Hierarchical porous carbons prepared from direct coal liquefaction residue and coal for supercapacitor electrodes

CARBON

5 5 ( 2 0 1 3 ) 2 2 1 –2 3 2

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Hierarchical porous carbons prepared from direct coal liquefaction residue and coal for supercapacitor electrodes Jianbo Zhang a, Lijun Jin

a,b

, Jie Cheng a, Haoquan Hu

a,*

a

State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China b Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, No. 130 Meilong Road, Shanghai 200237, China

A R T I C L E I N F O

A B S T R A C T

Article history:

Hierarchical porous carbons were prepared from a coal liquefaction residue (CLR) and two

Received 4 September 2012

coals, Shenhua (SH) coal with low and Shengli (SL) coal with high ash content, by KOH acti-

Accepted 17 December 2012

vation with the addition of some additives, and used as the electrode for supercapacitors.

Available online 24 December 2012

Two metal oxides (MgO and Al2O3) and three organic materials (sugar, urea and cetyltrimethylammonium bromide) were used as the additives, to investigate their effects on the structure and capacitive performance of the resultant carbons. The results show that the metal oxide and/or its salt formed by the reaction with KOH can serve as space fillers of nanopores in the carbonized carbon, while the gases produced by the decomposition of the organic additive can develop and/or widen some pores. Both help the carbon produced from CLR or the SH coal with low ash content to have additional meso- and macropores, but destroy the structure of the carbon from the SL coal with high ash content. Compared with the carbon without any additive, the optimized hierarchical porous carbon with each additive shows a smaller equivalent resistance, much higher capacitance in a wide range of charge–discharge rates and excellent cycle stability when the carbon was used as supercapacitor electrode. Ó 2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Supercapacitors, also called electrical double-layer capacitors, are regarded as a bridge between batteries and conventional dielectric capacitors and a promising solution to rapid storage and release of energy [1,2]. Because the capacitive behaviors of supercapacitors are closely related to the surface area of the formed interface between the electrode and electrolyte [1–3], the surface areas of the electrode materials play a great role on the performance of supercapacitors. Up to now, carbon materials [1,2], especially activated carbons (ACs) are the most widely used electrode materials for supercapacitors because of their large surface areas, high electronic conductivity, good

chemical stability and low cost. As known, the surface areas of ACs are mainly contributed by micropores. However, small pore sizes would increase the diffusion resistance of electrolyte ions through the pores, leading to a dramatic current-resistance drop of the electrode and further degrading the overall performance of supercapacitors [2,3]. In contrast, too many meso- or macropores in ACs result in a small surface area, thus the capacitance is not very high and the energy density is low [1–3]. Therefore, to lower the diffusion resistance of electrode materials and simultaneously to improve the overall performance of supercapacitors, hierarchical porous carbons (HPCs) with both micro– and meso–/macropores are promising electrode materials [4–6].

* Corresponding author: Fax: +86 411 84986157. E-mail address: [email protected] (H. Hu). 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.12.030

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However, the preparation strategy of HPCs is usually complicated and time-consuming due to the synthesis of some special nanostructure templates, polymerization, carbonization and additional activation [7–9]. As a result, the traditional preparation methods of HPCs show uncompetitive priceto-performance ratio compared with other electrode materials, and then restrict their commercial viability. We have so far pioneered a simple and effective method for preparing HPCs from direct coal liquefaction residue (CLR) by KOH activation with addition of some mineral matters (such as silica or silicate) [10]. Briefly, additional mineral salts or those formed by the reaction of the additive and KOH can serve as space fillers of nanopores in the thermoplastic CLR, and play a template role for the CLR-based porous carbon [10]. After washing off the mineral matters occupied the inner space of the carbonized sample, HPCs are formed with both micropores and mesopores. Herein, the previous work was extended and hierarchical micro–/macro–mesoporous carbons were prepared from CLR by KOH activation with addition of some other metal oxides, such as MgO and Al2O3, or organic materials that can easily be decomposed during heating, such as sugar, urea and cetyltrimethylammonium bromide (CTAB). And the preparation method was also applied to prepare coal-based carbons, i.e., two coals, Shenhua (SH) coal with low and Shengli (SL) coal with high ash content, were used as the carbon precursors. The resultant carbons were used as the electrodes for supercapacitors and their capacitive performances were investigated.

2.

Experimental

2.1.

Materials

SL coal, SH coal and the CLR from SH coal were used as the carbon precursors, respectively. They were crushed and sieved to be particle of 150–250 lm before use. Their proximate and ultimate analyses and ash compositions are listed in Tables 1 and 2. KOH was used as the activating agent. Two metal oxides, MgO (with an average particle size of 50 nm and Brunauer–Emmett–Teller (BET) surface area (SBET) of 20 m2/g from Nanjing High Technology Nano Material Co., China) and Al2O3 (with an average particle size of about 75– 150 lm from Sinopharm), and three organic materials, sugar (Tianjin Bodi Chemical Agent Co.), urea (Tianjin Damao Chemical Agent Co.) and CTAB (Tianjin Kermel Co.), were used as mineral and organic additives.

The additives selected in this work were based on the following factors. Al2O3 is similar to SiO2, knowing as an amphoteric oxide, and the latter has been confirmed to be an effective additive in KOH activation process for the CLR-based HPCs in our previous work [10], so it is easy to understand the utilization of Al2O3 as additive. MgO nanoparticles show chemical and thermal stability, no reaction with carbon up to the carbonization temperature and easy dissolution [11], and it has been testified to be an efficient template of porous carbons [12]. Therefore, MgO is expected to improve the CLR-based carbon pore structure and the capacitive performance in another way. Moreover, the utilization of organic materials, such as sugar, urea and CTAB, as the additives was inspired by the work of LozanoCastello´ et al. [13] that the release of some gaseous products (such as CO and CO2) from the carbon precursor during the carbonization process is beneficial for porosity development in the carbonized carbon. Because the CLR has a softening point of about 193 °C and the three organic materials show impressive devolatilization behaviors during the temperature between 200 and 400 °C (see the weight loss (thermogravimetric curve, TG) and weight loss rate (differential thermogravimetric curve, DTG) in Fig. 1, the three organic materials were used as additives and expected to play a role in improving the carbon structure.

2.2.

Preparation of HPCs

The HPCs were prepared by KOH activation with addition of the additive. The precursor and additive of the corresponding carbon samples are shown in Table 3. The synthetic procedure was similar to the traditional KOH activation process [14], involving mixing, carbonization and washing. Briefly, 5 g carbon precursor, 10 g KOH and certain amount of the additive were mixed in the mixed solvent containing tetrahydrofuran (THF, 50 ml) and deionized water (10 ml), stirring for 24 h at ambient temperature; then the mixture was evaporated in vacuum and dried at 120 °C overnight before carbonization. The carbonization was carried out in a horizontal tube furnace under nitrogen atmosphere (200 ml/min) with the following temperature program: heating from atmospheric temperature up to 400 °C with a heating rate of 2 °C/ min, followed by 1 °C/min to 600 °C and then keeping for 90 min, then 2 °C/min to 900 °C and holding for 240 min before cooling down. Finally, the carbonized samples were washed with 2 M NaOH, 2 M HCl and deionized water in sequence, and then dried at 120 °C. For comparison, the CLR

Table 1 – Proximate and ultimate analyses of the carbon precursors. Proximate analysis (wt.%, db)a

Sample

SH coal SL coal SH CLR a b c

Ultimate analysis (wt.%, daf)b

A

V

FCc

C

H

N

S

Oc

4.8 18.1 21.7

38.7 57.1 28.7

56.5 24.8 49.6

79.9 71.7 86.7

4.9 4.8 5.2

0.8 0.9 0.9

0.3 1.3 3.1

14.1 21.3 4.1

‘‘db’’ means dry basis. ‘‘daf’’ names dry ash-free basis. By difference.

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Table 2 – Ash compositions of the carbon precursors (wt.%). Sample

SiO2

Al2O3

CaO

SO3

Fe2O3

SH coal SL coal SH CLR

28.3 41.1 27.9

12.5 27.8 11.6

25.0 5.7 15.0

13.5 3.4 14.1

8.3 10.6 28.7

(a)

100

Na2O

Others

5.8 2.9 0.4

6.6 8.5 2.3

Table 3 – The precursor and additive of the carbon samples. Carbon samplea

Precursor

80

Additive Type

Mass (g)

– Al2O3 MgO CTAB Sugar Urea – Al2O3 MgO CTAB Sugar Urea – Al2O3 MgO CTAB Sugar Urea

– x x x x x – 3 2 0.5 1 3 – 3 2 0.5 1 3

Weight (%)

Sugar RC xAlRC xMgRC xCTAB–RC xSugar–RC xUrea–RC SHCC AlSHCC MgSHCC CTAB–SHCC Sugar–SHCC Urea–SHCC SLCC AlSLCC MgSLCC CTAB–SLCC Sugar–SLCC Urea–SLCC

60

Urea

CTAB

40

20

0 0

200

400

600

800

Temperature (oC)

Weight loss rate (% / oC)

(b)

0.00

-0.01

Sugar

Urea

a

The variable parameter, x, corresponding to the additive mass under constant carbon precursor mass of 5 g. For example, the sample 3AlRC means the carbon material prepared from 5 g CLR with addition of 3 g Al2O3 as the additive.

CTAB -0.02

-0.03

-0.04 0

200

400

600

800

Temperature (oC) Fig. 1 – TG (a) and DTG (b) analyses of the three organic additives under nitrogen of 60 ml/min.

or coal based carbon samples were also prepared with the same procedures as mentioned above but without any additive. In particular, all the samples were prepared with the same KOH dosage, and the resultant carbons were crushed and sieved to particle size of about 50 lm before use.

2.3.

SH CLR SH CLR SH CLR SH CLR SH CLR SH CLR SH coal SH coal SH coal SH coal SH coal SH coal SL coal SL coal SL coal SL coal SL coal SL coal

Characterization

Scanning electron microscope (SEM, NOVA NANO SEM450) was used to record the morphology of the sample. N2 adsorption/desorption isotherms of the carbons were measured at 196 °C on a physical adsorption apparatus (ASAP 2420) and the information on pore structure was obtained by using BET and Barrett–Joyner–Halenda (BJH) methods. The micropore volume (Vmic) was estimated by using the t-plot method,

and the microporosity was determined by the ratio of Vmic to the total pore volume (Vt), Vmic/Vt. The FT-IR spectrum was collected on an EQUINOX55 FTIR spectrometer. Ash contents of the carbons were determined by burning the samples at 900 °C under air flow of 60 ml/min in a thermogravimetric analyzer (Mettler Toledo TGA/SDTA851e). Ultimate analyses of the carbon samples were conducted on an elemental analyzer (Vario EL III, Elementar). The carbon yield was calculated as the percentage of the product weight versus the initial precursor weight.

2.4.

Electrochemical measurements

To prepare the electrode, the fine powders (50 lm) of HPCs were mixed with certain polytetrafluoroetylene (PTFE) as the binder along with the weight ratio of 95:5 (HPCs/PTFE). The mixture was pressed on a nickel foam current collector, with the ratio of the mass and the area at 10 ± 2 mg/cm2. And the mass of each electrode disc was kept at about 5 mg. A conventional three electrode system, with a platinum plate as the counter and saturated calomel electrode as the reference electrode, was used for cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) measurements. All the curves were recorded on

CARBON

5 5 ( 2 0 1 3 ) 2 2 1 –2 3 2

an electrochemical workstation (CHI760D), and the electrolyte was 6 M KOH aqueous solution. The CV and galvanostatic charge/discharge measurements were subjected with three cycles between 0.0 and 1.0 V, and the two segments of the third cycle were used. The specific capacitance based on the R dt , where, CV was determined by the equation [15–17], C ¼ i du C is gravimetric capacitance determined as the average value by integrating over the full CV curve, i is current density, t is the time and u is the potential.

3.

Results and discussion

3.1.

CLR-based carbons

3.1.1.

Capacitive performances

(a)

1.6

3Sugar-RC 1Sugar-RC

Current density (A/g)

224

0.8

5Sugar-RC 0.0

9Sugar-RC

7Sugar-RC -0.8

-1.6

3.1.2.

Surface and structural properties

Fig. 4 shows nitrogen adsorption/desorption isotherms and pore size distribution curves of the CLR-based carbon samples. All the isotherms are assigned to typical type IV, obviously exhibiting hysteresis loops at the relative pressure higher than 0.4, indicative of the carbon materials with mesopores. When the relative pressure is below 0.1, the existence of the distinctly adsorbed amount indicates the samples have

-1.0

-0.6

-0.8

-0.4

0.0

-0.2

Potential (V)

(b)

Capacitance (F/g)

To investigate the effect of the additives on the capacitances of the carbon materials, several groups of samples were prepared from the CLR with addition of different amounts or types of the additives, respectively. The resultant carbons were used as the electrode materials for supercapacitors, and the capacitance was measured by CV at a scan rate of 5 mV/s. Fig. 2a and b show the representative CV curves and the corresponding capacitances of the electrodes from CLRbased carbons prepared with different dosages of the sugar additive. From Fig. 2a, all the CV curves are nearly symmetric on the positive and negative sweeps and present a rectangular-like shape. In the middle potential region, no distinct peaks are presented on the CV curves, indicating the absence of strong pseudocapacitance (faradaic) phenomena [18]. At extreme potentials, close to 0.0 and 1.0 V, the electron transfer reactions are probably affected by the solvent or some surface functional groups of the electrode materials, which further contribute to the measured current [18]. However, the formed areas by the CV curves decrease with the increasing amount of the additive. This reflects that the additive overdose can affect the pore structure of the carbon electrode, and result in the increase of ohmic resistance for electrolyte motion in the carbon pores [19]. As a result, the capacitances obtained from the CV curves decrease from 169 to 126 F/g with the increasing additive dosage, see Fig. 2b. Clearly, 1Sugar–RC based electrode shows the highest capacitance, indicating the optimum additive amount is 1 g for xSugar–RC carbon group. The capacitance of 1Sugar–RC is 33% higher than that of the electrode prepared from CLR-based carbon RC, confirming the impressive contribution of the additive. Similarly, the electrodes obtained from 3AlRC, 2MgRC, 0.5CTAB–RC and 3Urea–RC exhibit the highest capacitance among their groups, see Fig. 3a and b, and the capacitances are up to 166, 185, 186 and 174 F/g, respectively, displaying 31%, 46%, 46% and 37% higher than that of the RC electrode.

200

150

100

50

0 x= 0

1

3

5

7

9

xSugar-RC Fig. 2 – CV curves (a) and the corresponding capacitances (b) of the electrodes from the CLR-based carbons prepared with different dosages of the sugar additive.

some micropores, especially the samples with the isotherms concave to the P/P0 axis [20,21]. Fig. 4b shows the pore size distribution curves obtained by the BJH method from desorption branches. The pore sizes of all the samples concentrate at about 3–5 nm except the 2MgRC which has a wide pore size distribution. Correspondingly, the textural properties of the carbon samples are listed in Table 4. Apparently, the CLR-based carbons show similar yields but different SBET and Vt, confirming the effect of the additive on the carbon structure. 3AlRC and 2MgRC show lower microporosities than RC, while 0.5CTAB– RC, 1Sugar–RC and 3Urea–RC show higher microporosities. This indicates that the mineral additives may be conducive to forming more meso- and/or macropores in the CLR-based carbons, but the organic additives are helpful for keeping and/or producing more micropores. The SEM images of CLR-based carbons prepared with different additives are shown in Fig. 5, indicating the changed carbon structures caused by the additives. Compared with the SEM image of RC (Fig. 5e) that shows no distinct pores under the magnification, many mesopores with a size of

200

150

(a)

C0 100

50

0 x= 1 2

3 4 5

800

600

400

200 0.0

1 2 3 4 5

xMgRC xAlRC Samples prepared with the mineral additive

(b)

225

5 5 (2 0 1 3) 2 2 1–23 2

Volume adsorbed (cm3/g, STP)

(a)

Capacitance (F/g)

CARBON

0.2

0.4

0.6

Relative pressure ( P/P0 )

(b)

200

RC 2.0

3AlRC 2MgRC

150

C0 100

50

0.5CTAB-RC

1.5

dV/dlogD

Capacitance (F/g)

1.0

0.8

1Sugar-RC 3Urea-RC 1.0

0.5

0.0

0

0 2. 5 1. 0 1. 5 0. 25 0.

x=

1

3

5

7

9

xCTAB-RC xUrea-RC Samples prepared with the organic additive

1

10

100

Pore diameter (D, nm)

Fig. 3 – The capacitances of the CLR-based carbon electrodes prepared with the mineral (a) or organic additives (b).

Fig. 4 – N2 adsorption/desorption isotherms (a) and the corresponding pore size distributions (b) of the CLR-based carbons prepared with/without the additives.

about 40 nm can be seen on the surface of 3AlRC (Fig. 5a), while 2MgRC sample (Fig. 5b) presents dispersed particles with sizes of about 40–500 nm. According to our previous works [10,14], the mineral salts formed during KOH activation process can play a template role for the CLR-based porous carbon, and serve as space fillers of nanopores in the thermoplastic CLR. After washing off the intrinsic and the additional mineral matters in the carbonized carbon, hierarchical porous structures were formed. Therefore, it is easy to understand the formation of the pore size of about 40 nm in 3AlRC (Fig. 5a), while some macropores are probably caused by the aggregation of some salt particles. Combined with the results from nitrogen adsorption analyses and the SEM image, it is clear that 3AlRC is a hierarchical micro–/ macro–mesoporous carbon. Although the MgO additive is not reacted with the carbon matrix or KOH [11], it has been confirmed to be an effective template of porous carbons [12]. However, because of the low density and relative large amount of the additional MgO and the good solubility of the CLR in the THF solution (about 50 wt.% of the CLR is soluble in THF according to the results of Soxhlet extraction), it can be inferred that the carbon matrix would be mixed evenly

with MgO particles, resulting in the carbon particles separated from each other after carbonization. As a consequence, the resultant carbon 2MgRC becomes dispersed particles (Fig. 5b) after washing off the MgO particles, and shows large external surface area of about 1922 m2/g (obtained by the tplot method). The hierarchical micro–/macro–mesoporous structure of 3AlRC and the large external surface area of 2MgRC are helpful for the electrolyte ion transfer and diffusion, and further promoting the overall performance of supercapacitors [3,23]. Additionally, SEM images in Fig. 5 also provide strong evidences for the effects of the organic additives on the carbon structures. Many meso- and macropores were formed on the surfaces of 3Urea–RC (Fig. 5c), 0.5CTAB–RC (Fig. 5d) and 1Sugar–RC (Fig. 5f). Specially, the emerged pores on the 3Urea–RC (Fig. 5c) are relatively uniform and highly dispersed, with the sizes of 20–50 nm; while hierarchical meso- and macropores were produced on 0.5CTAB–RC (Fig. 5d) and 1Sugar–RC (Fig. 5f). The differences are probably caused by the different decomposition rates of the additives (see Fig. 1) because the slower and more controlled evolution of gaseous products would correspond to develop more uniform and

226

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Table 4 – Yields and the textural properties of the CLR-based carbon samples. Sample RC 3AlRC 2MgRC 0.5CTAB–RC 1Sugar–RC 3Urea–RC a

Yield (%)

SBET (m2/g)

Smic (m2/g)

Vt (cm3/g)

Vmic (cm3/g)

Vmic/Vt

Dava (nm)

42 45 46 41 40 41

1634 1507 2061 1665 1597 1530

295 78 139 853 754 467

1.00 0.98 1.24 0.81 0.81 0.89

0.14 0.05 0.09 0.35 0.31 0.20

0.14 0.05 0.07 0.43 0.38 0.23

2.45 2.60 2.41 1.95 2.03 2.33

Average pore width, determined by the equation of 4Vt/SBET [22].

Fig. 5 – SEM images of the CLR-based carbons prepared with different additives: (a) 3AlRC, (b) 2MgRC, (c) 3Urea–RC, (d) 0.5CTAB–RC, (e) RC, (f) 1Sugar–RC, (g) 3Sugar–RC, (h) 5Sugar–RC, and (i) 7Sugar–RC.

highly dispersed porosity [13]. Combined with the micropores produced by KOH activation, hierarchical micro–/macro–mesoporous structure is obtained and provides favorable transport channels for electrolyte ions [4–6], further promoting the capacitive performance of the carbon electrode. However, the hierarchical porous structure would be destroyed by the rapid release of large amounts of gases in the carbonization process with the excess additive. For example, the obtained carbon 3Sugar–RC (Fig. 5g) shows more large pores on its surface than 1Sugar–RC (Fig. 5f), while 5Sugar–RC (Fig. 5h) and 7Sugar–RC (Fig. 5i) have the surface pores almost completely collapse. Consequently, an optimized amount of the additive exists for the carbons used as electrodes, which has been confirmed by the results shown in Figs. 2 and 3. Since the additive greatly affects the resultant carbon structure that is closely related to the capacitive performance of the carbon electrode [2,3], it is believed that good

capacitive performance can be obtained by well controlled utilization of the additive dosage. In the following sections, the optimized CLR-based carbons were taken as the examples to elucidate more information about the capacitive performance.

3.1.3.

Effect of heteroatom

Generally, the heteroatom, especially nitrogen- and/or oxygen-containing surface groups on carbon electrodes can introduce the acid–base properties to the carbon, and further give rise to Faradaic pseudocapacitive reactions [4,24,25]. Since nitrogen- and/or oxygen-containing groups are often present on the carbon surface as a result of activation processes or as a residue from the carbon precursor and/or the additive [25–27], the pseudocapacitive contribution to the overall capacitive performance of the carbon electrode should be taken into account.

CARBON

(a)

RC

25

2.0 1.5

20

-Z'' (Ohm)

3AlRC

Transmission (a.u.)

227

5 5 (2 0 1 3) 2 2 1–23 2

2MgRC 3Urea-RC

1.0 0.5

15 0.0

0

2

4

6

10

0.5CTAB-RC

5 1Sugar-RC

0

0

5

10

15

20

25

Z' (Ohm) 4000

3200

2400

1600

800

(b)

Wavenumbers (cm-1) Fig. 6 – FT-IR spectra of the CLR-based carbons prepared with/without the additives.

RC 3AlRC 2MgRC 1Sugar-RC 3Urea-RC 0.5CTAB-RC

180

Capacitance (F/g)

Fig. 6 shows the FT-IR spectra of the CLR-based carbons prepared with/without the additive. All the carbons display noticeable peaks at about 3400 and 1600 cm1. The former may belong to O–H or N–H stretching vibration, while the latter is probably ascribed to C@C or C@O stretching vibration. When the additives were used, the resultant carbons show new small peaks at about 1200 cm1 due to C–O–C or C–O stretching modes. The unique peaks at 1600 and 1200 cm1 may indicate the formation of ester group. However, element analyses in Table 5 show that the ratio of N/C or O/C in the carbons don’t increase distinctly but keep the initial level in spite of the additional additives. In contrast, some of the samples (such as 3AlRC and 2MgRC) have lower amount of oxygen. Additionally, the similar ash contents (no more than 0.60 wt.%, see Table 5) probably indicate that little mineral was introduced into the carbons after washing. More importantly, no conspicuous humps can be observed on the CV or charge–discharge curves, indicating that no oxidation–reduction reaction is caused by the residual heteroatom [4]. In other word, hardly any pseudocapacitance contributes to the total capacitances of the carbon electrodes although the additives were introduced.

240

120

60

0 -2

0

10

10

2

10

4

10

Frequency (Hz) Fig. 7 – Nyquist plots of electrochemical impedance (a) and the curves representing the capacitance-frequency dependence (b) of the CLR-based carbon electrodes prepared with/without the additives.

3.1.4.

Capacitive behaviors

To understand the effect of frequency on the carbon electrodes, EIS tests were carried out and the typical Nyquist diagrams are shown in Fig. 7a. A semicircle was observed at the high frequency region on each curve, and the diameter of the semicircle in the impedance spectra of the carbon electrode increases with the order of 1Sugar–RC < 3Urea–RC < 0.5CTAB–

Table 5 – Ultimate analyses of the CLR-based carbons prepared with/without the additives. Sample

RC 3AlRC 2MgRC 0.5CTAB–RC 1Sugar–RC 3Urea–RC a b c

Asha

Composition (wt.%, daf)

(wt.%)

C

H

N

S

O

0.54 0.60 0.57 0.59 0.58 0.53

85.89 90.33 87.47 86.18 84.70 85.02

1.30 1.48 1.57 1.65 2.38 2.04

0.16 0.21 0.20 0.22 0.17 0.25

0.11 0.09 0.12 0.08 0.12 0.10

12.54 7.89 10.64 11.87 12.63 12.59

Determined by TG analysis in air at 900 °C. By difference. The ratio of the weight.

N/Cc

O/Cc

0.002 0.002 0.002 0.003 0.002 0.003

0.15 0.09 0.12 0.14 0.15 0.15

b

5 5 ( 2 0 1 3 ) 2 2 1 –2 3 2

CARBON

RC < 2MgRC < 3AlRC < RC. Correspondingly, it can be assumed that their equivalent resistances increase in the same sequence [18], with the values of 0.57, 0.76, 0.80, 1.67, 3.62 and 4.25 O, respectively. At the low frequency region, the nearly perpendicular lines (except that of 3Urea–RC) show the pure capacitive behavior of the carbon electrode. The deviation observed for 3Urea–RC (a small high-frequency arc) probably indicates that the small and uniform pore size hinders the electrolyte ion diffusion within the pores to some extent [28]. To sum up, the utilization of the additive can reduce the equivalent resistance of the carbon electrode while the pure capacitive behavior is nearly kept. Fig. 7b shows the capacitance values of the carbon electrode as a function of frequency. The specific series capacitance (Cs) was calculated by the equation, Cs = 1/(2pfZ00 ), where f is the frequency and Z00 is the imaginary part of the impedance. As expected, all the curves show a typical drop of capacitance with frequency. Owing to the kinetic dependence of the ion accessibility to the porous network [28], the maximum capacitance occurs at the lowest frequency (about 0.01 Hz), with the value of 118 F/g for RC, 172 F/g for 3AlRC, 198 F/g for 2MgRC, 164 F/g for 1Sugar–RC, 196 F/g for 3Urea– RC and 205 F/g for 0.5CTAB–RC. These results are a little higher than those obtained from the CV curves mentioned above, which may be attributed to the high scan rate (5 mV/s) for CV curves since quick scan rates will lead to some diffusion limitation in some small micropores [29]. It looks that for

Current density (A/g)

(a)

40

200 mV/s

the carbons prepared with the additives the capacitance values even at 1 Hz still maintain higher than 90 F/g, especially those of 2MgRC and 0.5CTAB–RC higher than 150 F/g. That will be very interesting from practical point of view [29,30]. When the frequency is higher than 50 Hz, all the capacitances drop to lower than 30 F/g and become insignificant. The carbon 2MgRC was taken as an example to further study the capacitive behaviors by CV at different scan rates, galvanostatic charge–discharge measurements and cycle performance, and the results are shown in Fig. 8. From Fig. 8a, rectangular CV curves were observed at the scan rates no more than 20 mV/s, and the shape became slim and leaf-like with the increasing rates. The changes in the shapes of CV curves are caused by the finite time constant of the charging electrical double-layer capacitance [23]. Corresponding to the CV curves in Fig. 8a, the capacitances are shown in Fig. 8b, decreasing from 185 F/g at 5 mV/s to 100 F/g at 200 mV/s. Clearly, the rapid scan rate may make ions discharged or consumed only on shallow positions of the pores [23,30]. The capacitance is still as high as 100 F/g at the scan rate of 200 mV/s, indicative of the excellent capacitive behavior. As for the galvanostatic charge–discharge profiles in Fig. 8c, all the charge and discharge curves are very linear with negligible curvature at the current density ranging from 1 to 10 A/ g, with a typical triangular shape, further indicating the good capacitive properties. The specific capacitances are determined by the equation of Csp = IDt/(DVm), where I is the

(b)

200

Capacitance (F/g)

228

150

100 mV/s 20

80 mV/s 0

5 mV/s

20 mV/s -20

-40

-1.0

-0.8

-0.6

-0.4

-0.2

100

50

0

0.0

5

Potential (V)

(d)

0.0

Potential (V)

-0.2

5 A/g

10 A/g

-0.6 -0.8 -1.0

0

100

100

200

150

125

1 A/g -0.4

80

Scan rate (mV/s)

Capacitance (F/g)

(c)

20

200

Time (s)

300

400

100

75

50

0

1500

3000

4500

6000

Cycle number

Fig. 8 – Capacitive behaviors of the 2MgRC electrode: (a) CV curves at different scan rates, (b) the capacitances corresponding to the different scan rates, (c) galvanostatic charge–discharge profiles, and (d) variations of the specific capacitance versus cycle number (obtained from the CV curves at a scan rate of 200 mV/s).

CARBON

discharge current, Dt is the discharge time, DV is the potential range and m is the mass of the carbon used for the electrode. They are 190 F/g at 1 A/g, 157 F/g at 5 A/g and 137 F/g at 10 A/g, respectively, agreeing with the values obtained from the CV curves and EIS test. To clarify the cycling stability of the carbon electrode, 2MgRC endured 6000 cycles charge–discharge (shown in Fig. 8d) with a scan rate of 200 mV/s. The decrease of the capacitance, which is usually caused by the deterioration of irreversible side reactions, is not present. In contrast, the specific capacitance was found to keep increasing from the initial value of 100 to about 118 F/g after 2250 cycles, and then remain almost constant for the next 3750 cycles. Similarly, another two carbon electrodes from 3Urea–RC and 0.5CTAB–RC in the study were also found to show the increased capacitance in different degrees at the scan rate of 200 mV/s. However, the phenomena would be gone at lower scan rates. Based on the literature [31], one potential interpretation is that the increase of the capacitance is caused by the continuous impregnation of the electrolyte ions in the nanopores of the carbon electrode during the charge–discharge processes. As for the detailed reason, it will be investigated further in the future work. Considering the high capacitance in the wide range of charge–discharge rates and the pretty good cycle performance, 2MgRC presents as a promising

candidate for supercapacitor electrode. This excellent performance is closely correlated with the integrated advantage of KOH activation with addition of the additive, i.e., the combination of the large surface area, hierarchical porous structure, and the negligible effect of the heteroatom.

3.2.

Coal-based carbons

In the following section, the preparation method was expanded to coal-based carbons. SH coal with low (about 4.8 wt.%, see Table 1) and SL coal with high ash content (about 18.1 wt.%), were used as the carbon precursors to prepare carbons according to the optimized conditions mentioned above for supercapacitor electrodes.

3.2.1.

Capacitive performances

When SH coal was used as the carbon precursor, all electrodes prepared from the resultant carbon, AlSHCC, MgSHCC, Urea–SHCC, Sugar–SHCC and CTAB–SHCC show higher capacitances than that from SHCC (Fig. 9a), up to 147%, 143%, 160%, 157% and 123%, respectively. However, when SL coal was used as the precursor, none of the resultant carbon electrodes

(a)

150

CTAB-SHCC

Sugar-SHCC

Urea-SHCC

SHCC

50

MgSHCC

100

Volume adsorbed cm3/g, STP

200

AlSHCC

Capacitance (F/g)

(a)

229

5 5 (2 0 1 3) 2 2 1–23 2

800

600

400

200

0 0.0

SHCC series

(b)

0.2

0.4

0.6

(b)

200

dV/dlogD CTAB-SLCC

Sugar-SLCC

Urea-SLCC

SLCC

MgSLCC

100

AlSLCC

Capacitance (F/g)

1.0

2.0 SHCC AlSHCC Sugar-SHCC SLCC AlSLCC Sugar-SLCC

1.5 150

50

0.8

Relative pressure P/P0

0

1.0

0.5

0.0 1

0

SLCC series Fig. 9 – The capacitances of the coal-based carbon electrodes prepared with/without the additives: (a) SHCC series, and (b) SLCC series.

10

100

Pore diameter (D, nm) Fig. 10 – N2 adsorption/desorption isotherms (a) and the corresponding pore size distributions (b) of the coal-based carbons prepared with/without the additives.

230

CARBON

5 5 ( 2 0 1 3 ) 2 2 1 –2 3 2

Table 6 – Yields and the textural properties of the coal-based carbon samples. Sample SHCC AlSHCC Sugar–SHCC SLCC AlSLCC Sugar–SLCC a

Yield (%)

SBET (m2/g)

Smic (m2/g)

Vt (cm3/g)

Vmic (cm3/g)

Vmic/Vt

Dava (nm)

56 53 53 42 43 41

1600 1868 2018 1434 662 1366

798 381 530 493 320 399

0.78 1.07 1.13 1.15 0.39 0.83

0.33 0.17 0.23 0.22 0.14 0.17

0.42 0.16 0.20 0.19 0.36 0.20

1.95 2.29 2.24 3.21 2.36 2.43

Average pore width, determined by the equation of 4Vt/SBET [22].

prepared with the additives shows higher capacitance than that without any additive (Fig. 9b). The AlSLCC, MgSLCC, Urea–SLCC, Sugar–SLCC and CTAB–SLCC carbon based electrodes only have the capacitances up to about 70%, 95%, 81%, 94% and 85% of the SLCC based electrode, respectively. To better understand the coal-based carbon capacitive performance, the surface and structural properties of some carbons were also investigated.

3.2.2.

Textural properties

All the isotherms in Fig. 10a are typical type IV, exhibiting the HPC characteristics similar as the results obtained from Fig. 4. The hysteresis loop of SLCC isotherms is much wider than the others, indicating its different pore structure. And in Fig. 10b, the SL coal-based carbons show more concentrated pore sizes at around 3–5 nm than the SH coal-based carbons. According to the detailed data in Table 6, AlSHCC and Sugar–SHCC have larger SBET and Vt but smaller Smic and Vmic than SHCC. Obviously, the initial pores have a significantly expanded window size due to the effect of the additional mineral or organic additives. When SL coal was used as the carbon precursor,

(a)

AlSLCC and Sugar–SLCC show smaller SBET and Vt than SLCC, probably caused by the collapse of the pore structure. In other word, the additives do harm to the carbon from the coal with high ash content. Fig. 11 can further confirm the great effect of the additive on the pore structures of the coal-based carbons. SHCC (Fig. 11a) only shows some messy pores of 200–1000 nm on its surface. With the effect of the additive, AlSHCC (Fig. 11b) and Sugar–SHCC (Fig. 11c) exhibit much more uniform and highly dispersed mesopores on their surfaces. SLCC (Fig. 11d) shows more and smaller pores on its surface than SHCC despite that SL coal has higher ash content than SH coal. However, under the influence of the additive, the pore structure of AlSLCC (Fig. 11e) is destroyed while the pores on the surface of Sugar–SLCC (Fig. 11f) are broadened from the size of smaller than 300 to about 380 nm. Both of the two coals have lower ash contents than the CLR, but the additives show different effects on the resultant carbons. It is considered that the higher pyroplasticity of the CLR may enhance the liquidity of the carbon matrix during the heat treatment process, helping the homogeneous mixing of the carbon

(b)

1 µm

(d)

(c)

500 nm

(e)

1 µm

500 nm

(f)

1 µm

1 µm

Fig. 11 – SEM images of the coal-based carbons prepared with/without the additives: (a) SHCC, (b) AlSHCC, (c) Sugar–SHCC, (d) SLCC, (e) AlSLCC, and (f) Sugar–SLCC.

CARBON

5 5 (2 0 1 3) 2 2 1–23 2

matrix and the additive and further developing more uniform and highly dispersed pores. Otherwise, the additive only plays a positive role on the pore structure when the precursor has relative low ash content. Because the intrinsic mineral matters can also serve as the template of the resultant carbon pore structure [14], collapse of the pore structure would be caused by aggregation of the formed pores especially for randomly mixing the carbon matrix and the additive. Obviously, the addition of the organic materials into the carbon precursor will not increase the mineral content, but the gas releasing from the additive decomposition would result in wider pores and smaller SBET. As a result, the same additive is helpful for promoting the capacitances of SH coal-based carbon electrodes (see Fig. 9a) but suppressing those of SL coal-based carbon electrodes (see Fig. 9b) owing to the smaller accessible surface areas.

4.

Conclusions

HPCs can be prepared from CLR or the coal with low ash content by KOH activation with the addition of some mineral or organic additives. The additive type and amount have great effect on the carbon structure and capacitive performance. The metal oxide and/or its salt formed with KOH can serve as space fillers of nanopores in the carbonized carbon, while the gases produced by the decomposition of the organic additive can develop and/or widen some pores. But the additive will destroy the structure of the carbon from the coal with high ash content. As for the CLR-based carbon, 3AlRC, 2MgRC, 1Sugar–RC, 0.5CTAB–RC and 3Urea–RC show the optimized addition amounts of the additives, and their corresponding electrode capacitances are up to 166, 185, 169, 186 and 174 F/g at the scan rate of 5 mV/s, respectively, displaying 31%, 46%, 33%, 46% and 37% higher than the electrode prepared from the carbon RC without additive. It is attributed to that the hierarchical porous structures lead to smaller equivalent resistances. The capacitance values of electrodes from 2MgRC and 0.5CTAB–RC can maintain higher than 150 F/g even at 1 Hz. Further study shows that the 2MgRC based electrode presents the capacitance up to 100 F/g at 200 mV/s, 137 F/g at 10 A/g and excellent cycle stability after 6000 cycles. Higher capacitances can be obtained when the optimized preparation conditions were used for SH coal-based carbons, but it is in vain and even worse for the carbons from SL coal with high ash content. Additionally, hardly any pseudocapacitance contributes to the total capacitances of the carbon electrodes although the additives were introduced.

Acknowledgements This work was supported by the Natural Science Foundation of China (Nos. 20906009, 51134014), the National Basic Research Program of China (973 Program), the Ministry of Science and Technology, China (No. 2011CB201301), and the Fundamental Research Funds for the Central Universities (No. DUT12JN05).

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