied by the X-ray photoelectron spectroscopy (XPS). Table 4 presents the elemental composition of the surfaces of the carbon samples compared with that of the bulk of the samples. The contents of particular elements on the surface and in the bulk are similar. The ammoxidation of the active carbon (SDCA) upon which significant amounts of nitrogen are introduced in the bulk and on the surface (SDCAN) results in a decrease in the content of oxygen on the surface and its increase in the bulk. This result suggests that the bulk of the sample undergoes oxidation, while the surface does not, although the access to the air–ammonia mixture of the surface is much easier. It is common knowledge that the XPS method examines only the surface of the solid phase and a narrow near-surface region (2–20 monolayers), while the other results refer to the whole solid phase. Therefore two situations are possible: (a) weakly oxidised surface and (b) a surface so strongly oxidised that the outer layers are gasified and the deeper nonoxidised layer is uncovered and the latter situation takes place in our experiment. The two phenomena give a similar XPS image. Therefore, the results of the surface study should be interpreted together with the results for the whole solid phase. These differences can be explained on the basis of our earlier studies [17] whose results proved that nitrogen species are incorporated into the coal structure through the oxygen species. Therefore, it is clear that the coal surface also undergoes oxidation upon ammoxidation leading to the formation of oxygen species through which the nitrogen species are absorbed because of the easy access to ammonia. The same process takes place in the bulk of the samples, but because of the more difficult access to ammonia the process of nitrogen species introduction is slower (the air–ammonia mixture contains less nitrogen than air), which leads to a greater content of oxygen and lower content of nitrogen in the bulk. The smallest differences in the elemental composition of the surface and in the

Table 3 Acid–base properties of the modified carbons (mmol/g). Sample

Carboxyl groups

Lactone groups

Phenol groups

Total acidic groups

Total basic groups

SDCA SDCAN1 SDCAN2 SDCN1A SDCN2A SDN1CA SDN2CA

0.40 0.15 0.20 0.21 0.00 0.68 0.15

0.43 0.42 0.42 0.32 0.52 0.15 0.36

0.32 0.26 0.36 0.46 0.51 0.38 0.74

1.15 0.83 0.98 0.99 1.03 1.21 1.25

0.65 1.45 1.37 1.24 1.37 1.74 1.71

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Table 4 Elemental composition of the surface and in the bulk of the carbon samples studied (C + O + S + N = 100 at.%). Coal

SDCA SDCAN1 SDCAN2 SDCN1A SDCN2A SDN1CA SDN2CA

Surface

Bulk

C

O

N

S

C

O

N

S

94.1 89.2 86.8 94.0 93.9 92.1 95.2

5.4 3.5 3.9 4.8 4.7 5.0 3.5

0.5 7.3 9.3 1.2 1.4 2.9 1.3

0.0 0.0 0.0 0.0 0.0 0.0 0.0

90.0 87.9 86.8 94.7 94.6 92.5 93.8

3.6 5.2 5.6 4.0 4.5 5.6 5.0

0.4 6.9 7.6 1.2 0.9 1.8 1.1

0.0 0.0 0.0 0.1 0.0 0.1 0.1

bulk are observed for the samples ammoxidised at the stage of the carbonisate and then activated (SDCNA). Significant differences in the surface contents of oxygen and nitrogen are noted for the samples ammoxidised at different temperatures at the stage of the precursor and then carbonised and activated (SDNCA). The sample ammoxidised at 300 °C (SDN1CA) has much greater contents of oxygen and nitrogen than that ammoxidised at 350 °C. Slightly reduced differences are also noted in the bulk of the samples. This observation is consistent with the analytical data presented in Table 2, indicating that sample SDN1CA has much greater content of nitrogen and oxygen than SDN2CA. According to Table 5, the surface of SDN2CA shows much greater content of oxidised nitrogen species (N-Ox) than SDN1CA, which means that much more oxidised nitrogen species have been removed from the surface of the latter sample in particular processes. Analysis of other data from Table 5, showing the contribution of different nitrogen species in relative percent, implies that the nonammoxidised active carbon (SDCA) contains nitrogen mainly in the forms of N-5 (pyrrole, pyridone) and N-6 (pyridine), then in the form of N-Q (in graphene layers) and pyridine N-oxides. It also contains small amounts of imines, nitriles and N-ox. Ammoxidation of the active carbon (SDCAN) permits the introduction of 7.4 wt.% of Ndaf at 300 °C and 8.1 wt.% Ndaf at 350 °C (Table 2) mainly in the form of imines and nitriles, whose content significantly increases relative to that in the unmodified active carbon. Samples SDCAN also show an increased content of (N-Ox) – as a consequence of the reaction conditions – and a small increase in the content of N-6 species. The content of the other nitrogen species decreases, and the decrease is the greatest in the content of N-Q species. As follows from Table 5, the dominant nitrogen species on the surface of the active carbons ammoxidised at the stage of precursor (SDNCA) and carbonisate (SDCNA) are N-5 ones. However, from the samples ammoxidised at the stage of carbonisate, the content of N5 species is greater in that modified at 350 °C (SDCN2A), while from the samples ammoxidised at the stage of precursor – in the one treated at 300 °C (SDN1CA). From among the other nitrogen species present in these samples, the content of (N-6) is the greatest but in all samples except SDN2CA, it is twice or more than twice smaller than that of N-5 species. Although the literature data [22,28] imply

that with increasing temperature the content of N-5 species decreases while that of N-6 species increases, which is in contrast to the results of our study, however, our result are consistent with those obtained earlier [17]. According to our earlier results on introduction of nitrogen from urea, the increase in the content of N-5 species is related mainly to the increasing number of pyridone groups formed as a result of pyridinium transformation and the contribution of nitrogen from these groups and not from pyrrolic ones to the peak assigned to N-5 species is dominant. As to the content of N-6 species, it is lower in the samples ammoxidised at the lower temperature (SDCN1A and SDN1CA) than in those modified at the higher one (SDCN2A and SDN2CA). Moreover, the contents of N-6 species in the samples ammoxidised at the same temperature but at different stages are similar (SDCN1A 20.5% and SDN1CA 19.4%, SDCN2A 30.4% and SDN2CA 27.3%), the differences in the contents of N-5 species are much greater. As mentioned above, the ammoxidation of the active carbon leads to a significant increase in the content of imine and nitrile groups. These species are unstable and decompose above 400 °C [21], which is consistent with our results indicating a significant or total decrease in the content of these groups on activation of the ammoxidised carbonisate or on carbonisation and activation of the ammoxidised precursor. Very interesting but difficult to interpret at this stage of the study is the total disappearance of imine and nitrile groups from the samples ammoxidised at the higher temperature at the stage of precursor or carbonisate (SDCN2A and SDN2CA), which is not observed in the samples modified at the lower temperature (SDCN1A and SDN1CA). The most pronounced changes in the contents of nitrogen species are observed to occur upon the ammoxidation of precursor at 350 °C followed by its carbonisation and activation (SDN2CA). As a result of these processes the greatest amount of nitrogen is incorporated into the graphene structure (N-Q) and the greatest amount of oxidised nitrogen species (N-Ox) is formed. The changes are particularly pronounced in the typical XPS spectra of N1s for all samples, shown Fig. 1. 3.2. Textural studies of carbons The surface areas of all active carbon samples studied were measured and the results are collected in Table 6. The pore size distributions in the individual samples are presented in Fig. 2. All the active carbon samples obtained have very well-developed surface area with the dominant contribution of micropores. The stage of the sample processing at which nitrogen was introduced and the temperature of this process have a considerable influence on the textural parameters of the active carbon samples. The application of ammoxidation at the last stage of processing (SDCAN) leads to a decrease in the surface area and the micropores area relative to those in the unmodified active carbon (SDCA). In spite of this, with increasing temperature of ammoxidation, the contribution of the micropores area in the total area of the sample increases. This process is first of all related to a significant decrease in the volume of mesopores relative to that of micropores in the total pore volume.

Table 5 The contribution of nitrogen species to N1s peak (%). Coal

Imines, nitriles

Pyridine N-6

Pyrrole, pyridone N-5

In graphene layers N-Q

Pyridine N-oxides, ammonia

N-Ox

SDCA SDCAN1 SDCAN2 SDCN1A SDCN2A SDN1CA SDN2CA

5.0 27.5 28.7 11.2 0.0 17.8 0.0

26.7 30.2 29.5 20.5 30.4 19.4 27.2

33.6 21.7 22.4 47.6 56.6 41.5 35.2

17.3 7.0 7.5 6.5 0.0 9.5 23.4

14.3 7.1 6.1 8.5 13.0 8.3 1.4

3.1 6.5 5.8 5.7 0.0 3.5 12.8

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R. Pietrzak et al. / Fuel 89 (2010) 3457–3467 N1s SDCA imines, nitriles N-6 N-5 N-Q pyridine-N-ox, ammonia N-Ox

408

406

404

402

N1s SDCAN1

400

398

396

408

394

406

Binding Energy [eV]

404

402

400

402

398

398

396

408

408

394

406

406

Binding Energy [eV]

404

404

402

400

402

398

400

398

396

394 408

406

394

N1s SDN2CA N-6 N-5 N-Q pyridine-N-ox ammonia N-Ox

imines, nitriles N-6 N-5 N-Q pyridine-N-ox, ammonia N-Ox

Binding Energy [eV]

396

Binding Energy [eV]

N1s SDN1CA

N-6 N-5 pyridine-N-ox, ammonia

400

396

Binding Energy [eV]

imines, nitriles N-6 N-5 N-Q pyridine-N-ox, ammonia N-Ox

406

404

imines, nitriles N-6 N-5 N-Q pyridine-N-ox, ammonia N-Ox

N1s SDCN2A

N1s SDCN1 A

408

N1s SDCAN2

imines, nitriles N-6 N-5 N-Q pyridine-N-ox, ammonia N-Ox

404

402

400

398

396

394 408

Binding Energy [eV]

406

404

402

400

398

396

394

Binding Energy [eV]

Fig. 1. N1s peak for active carbons studied.

Table 6 Structural parameters for active carbons studied. Sample

SDCA SDCAN1 SDCAN2 SDCN1A SDCN2A SDN1CA SDN2CA

Surface area (m2/g)

Pore volume (cm3/g)

Total surface area (BET)

Micropore area

Vmic

Vmeso

Vt

1950 1722 1577 2386 2505 1996 2510

1877 1666 1532 2246 2163 1846 2221

0.87 0.77 0.71 1.04 1.03 0.88 1.05

0.08 0.06 0.05 0.13 0.30 0.15 0.27

0.95 0.83 0.76 1.17 1.33 1.03 1.32

Fig. 2. Pore size distributions for active carbons studied.

This fact is confirmed by the pore volume distribution. The effect of the ammoxidation temperature has insignificant effect on the micropore volume: when performed at 300 °C (SDCAN1) it causes

Vmic/Vt

Average pore diameter (nm)

0.92 0.93 0.94 0.89 0.77 0.86 0.80

1.95 1.92 1.93 1.96 2.12 2.05 2.10

a decrease in the micropore volume by 12% relative to that in SDCA and when performed at 350 °C (SDCAN2) – by about 18%. The effect of the ammoxidation temperature on the volume of mesopores is greater: for SDCAN1 the volume of mesopores decreases by 27%, while for SDCAN2 it reaches 37% relative to that in SDCA. The results indicate that the nitrogen introduced in the process mainly blocks the greater size pores. The situation is different for the samples ammoxidised at the stage of demineralised coal (SDNCA) or carbonisate (SDCNA). The surface areas and the micropore areas of these samples (except SDN1CA) are much greater than those in the not ammoxidised sample (SDCA). Irrespective of whether ammoxidation was performed at the precursor or carbonisate stage, increased temperature of the process favoured development of the area of the active carbon samples obtained. The same is true for the micropores area in the samples ammoxidised at the precursor stage. For the samples ammoxidised at the carbonisate stage, the reverse is true – the lower temperature of the process favours development of the micropores area. Despite these differences, for these two series of samples the contribution of micropores area in the total area of the samples significantly decreases with increasing ammoxidation temperature. Moreover, in contrast to the situation in the samples ammoxidised at the last stage of processing, in

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Table 7 Capacitance parameters of the samples studied in acidic medium (4 M H2SO4). Sample

SDCA SDN1CA SDN2CA SDCN1A SDCN2A SDCAN1 SDCAN2

C (F/g)

Cs (lF/cm2)

DC25 (%)

fg (Hz)

SD10 (%)

ILC (mA/g)

{+}

{ }

{+}

{ }

{+}

{ }

{+}

{ }

{+}

{ }

282 277 276 278 288 201 183

277 276 319 279 288 236 234

10 14 18 21 14 19 8

21 22 21 21 22 4 11

14.5 13.9 11.0 11.7 11.5 11.7 11.6

14.2 13.8 12.7 11.7 11.5 13.7 14.8

10.0 4.0 0.1 4.0 6.3 0.1 1.6

1.6 2.5 10.0 6.3 10.0 6.3 4.0

12 13 11 16 12 11 20

7 8 6 11 9 9 17

5.6 5.3 5.9 5.8 6.1 3.8 3.7

C (F/g) – average capacitance from galvanostatic and voltammetry measurements; DC25 (%) – capacitance loss with enhanced voltammetry sweep from 1 mV/s to 25 mV/s; Cs (lF/cm2) – surface specific capacitance; fg (Hz) = (dC/df)max – frequency limit of the capacitance; SD10 (%) – self-discharge during 10 h after full charging of a capacitorILC (mA/ g) – leakage current after 2 h; {+} and { } – positive and negative electrode, respectively.

Table 8 Capacitance parameters of the samples studied in alkaline medium (7 M KOH). Sample

SDCA SDN1CA SDN2CA SDCN1A SDCN2A SDCAN1 SDCAN2

C (F/g)

Cs (lF/cm2)

DC25 (%)

fg (Hz)

SD10 (%)

ILC (mA/g)

{+}

{ }

{+}

{ }

{+}

{ }

{+}

{ }

{+}

{ }

215 188 225 219 213 196 182

348 287 336 286 317 319 299

16 24 25 23 20 18 24

9 34 9 2 10 8 9

11.0 9.4 9.0 9.2 8.5 11.4 11.5

17.8 14.4 13.4 12.0 12.7 18.5 19.0

4.0 0.6 0.4 4.0 6.3 0.1 0.1

6.3 0.1 16.0 2.5 6.3 2.5 6.3

16 15 11 14 16 15 14

12 12 7 12 11 9 10

3.4 5.1 5.1 6.2 7.0 4.9 4.1

C (F/g) – average capacitance from galvanostatic and voltammetry measurements; DC25 (%) – capacitance loss with enhanced voltammetry sweep from 1 mV/s to 25 mV/s; Cs (lF/cm2) – surface specific capacitance; fg (Hz) = (dC/df)max – frequency limit of the capacitance; SD10 (%) – self-discharge during 10 h after full charging of a capacitor; ILC (mA/g) – leakage current after 2 h; {+} and { } – positive and negative electrode, respectively.

the samples ammoxidised at the precursor or carbonisate stage the general pore volume increased relative to that in the unmodified carbon. However, the volume of mesopores increases much more than that of micropores. The exception is SDN1CA whose structural parameters are close to those of SDCA. As follows, the ammoxidation of the precursor at 300 °C, then its carbonisation and activation do not change significantly the coal structure. Significant differences relative to the initial coal are notable only in the volume of mesopores, which is almost twice greater in the ammoxidised sample. It is most probably a result of activation. This process caused first of all the opening of the pores blocked by amorphous carbon formed on carbonisation and partial removal of nitrogen, but only from mesopores. The walls of the mesopores were destroyed in the process and the volume of the mesopores increased. This interpretation is confirmed by the presence of a considerable amount of Ndaf in SDN1CA (2.1 wt%, Table 2) relative to that in the other samples (SDN2CA – 1.2 wt% Ndaf Table 2, SDCN1A – 1.4% and SDCN2A – 1 wt% Ndaf Table 2). In the other samples the processes were more intense leading to greater development of the surface area and pore volume. 3.3. Electrochemical characteristics Different ammoxidation procedures employed during the preparation of active carbons allowed obtaining samples significantly varying in the content and types of nitrogen species contained in the material from the same precursor. Unfortunately, these samples seriously differed in porous structure, surface area as well as the degree and mode of oxidation. In consequence, it is difficult to evaluate precisely how the presence of nitrogen heteroatoms affects the capacitance parameters of the active carbon samples. Electrochemically determined capacitance of each sample includes the electric double layer component, proportional to the pore surface available to electrolyte and the pseudocapacitance component

induced by the presence of the nitrogen and oxygen heteroatoms. Contributions of these two components cannot be easily separated since, for example carbon wettability, and hence its active surface area, are affected both by the structural parameters and the presence of certain functional groups. In this respect, beneficial effect is brought by acidic oxygen groups and nitrogen groups [29,30]. Basic functional groups, which predominate in the ammoxidised materials studied (Table 3), are rather responsible for hydrophobic character of the carbon surface [30]. Pseudocapacitance can also be split into two components. One of them depends on the content of nitrogen and oxygen functional groups, capable of taking part in fast Faraday redox reactions at adequate potentials, which is especially feasible if they exist in the form of surface functional groups. The second component depends on the types and quantity of heteroatoms and their positions in the graphene planes since they affect the energy levels of the highest occupied and lowest unoccupied molecular orbitals (EHOMO and ELUMO) as well as on the band gap DE = ELUMO EHOMO, inducing changes in the electron donor properties of the carbon samples [12,31,32]. The contribution of this component is particularly difficult to estimate because, as opposed to the one, it depends not only on the amount of the heteroatoms but also on their distribution in the crystalline lattice and on the size of the graphene planes. Theoretically, the influence of electron donicity on pseudocapacity is the highest at rather relatively low content of heteroatoms, at a level of ca. 2–3 at.% [31,32] and should be the opposite for capacitor electrodes of the opposite polarity. Taking this into account, electrochemical GC and CV measurements were performed in the three-electrode system. The performance of the positive {+} and negative { } electrodes versus the reference electrode was investigated in the capacitor operated at the full voltage range of 50–1050 mV, i.e. within 1 V, irrespective of the type of electrolyte. EIS measurements were performed in the same electrode configuration with the capacitor charged at half

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SDCA SDN1CA SDN2CA SDCN1A SDCN2A SDCAN1 SDCAN2

400

{+} 350 300

C (F/g)

250 200 150 100 50 0 0.0001

0.001

0.01

0.1

1

10

100

f (Hz)

SDCA SDN1CA SDN2CA SDCN1A SDCN2A SDCAN1 SDCAN2

400

{-}

350 300

C (F/g)

250 200 150 100 50 0 0.0001

0.001

0.01

0.1

1

10

100

f (Hz) Fig. 3. Capacitance frequency spectra of the investigated samples used as positive {+} or negative { } electrodes in acidic electrolyte (4 M H2SO4).

of its maximum voltage, i.e. up to 0.52 V. Specific capacities of particular samples determined by the three methods of measurements were consistent enough to be considered in the discussion only as the mean values. Tables 7 and 8 show the average capacitance values in relation to the mass of the active carbon in electrode of a defined polarity (C) or to its BET surface (Cs). Figs. 3 and 4 present the capacitance frequency spectra of all samples tested as the positive or negative electrodes in acidic and alkaline electrolyte, respectively. The most characteristic voltammograms of these two media are shown in Figs. 5 and 6. The results of electrochemical measurements (Tables 7 and 8) are in agreement with the results of our earlier investigation on other coal-based active carbons enriched with nitrogen by ammoxidation [14,19] or in the reaction with urea [33], namely, in acidic capacitors, the electrodes of the opposite polarity reveal similar capacitances, while in alkaline capacitors higher values are obtained for the negative electrodes. Similar to the earlier studied materials, also in these studied here, it is difficult to find a correla-

tion between the nitrogen content and the capacitance. It is also not easy to determine the dependence of capacitance on structural parameters (Tables 6–8). However, it is possible to correlate the capacitance of specified samples with the types of nitrogen and oxygen species (Tables 3–5 and Figs. 3–6). In order to analyse the influence of chemical composition on capacitance, it would be convenient to divide the samples into three groups: (i) SDN1CA, material ammoxidised as a precursor, at lower temperature, and having structural parameters very close to those of the unmodified sample SDCA; (ii) samples with structure additionally developed by ammoxidation – both samples SDCNA ammoxidised after carbonisation and SDN2CA ammoxidised as precursor but at the higher temperature; (iii) samples with limited porous structure due to ammoxidation – the two SDCAN samples ammoxidised after activation. Sample SDN1CA is certainly a microporous material like unmodified SDCA but its mesopores volume is two times higher. In acidic medium, these two samples reveal almost the same specific

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SDCA SDN1CA SDN2CA SDCN1A SDCN2A SDCAN1 SDCAN2

400

{+} 350 300

C (F/g)

250 200 150 100 50 0 0.0001

0.001

0.01

0.1

1

10

100

f (Hz) SDCA SDN1CA SDN2CA SDCN1A SDCN2A SDCAN1 SDCAN2

400

{-} 350 300

C (F/g)

250 200 150 100 50 0 0.0001

0.001

0.01

0.1

1

10

100

f (Hz) Fig. 4. Capacitance frequency spectra of the investigated samples used as positive {+} or negative { } electrodes in alkaline electrolyte (7 M KOH).

capacity C for positive and negative electrodes (Table 7) although the SDN1CA has five times higher nitrogen content and two times higher oxidation degree (Table 2). Differences in the capacitance of these samples are distinct only in the capacitance frequency spectra at the intermediate frequency range, and are manifested by slightly better charge exchange dynamics of the negative electrode made of ammoxidised material and, unfortunately, significantly worse for the positive electrode (Fig. 3). However, it is worth noting that the ammoxidation process has a higher or at least equal influence on the capacitance of all other samples. In alkaline environment the performance of SDN1CA as compared to SDCA is by far less beneficial (Table 8). Deterioration of the negative electrode impedance characteristics is exceptional in comparison with all other materials studied (Fig. 4). This effect cannot be due to 60% higher oxidation or almost three times higher alkalinity on the surface because sample SDN2CA, having almost the same chemical composition and being ammoxidised in a similar way at a higher temperature, shows the best capacitance char-

acteristics of all the materials studied, also the best charge exchange dynamics in both media (Figs. 4–6). Therefore, poor capacitance of SDN1CA versus SDCA must be rather due to less advantageous configuration of nitrogen heteroatoms in the graphene structures and excessive content of oxygen heteroatoms. SDN1CA as compared to SDCA contains much more nitrogen in the surface functional groups, such as aromatic N-imines and/or nitriles, and less nitrogen in the form of N-Q incorporated in the graphene structures (Table 5). Also, oxygen heteroatoms contained in this sample form functional groups of negligible contribution to pseudocapacitance [7] (Table 3). The characteristic feature of sample SDN2CA with the best capacitance is the predominant number of phenolic groups favouring higher pseudocapacity [7]. Trying to explain this high performance by chemical structure, it would be worth comparing SDN2CA to SDCN2A sample with similarly well-developed BET surface (over 2500 m2/g) and high contribution of mesopores (Table 6) but much worse electrochemical properties. Both samples reveal

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400 300 200 100

C (F/g)

{-} -0.8

-0.7

-0.6

-0.5

-0.4

{+}

0

-0.3

-0.2

-0.1 0 -100

0.1

0.2

0.3

0.4

0.5

0.6

-200 -300

SDCA SDN2CA SDCAN1

-400 -500

Potential vs. Hg/Hg2SO4 (V) Fig. 5. Voltammograms for selected samples with extremely different charge exchange dynamics of positive and negative electrodes in acidic environment (4 M H2SO4). Potential sweep: 5 mV/s.

400 300

C (F/g)

200

{-} -1.1

-1

-0.9

-0.8

-0.7

{+} -0.6

-0.5

-0.4

-0.3

-0.2

100 0 -0.1 0 -100

0.1

0.2

-200

SDN1CA SDN2CA SDCAN2

-300 -400 -500

Potential vs. Hg/HgO (V) Fig. 6. Voltammograms for selected samples in alkaline environment (7 M KOH). Potential sweep: 5 mV/s.

similar surface alkalinity, but SDN2CA is more oxidised since it contains about 20% more oxygen functional groups (Table 3) with ca. 45% higher content of phenol groups. Differences between samples SDN2CA and SDCN2A are less distinct in the content and type of nitrogen species (Table 5). Both samples do not contain nitrogen in the surface groups with binding energy of ca. 398.1 eV. Therefore, significantly lower capacitance of SDCN2A could be explained by the lack of N-Q and N-Ox groups of strongly electron-donating effect in graphene structures [12,31]. Considerations of the electrochemical properties of materials with well-developed surface should also take into account the sample SDCN1A since its BET surface is close to 2400 m2/g. While its microporosity is comparable to that of high surface materials, its mesopore structure resembles rather that of SDN1CA sample, discussed earlier. Also in respect of chemical structure, this sample differs from the other high surface materials. Although its nitrogen content (1.4 wt%) is similar to high surface samples SDN2CA and SDCN2A (1.2 and 1.0 wt%, respectively), the heteroatoms occur in a different form, mostly as N-5 species of beneficial influence on electron donicity. On the other hand, SDCN1A contains a relatively large amount of nitrogen in the surface functional groups (imines and nitryles) and only few exist as N-Q and N-Ox species, as similar

as it is for the sample SDN1CA (Table 5), which is rather not in favour of electron donicity. Moreover, it is characterised by lower degree of oxidation (Table 2), lower alkalinity of surface and balanced contribution of individual acidic groups with slight domination of phenolic groups (Table 3). The intermediate structural and chemical properties of this material are accompanied by the intermediate capacitance performance. Despite a much better developed BET surface area, its capacitance is only slightly higher than the lowest one obtained for SDN1CA. However, its charge exchange dynamics is much better than that of the latter sample if used as the negative electrode material, particularly in an alkaline environment. Active carbons ammoxidised at the last stage of processing, i.e. after activation (SDCAN) make another group of samples. They differ from the other materials both in the structure and the capacitance performance because the nitrogen introduced into these samples was not modified at higher temperatures. As a result, they contain much more nitrogen and only a small part of it is incorporated into the graphene structures. Instead, nitrogen heteroatoms occur mainly in the form of surface species confirmed by the XPS peak at 398.1 eV, i.e. aromatic imines and/or nitriles. It should be assumed that also nitrogen with the binding energy of ca. 399 eV, most frequently identified as N-6 type, occurs in these

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0.35 0.3 0.25

2000

0.2 1500 0.15 1000

Vmeso (cm3/g)

Specific surface (m2/g)

2500

0.1

500

0.05

0

0 0

5

10

15

20

25

30

Surface functional groups of nitrogen (%) Fig. 7. Variations of BET surface (D), micropore surface (h) and mesopore volume (s) with nitrogen content in surface functional groups of binding energy 398.1 ± 0.1 eV (aromatic imines and/or nitriles) for nitrogen-enriched samples.

samples rather in the form of surface amines and amides of a similar binding energy (Table 5). Thus, both samples ammoxidised after activation differ from all the other ones mainly by the fact that they contain a large amount of nitrogen in form of surface functional groups and the content of pyrol nitrogen N-5, responsible for effective electron donicity of the graphene structures, is much lower. Moreover, they also contain the lowest amount of oxygen phenolic functional groups favouring the electron donor properties [12] (Table 3). Unfavourable chemical structure of the samples is accompanied by poorly developed porous structure (Table 6). Besides the limitations in BET surface area and microporous structure, also mesopores are substantially blocked in these samples. As follows from the relations presented in Fig. 7, the structural limitations can be a consequence of the presence of the nitrogen surface species of the binding energy of ca. 398.1 eV. Because of their specific structure and chemical composition, the two samples ammoxidised after activation (SDCAN) show the lowest specific capacity C, particularly in acidic environment, despite almost six-times higher nitrogen content. Voltammograms in Fig. 5 show that one of the reasons for capacitance deterioration in the acidic environment can be the suppression of Faraday redox reactions, manifested by the lack of the characteristic peaks in the voltammograms, which were observed for all other nitrogen-enriched materials in electrodes of both signs. It is worth noting that in spite of the deterioration of C, the specific surface capacity of the materials Cs is not decreased. Moreover, these samples reveal beneficial rectangular shape of the voltammograms and their leakage currents are much lower than those of the other ammoxidised samples (Table 7). In alkaline environment, the capacitance of samples ammoxidised after activation (SDCAN) is also aggravated as in the acidic environment, however, it is of less concern to the negative electrodes (Table 8 and Fig. 4). Since the shapes of voltammograms obtained for the positive electrodes in alkaline environment are similarly unsatisfactory as for all samples studied (Fig. 6), it seems that their limited capacitance is a result of less favourable matching of the porous structure and electrolyte. Nevertheless, since the differences in capacitance are not proportional to the textural changes in the samples, we might expect that the chemical composition of active carbons is also important. Voltammograms of the negative electrodes of SDCAN look like the extension of curves determined for the positive electrodes, so they are in contrast to those of all the remaining materials. Therefore, in contrast to the positive electrodes, their capacity in alkaline medium advantageously increases with the increasing charge level of the capacitor.

Similar to the acidic environment, these samples do not show peaks assigned to the Faraday redox reactions, which are quite apparent in all other materials, except SDN1CA, at about 0.78 V in the anodic potential sweep direction (Fig. 6). Applying the capacitance and charge exchange dynamics criteria to all materials investigated, apparently the best performance is obtained in alkaline environment for the negative electrode made of SDN2CA. Unfortunately, positive electrode of the same material reveals much worse charge exchange dynamics. The superiority of the negative electrode is manifested by peculiar peaks in the voltammogram during the change in the current direction (Fig. 6), resulting from non-uniform voltage distribution between the capacitor electrodes [33]. Sample SDCN2A is the most suitable material for positive electrode of an alkaline capacitor. In an acidic capacitor, SDN2CA is the best material for negative electrode, while SDCN2A or SDN2CA could be used for positive electrode, although for this application non-ammoxidised sample SDCA seems to be a much better choice.

4. Conclusions According to the above-discussed results, the amount of nitrogen incorporated into the carbon structure depends on the stage at which the ammoxidation is performed and the temperature of the process. The greatest amount of nitrogen is incorporated into the demineralised coal, less into the activated one and the least to the carbonised one. In the case of activated and carbonised materials (in contrast to demineralised coal), nitrogen enrichment is more efficient at higher ammoxidation temperature of 350 °C than at lower of 300 °C. The stage of the coal processing at which the ammoxidation is performed and also the temperature of the process influence the acid–base properties of the products. The XPS results have shown that in the samples ammoxidised as a precursor and carbonisate the dominant nitrogen species is N-5 type (pyrrole and pyridone), while in the samples ammoxidised after activation, the surface nitrogen species, such as imines and/or nitriles, are predominating. The greatest changes in the composition of nitrogen species take place in the samples ammoxidised as a precursor, at 350 °C, followed by carbonisation and activation. In this sequence of processes, the highest amount of nitrogen, with regard to all the samples studied, is incorporated into graphene planes in the form of N-Q and the largest number of oxidised (N-Ox) species is formed.

R. Pietrzak et al. / Fuel 89 (2010) 3457–3467

All the active carbon samples obtained are characterised by a well-developed surface area with the dominant micropores. Structural studies revealed that samples ammoxidised at the stage of demineralisation or carbonisation have much higher BET surface area and micropore surface than the unmodified sample. Irrespective of the ammoxidation mode, i.e. whether performed on precursor or carbonisate, the increased temperature of this process enhances the development of surface area. If ammoxidation is applied in the last stage of the processing, the BET surface area and the surface of micropores decrease as compared to the unmodified active carbon. Electrochemical behaviour of the materials studied depends on their spatial structure and chemical composition. Because of the presence of heteroatoms in carbon structures, the type of species formed is more important than their amounts. Nitrogen heteroatoms introduced correctly into carbon structures are responsible for increased capacity and improved charge exchange dynamics. The N-5 and N-Ox nitrogen species are the most effective configurations in this respect. The presence of nitrogen in the form of surface groups, particularly those with the binding energy of 398.1 eV, seems to exert negative influence on the BET surface area and, in consequence, contribute to the deterioration of capacitance behaviour of the amoxidised materials. Oxygen heteroatoms are important in the modification of capacitance characteristics through pseudocapacitance as well, especially if they exist as phenolic or/ and alkaline functional groups. The results clearly show that the investigation of the influence of heteroatoms, particularly nitrogen heteroatoms, on the capacitance of carbon materials and optimisation of design of capacitors which employ the effect of pseudocapacitance require individual approach to the electrodes of opposite polarities. Acknowledgement This work was supported by The Polish Ministry of Science and Higher Education Projects No. N N204 056235 and BW 31-161/ 2008. References [1] Pandolfo AG, Hollenkamp AF. Carbon properties and their role in supercapacitor. J Power Sources 2006;157:11–27. [2] Kierzek K, Fra˛ckowiak E, Lota G, Gryglewicz G, Machnikowski J. Electrochemical capacitors based on highly porous carbon prepared by KOH activation. Electrochim Acta 2004;49:515–23. [3] Zeng X, Wu D, Fu R, Lai H, Fu J. Preparation and electrochemical properties of pitch-based activated carbon aerogels. Electrochim Acta 2008;53:5711–5. [4] Ruiz V, Blanco C, Granda M, Santamaria R. Enhanced life-cycle supercapacitors by thermal treatment of mesophase-derived activated carbons. Electrochim Acta 2008;54:305–10. [5] Lota G, Centeno TA, Frackowiak E, Stoeckli F. Improvement of the structural and chemical properties of a commercialactivated carbon for its application in electrochemical capacitors. Electrochim Acta 2008;53:2210–6. [6] Kim W, Joo JB, Kim N, Oh S, Kim P, Yi J. Preparation of nitrogen-doped mesoporous carbon nanopipesfor the electrochemical double layer capacitor. Carbon 2009;47:1407–11. [7] Ruiz V, Blanco C, Raymundo-Pinero E, Khomenko V, Beguin F, Santamaria R. Effects of thermal treatment of activated carbon on the electrochemical behaviour in supercapacitors. Electrochim Acta 2007;52:4969–73.

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[8] Hulicova D, Kodama M, Hatori H. Electrochemical performance of nitrogenenriched carbons in aqueous and non-aqueous supercapacitors. Chem Mater 2006;18:2318–26. [9] Bleda-Martinez MJ, Macia-Agullo JA, Lozano-Castello D, Morallon E, CazorlaAmoros D, Linares-Solano A. Role of surface chemistry on electric double layer capacitance of carbon materials. Carbon 2007;43:2677–84. [10] Bakhmatyuk BP, Venhryn BY, Grygorchak II, Micov MM. Influence of chemical modification of activated carbon surface on characteristics of supercapacitors. J Power Sources 2008;180:890–5. [11] Biniak S, S´wia˛tkowski A, Pakuła M. Electrochemical studies of phenomena at active carbon-electrolyte solution interfaces. In: Radovic JL, editor. Chemistry and Physics of Carbon. New York/Basel: Marcel Dekker; 2001. p. 125–225. [12] Jankowska H, S´wia˛tkowski A, Starostin L, Lawrynienko-Omiecyn´ska J. Adsorption of ions on activated carbon. Warsaw: PWN Publishing House; 1991 (in Polish). [13] Jurewicz K, Babeł K, Ziółkowski A, Wachowska H. Ammoxidation of active carbons for improvement of supercapacitor characteristics. Electrochim Acta 2003;48:1491–8. [14] Jurewicz K, Babeł K, Ziółkowski A, Wachowska H. Capacitance behaviour of the ammoxidised coal. J Phys Chem Solids 2004;65:269–73. _ [15] Bimer J, Sałbut PD, Berłozecki S, Boudou JP, Broniek E, Siemieniewska T. Modified active carbons from precursors enriched with nitrogen functions: sulfur removal capabilities. Fuel 1998;77:519–25. [16] Bagreev A, Menendez JA, Dukhno I, Tarasenko Y, Bandosz TJ. Bituminous coalbased activated carbons modified with nitrogen as adsorbents of hydrogen sulfide. Carbon 2004;42:469–76. [17] Pietrzak R, Wachowska H, Nowicki P. Preparation of nitrogen-enriched activated carbons from brown coal. Energy Fuels 2006;20:1275–80. [18] Pietrzak R, Wachowska H, Nowicki P, Babeł K. Preparation of modified active carbon from brown coal by ammoxidation. Fuel Process Technol 2007;88:409–15. [19] Pietrzak R, Jurewicz K, Nowicki P, Babeł K, Wachowska H. Microporous activated carbons from ammoxidised anthracite and their capacitance behaviours. Fuel 2007;86:1086–92. [20] Jansen RJJ, Bekkum H. Amination and ammoxidation of activated carbons. Carbon 1994;32:1507–16. [21] Jansen RJJ, Bekkum H. XPS of nitrogen-containing functional groups on activated carbon. Carbon 1995;33:1021–7. [22] Pels JR, Kapteijn F, Moulijn JA, Zhu Q, Thomas KM. Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis. Carbon 1995;33:1641–53. [23] Radmacher W, Mohrhauer O. Demineralizing of coal for analytical purposes. Brennstoff-Chemie 1956;37:353–8. [24] Boehm HP, Diehl E, Heck W, Sappok R. Surface oxides of carbon. Angew Chem, Int Ed Engl 1964;3:669–77. [25] Scofield JH. Hartree–Slater subshell photoionization cross-sections at 1254 and 1487 eV. J Electr Spectr 1976;8:129–37. [26] Biniak S, Szyman´ski G, Siedlewski J, S´wia˛tkowska A. The characterization of activated carbons with oxygen and nitrogen surface groups. Carbon 1997;35:1799–810. [27] Kapteijn F, Moulijn JA, Matzner S, Boehm HP. The development of nitrogen functionality in model chars during gasification in CO2 and O2. Carbon 1999;37:1143–50. [28] Stan´czyk K, Dziembaj R, Piwowarska Z, Witkowski S. Transformation of nitrogen structures in carbonization of model compounds determined by XPS. Carbon 1995;33:1383–92. [29] Qu D. Studies of the activated carbons used in double-layer supercapacitors. J Power Sources 2002;109:403–11. [30] Menendez JA, Phillips J, Xia B, Radovic LR. On the modification and characterization of chemical surface properties of activated carbon: in the search of carbons with stable basic properties. Langmuir 1996;12:4404–10. [31] Strielko VV, Kuts VS, Thrower PA. On the mechanism of possible influence of heteroatoms of nitrogen, boron and phosphorus in carbon matrix on the catalytic activity of carbons in electron transfer reactions. Carbon 2000;38:1499–503. [32] Strielko VV, Kartel NT, Dukhno IN, Kuts VS, Clarkson RB, Odintsov BM. Mechanism of reductive oxygen adsorption on active carbons with various surface chemistry. Surf Sci 2004;548:281–90. [33] Jurewicz K, Pietrzak R, Nowicki P, Wachowska H. Capacitance behaviour of brown coal based active carbon modified through chemical reaction with urea. Electrochim Acta 2008;53:5469–75.