Tunable N-doped or dual N, S-doped activated hydrothermal carbons derived from human hair and glucose for supercapacitor applications

Electrochimica Acta 107 (2013) 397–405

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Tunable N-doped or dual N, S-doped activated hydrothermal carbons derived from human hair and glucose for supercapacitor applications Weijiang Si a , Jin Zhou a,∗ , Shumei Zhang a , Shijiao Li a , Wei Xing a,b , Shuping Zhuo a,∗ a b

School of Chemical Engineering, Shandong University of Technology, Zibo 255049, PR China School of Science, China University of Petroleum, 66 Changjiang West Road, Qingdao 266580, PR China

a r t i c l e

i n f o

Article history: Received 21 May 2013 Received in revised form 18 June 2013 Accepted 19 June 2013 Available online 27 June 2013 Keywords: Supercapacitor Hydrothermal carbon N-doped S-doped Human hair

a b s t r a c t In this work, human hair was firstly used to prepared heteroatom-doped carbon materials. Tunable Ndoped or dual N, S-doped microporous carbons were successful prepared using glucose and human hair as carbon precursors via a method of hydrothermal carbonization procedure and sequent KOH activation. The electrochemical capacitive performance of the prepared carbons was investigated in KOH electrolyte. The morphology, structure and surface properties of the carbon materials are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption, X-ray photoelectron spectroscopy (XPS), Energy Dispersive Spectroscopy (EDS) and Fourier transform infrared spectroscopy (FT-IR). The N- or S-doping could be tuned by controlling the dosage of human hair additives. Due to the synergistic effect of multi N, O, S-doped species, the prepared carbons showed large pseudo-capacitance. The capacitance of AHC-4 could reach 264 F g−1 in KOH electrolyte. AHC-1 shows an acceptable cycle life with relatively high capacitance value of 154 F g−1 after 1000 cycles. The present preparation method is attractive and inspiring for preparation of tunable dual heteroatom-doped carbon materials with high surface area and capacitive performance using renewable biomass. Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction Supercapacitors have attracted much interest and attention due to their high power density, long cycle life and reversibility [1–3]. Based on the charge storage mechanism, supercapacitor can be divided into electrochemical double layer capacitors (EDLCs) and pseudo-capacitors. Energy storage in EDLCs depends on the charge uptake at the electrolyte/porous electrode interfacial regions. EDLCs require electrode materials with high effective surface area and pore adapted to the size of ions. In pseudo-capacitor, the energy is stored through reversible redox reactions of the electroactive species. Pseudo-capacitive materials, mainly metal oxides and electrical conductive polymers, have been extensive used to deliver high specific capacitance. However, poor cycle stability, low conductivity and high price have limited the practical application of pseudo-capacitive materials [4]. Porous carbon materials are the most promising electrode for EDLCs due to their high electrical conductivity, electrochemical inertness and high specific surface area [5]. Consequently, carbonbased EDLCs play a main part in the commercial supercapacitors.

∗ Corresponding authors. Tel.: +86 533 2781664; fax: +86 533 2781664. E-mail addresses: [email protected], [email protected] (J. Zhou), zhuosp [email protected] (S. Zhuo).

Various carbon materials with tailored porosity, including activated carbons [6–9], ordered micro/mesoporous carbons [10–12], hierarchical porous carbons [13,14], carbide-derived carbons [15] and graphene-based materials [16–19], have been applied in EDLCs. However, the preparation for these carbon materials required expensive and non-renewable raw materials, costly inorganic templates, tedious preparation step, or lots of time and energy. For example, the activated carbons are typically synthesized from special petroleum coke, the ordered micro/mesoporous carbons are prepared by a nanocasting method using expensive zeolites or mesoporous silica oxides as templates [20,21], Phenolformaldehyde based ordered mesoporous carbons prepared by soft-template method need chemically synthesized amphiphilic copolymer [22,23], and the carbide-derived carbons are prepared by demetallation of metal carbides under high temperature chlorine treatment (Toxic!) [24]. Considering the potential scale of supercapacitor applications, the development of low-cost carbon materials from renewable raw materials is even more worthwhile. Biomass materials are renewable, easily available and very cheap, which are potential raw materials for the preparation of porous carbons with good capacitive performance [25–28]. Furthermore, heteroatom rich carbons can be obtained by the selecting a suitable oxygen or nitrogen containing biomass as carbon precursors, such as sodium alginate [29] and bean dregs [30]. That is very attractive because recent

0013-4686/$ – see front matter. Crown Copyright © 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.06.065

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studies have proved that the heteroatom species in the carbon framework, such as oxygen or nitrogen species could induce additional pseudo-capacitance via revisable redox reaction and improve the wettability between the electrodes and electrolytes (usually hydrophilic electrolyte), consequently improve the whole capacitive performance of the supercapacitor [31–34]. Biomass could be converted into valuable carbon materials via hydrothermal carbonization approach using mild processing temperature and water as the carbonization medium under selfgenerated pressure [35–37]. Hydrothermal carbons are always oxygen-rich due to the nature of biomass precursor. Other functionalities, like nitrogen species, can be induced in the final carbonaceous materials by selecting appropriately the initial carbon precursor or additional additives [38,39]. Human hair is a filamentous biomaterial that ceaselessly grows from follicles of dermis in the major part of lifetime. Hair is one of the defining characteristics of mammals. Most common interest in hair is focused on hair growth, hair types and hair care, but hair is also an important renewable biomaterial primarily composed of protein, notably keratin. Keratins have large amounts of cysteine, a sulfur-containing amino acid (more than 10 wt% of hair). Therefore, nitrogen or sulfur species may be introduced into the target carbon product when using human hair as additives in a hydrothermal carbonization approach. In the present study, human hair was used to prepare heteroatom-doped activated hydrothermal carbon materials for the first time. KOH activation was employed to develop the porosity after the hydrothermal carbonization procedure. Tunable N-doping or dual N, S-doping of the activated carbons could be easily achieved by adding different dosage of human hair as additive in the hydrothermal carbonization process. The large heteroatomdoping exhibits positive synergistic effect on the whole capacitive performance of the prepared carbons. The prepared carbons show excellent gravimetric capacitance, up to 264 F g−1 in 6 mol L−1 KOH electrolyte. 2. Experimental 2.1. Material preparation Chinese hairs were collected from barbershop and were prior washed to remove the grease on the hair surface. All of the other chemicals were analytical grade and were purchased from Sinopharm Chemical Reagent Corporation without further treatment. Firstly, 6 g of the clean hairs were dissolved in 36 mL of 2 mol L−1 NaOH solution. This solution was filtered to remove the deposition. 2 g of glucose and a certain volume of the hair solution (2, 4, 6, or 8 mL) were mixed together, and were subsequently diluted to 20 mL. The as-prepared mixture was sealed into Teflonlined stainless steel autoclave followed by hydrothermal treatment at 180 ◦ C overnight. The obtained dark brown powder was collected and washed with deionized water for several times, and dried overnight. The hydrothermal carbonaceous samples were chemically activated by KOH with a weight ratio of 1:2 in a tubular furnace under N2 flow at 600 ◦ C for 3 h. Finally, the resulting samples were washed with water until neutral pH was reached. For convenience, the activated hydrothermal carbon samples were named AHC-x, where x (x = 1, 2, 3 or 4) stands for the hair solution/glucose ratio. For comparison, a non-activated hydrothermal carbon sample named as HC-1 was prepared with a hair solution/glucose ratio of 1:1. 2.2. Material characterization The morphology of the carbon samples were observed with a scanning electron microscope (SEM, Sirion 200 FEI Netherlands)

and a transmission electron microscope (TEM, H800, Hitachi, Japan). The element composition and atom binding states were characterized by Energy Dispersive Spectroscopy (EDS, INCA Energy spectrometer), X-ray photoelectron spectroscopy (XPS, Escalab 250, USA) and Fourier transform infrared spectroscopy (FT-IR, Nicolet 5700, USA). Nitrogen sorption measurements were performed at −196 ◦ C using an ASAP 2020 system (Micrometitics, USA). The carbon materials were degassed at 350 ◦ C overnight before sorption measurements. Brunauer–Emmett–Teller (BET) surface area (SBET ) was calculated using the N2 adsorption isotherm data within the relative pressure of 0.05–0.25. Total pore volume (VT ) was obtained at P/P0 = 0.995. Micropore volume (Vmicro ) was calculated by t-plot method. Mesopore volume (Vmeso ) was determined by subtracting the micropore volume from the total pore volume. Pore size distributions (PSDs) were obtained from the adsorption isotherms using the nonlocal density functional theory (NLDFT) model, assuming a slit-shape pore. 2.3. Electrochemical measurements Working electrodes were made by pressing a mixture of carbon material and PTFE binder with a weight ratio of 95:5 onto nickel foam at 15 MPa. The electrode was dried at 120 ◦ C for 10 h. All electrochemical tests, including cyclic voltammetry (CV), galvanostatic charge/discharge measurements and electrochemical impedance spectroscopy (EIS), were carried out using a three electrode system in 6 mol L−1 KOH electrolyte on a CHI660D electrochemical testing station (Chenhua Instruments Co. Ltd., Shanghai). A platinum plate electrode and a saturated calomel electrode (SCE) were used as the counter and reference electrode, respectively. The galvanostatic charge/discharge test was used to determine the specific capacitance of the prepared carbons at current density ranging from 0.25 to 6 A g−1 . The specific capacitance is calculated by the following equation: Cm =

I×t V × m

(1)

where Cm (F g−1 ) is the gravimetric specific capacitance of the carbon samples, I (A) is the discharge current, t (s) is the discharge time, V (V) is the potential window (0.9 V in this study), and m (g) is the mass of active material in working electrode. The energy density (E) and powder density (P) could be calculated from the galvanostatic charge/discharge test using the equations of E=

1 × Cm × V 2 2

(2)

E t

(3)

and P=

where the Cm , V and t are the specific capacitance, discharge voltage decrease (0.9 V in this case) and discharge time, respectively. 3. Results and discussion 3.1. Characterization of the carbon samples Fig. 1 shows the morphology of the hydrothermal carbonaceous sample and some hydrothermal carbon samples. Typically spherical micro-sized particles are generally obtained for hydrothermal carbonization technique [36,40,41]. It is observed that the shape of the hydrothermal carbonaceous sample prepared in this work is irregular sphere with uneven size, and the boundary of this sample is unclear indicating its poor conductivity (Fig. 1a). Compared to the hydrothermal carbonaceous sample, the non-activated hydrothermal carbon of HC-1 presents much more clear SEM

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Fig. 1. SEM: (a) hydrothermal carbonaceous sample prepared with 1:1 ratio, (b) HC-1, (c) AHC-1, (d) AHC-2. TEM (e) and HR-TEM (f) of AHC-1.

picture indicating that the pyrogenic step leads to the conductivity sharp increasing (Fig. 1b). Obviously, the morphology of activated hydrothermal carbons is very different with the non-activated hydrothermal carbon. The activated hydrothermal carbons looks composed of lots of smaller carbon fragments because the hydrothermal microspheres are oxidized and broke down during the severely chemical activation process (Fig. 1c and d). TEM and HR-TEM images (Fig. 1e and f) clearly present worklike micropores formed by stacking curved graphene micro-layers in the activated hydrothermal carbons. Fig. 2 presents the N2 sorption isothermals and PSDs calculated by NLDFT model of the prepared carbons. All the carbons present a sorption isotherm of type I which achieves a high adsorption plateau at very low relative pressure, indicating microporous nature of these carbons. The non-activated hydrothermal carbon (HC-1) shows a very low amount of adsorbed gas, indicating the

lack of developed porosity (BET specific surface area and pore volume are 109 m2 g−1 and 0.06 cm3 g−1 , respectively). All isotherms of the activated carbons emerge a slightly upswept rear edge at high relative pressure (p/p0 > 0.9), suggesting the existence of largemesopores or macropores. As shown apparently in the PSDs of the activated carbons (Fig. 1b), the pore texture of the activated carbons possesses a typical microporous structure. It should be noted that, for the activated carbons prepared in this work, micropores are dominant and the ultra-micropores (<1.5 nm) occupies a significant fraction in the total micropore volume. These carbons also have a few large-mesopore or macropore with the size between 20 and 150 nm, which result from the aggregation of the carbon fragments. The activated carbons prepared by using different amount of hair solution but the same KOH dosage shows the similar pore size distribution. As shown in Table 1, the activated carbons show moderate specific surface area and pore volume. The difference of

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Fig. 2. N2 sorption isothermals (a) and PSDs derived from NLDFT model (b) of the prepared carbons.

specific surface area and pore volume between these carbons is narrow. The similar porosity properties of the activated carbons demonstrate that the dosage of activated agents may be the main factor that determines the porosity of the activated carbons prepared in this work. Hair is primarily composed of protein, notably keratin. Keratins have large amounts of the sulfur-containing amino acid cysteine. Therefore, N or S doping may be induced into the target carbon product when using human hair as additive in hydrothermal carbonization process. The surface element compositions of the prepared carbons were investigated by XPS (Table 1 and Fig. 3). Apparently, the oxygen content of the prepared carbons is very high, more than 10 at%, due to the lower carbonization temperature. The activated carbon AHC-1 has more nitrogen species than the non-activated HC-1, which may be caused by the loss of other atoms, such as oxygen. It is seen that the nitrogen content of the activated carbons gradually increases with the increasing of hair dosage. For the activated carbons of AHC-3 and AHC-4, sulfur doping is observed. The AHC-4 carbon is typical dual N, S-doped carbon materials, which possesses 3.1 at% of N-doping and 1.2 at% of S-doping on the surface. The S-doping is also proved by EDS measurement. About 0.4 at% and 1.2 at% of S-doping were observed for AHC-3 and AHC-4 determined by EDS, respectively, which is very close to the values determined by XPS measurement. The distribution of sulfur in AHC-4 is homogeneous, as was confirmed by EDS mapping (Fig. S1). The above experimental facts indicates that tunable N-doped or dual N, S-doped activated carbons could be obtained by controlling the dosage of human hair as additive. In order to further investigate the atom binding states of the prepared carbons, the high resolution XPS were carried out (Fig. 3). The peak assignments for high resolution XPS of the prepared carbons are summarized in Table 2. C1s scans show several peaks with varying contributions. In general, the peak at around 284.7 eV can be assigned to sp2 hybridized carbon in an aromatic environment. The shoulder peak at 285.4 eV corresponds to carbon atoms single bonded to sulfur, nitrogen or oxygen in the form of thioether, thiophene, pyrrolidonic, phenol or ether. Weak peaks at 286.4 eV (assigned to carbonyl or amide groups) and 289 eV (assigned to ester or carboxylic groups) are also observed. Three types of

O-containing groups could be verified on the surface of the prepared carbons, including C O, O C O, and O C O, corresponding to the peaks at 531.2, 532.3 and 533.5 eV, respectively (Fig. 3c) [42]. In case of nitrogen, three types of N-containing groups could be verified on the surface of AHC-1, including pyridinic N (N-6), pyrrolic N (N-5) and quaternary N (N-Q), corresponding to the peaks at 398.2, 400.4 and 401.2 eV, respectively (Fig. 3d) [43]. Besides the N-6, N-5 and N-Q groups, a small fraction of pyridinic N-oxide group (NX) is determined on the surface of AHC-4, corresponding to the peak at 402.5–403.4 eV [43]. In the case of sulfur, the majority of sulfur species are present as C S C species contain sulfur atoms that form thiophenic structures with neighboring carbon atoms (163.8–164.7 eV) [44]. Some oxidized sulfur species appear (38.3% for AHC-4, corresponding to the peaks of 168–169.5 eV) which may be the result of the reaction of surface sulfur with adjacent oxygen molecules [45]. The heteroatom-doping is also analyzed by the Fourier transform infrared spectroscopy (Fig. S2). The wide adsorption bands at 1000–1150 cm−1 is assigned to the C O groups in ethers or phenols. The N-doping is proved by the presence of adsorption peaks at about 1250 cm−1 (assigned to the C N stretching vibration) and 1550 cm−1 (assigned to pyridinic N or pyrrolic N groups) [46]. For AHC-4, a sharp adsorption peak were observed at around 620 cm−1 (assigned to thiophenic sulfur), further indicating the success of sulfur doping. In brief, XPS, EDS and FTIR analysis suggest that Ndoping or dual N, S-doping carbon materials were prepared, and the amount of heteroatom-doping could be tuned by controlling the dosage of additive human hair. 3.2. Electrochemical performance in 6 mol L−1 KOH electrolyte The obtained hydrothermal carbons were further investigated as the electrodes of supercapacitors in a three-electrode cell. The testing of electrochemical performance was carried out on in a 6.0 mol L−1 KOH electrolyte and a potential window from −0.9 V to 0.0 V. Fig. 4 shows the cyclic voltammetry (CV) curves, galvanostatic charge/discharge curves of the prepared carbons, and specific capacitance at current densities in the range of 0.25–6.0 A g−1 . The prepared carbons show almost

Table 1 Porosity parameters and element composition by XPS of the carbon samples. Sample

HC-1 AHC-1 AHC-2 AHC-3 AHC-4

Porosity parameters

Element composition by XPS (at%)

SBET (m2 g−1 )

VT (cm3 g−1 )

Vmeso (cm3 g−1 )

Vmicro (cm3 g−1 )

C

O

N

S

109 853 1104 897 849

0.06 0.49 0.62 0.50 0.48

0.01 0.07 0.14 0.05 0.08

0.05 0.45 0.55 0.48 0.44

74 85.2 83.5 84.6 84.9

24.8 12.8 13.3 12.6 10.7

1.2 2.0 2.2 2.6 3.1

0 0 0 0.2 1.3

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Fig. 3. XPS spectra of AHC-1 and AHC-4 (a) XPS survey, (b) C 1s, (c) O 1s, (d) N 1s, (e) S 2p, (f) different nitrogen species.

Table 2 Peak assignment of C 1s, O 1s, N 1s and S 2p for the prepared carbons. Peak

Binding energy (eV)

Assignment

2

Fraction of species (%) HC-1

AHC-1

AHC-2

AHC-3

AHC-4

C 1s

284.7 285.4 286.4 289

sp C C O

C C C/C O/C N O/C N C O

63.6 23.4 10.2 3.0

70.3 20.8 7.8 1.1

56.0 13.1 15.4 7.6

65.6 18.8 12.0 3.4

68.0 21.0 7.5 3.4

O 1s

531.2 532.3 533.5

C O O C O/C OH O C O

42.0 31.9 26.1

38.1 36.6 25.3

25.8 27.1 47.0

42.2 33.2 24.5

42.7 34.6 22.7

N 1s

398.2 400.4 401.2 402.5–403.4

N-6 N-5 N-Q N-X

42.1 45.8 10.0 –

20 69.5 10.5 0

17.0 47.7 24.2 16.9

40.9 40.7 18.5 –

15.9 67.8 12.8 3.4

S 2p

163.8–164.7 168.0–169.5

Thiophenic (C S C) Oxidized sulfur

– –

– –

– –

36.0 64.0

61.7 38.3

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Fig. 4. Electrochemical capacitance of the carbons in 6 mol L−1 KOH electrolyte (a) cyclic voltammograms curves at 5 mV s−1 , (b) cyclic voltammograms curves of AHC-1 at different scan rate, (c) galvanostatic charge/discharge curves at 0.25 A g−1 , (d) specific capacitance at current densities in the range of 0.25–6.0 A g−1 .

the same shape of distorted square CV curves. Obviously, the current density (thus the capacitance) changed with the applied potential, and a pronounced increase of current densities in potential range of −0.4 to −0.9 V was observed, indicating a large pseudo-capacitance contribution to the overall capacitance linked to the surface functional groups. Compared to the non-activated hydrothermal carbon (HC-1), the activated carbons show much larger response current on charge or discharge, reflecting the great increasing of specific capacitance after KOH activation. CV curves of AHC-1 at scan rates between 5 and 200 mV s−1 are shown in Fig. 4b. The gradually distorted shape of the CV curves as a function of scan rate indicates that a longer response time is required for fast charging and discharging. Charge/discharge branches of the carbons slightly deviate from a linear shape, further confirming the existence of substantial faradaic capacitance for the prepared carbons (Fig. 4c). Current enlargement spread over a wide potential range (from −0.40 to −0.9 V versus SCE) on the CV curves of the prepared carbons evident that complex reversible redox reactions occur and overlap while the electrode is polarized. It has been widely accepted that the heteroatom-containing groups are positive to the capacitive performance in aqueous electrolyte. Recent studies have shown that the heteroatom species play multiple roles on the electrochemical performance. The conductivity of the carbons could be improved by doping nitrogen into the carbon skeleton [40]. The hydrophilic nitrogen, oxygen or sulfur species promote the wettability of carbon pore surface to the aqueous electrolyte. As very electrochemical active sites, the nitrogen-containing groups can induce extra pseudo-capacitance due to faradaic redox reactions, which are related with type of N-containing groups. However, the contribution of O-containing groups to pseudo-capacitance should be prior considered because all the prepared carbons possess larger amount of oxygen species (more than 10 at%). A redox mechanism for a carbonyl or quinone-type group has been proposed [47]. C X O + e− + K + ⇔

CX − OK

(4)

where Cx OK represents a phenol- or hydroquinone-type groups. This reaction should make large contribution to the

pseudo-capacitance of the prepared carbons. The contribution of sulfur species on the capacitive performance will be discussed below. As shown in Table 3, very high values of specific capacitance are obtained at a current density of 0.25 A g−1 for the activated carbons. AHC-4 shows the highest specific capacitance, up to a very high value of 264 F g−1 . As a consequence of moderate surface area, the prepared carbons show high specific capacitance per surface area (Cs ). Although the non-activated carbon of HC-1 shows much lower specific capacitance than the activated ones, this carbon possesses much higher specific capacitance per surface area (Cs , about 53 ␮F cm−2 ) than the activated ones. The high Cs of the prepared carbons may be due to synergistic effect of the heteroatom-doped species and the ultramicropores. The revisable redox of the heteroatom species induces the extra pseudo-capacitance resulting in a high Cs . In the ultra-micropores (<1 nm), the ions are much closer to the carbon surface, which could also lead to improved specific capacitance per surface area [48]. AHC-4 shows much higher Cs than the other activated carbons. That seems not be solely attributed to the increasing of N-doping because the increase of N-doping is low and even much lower than the decrease of Odoping. Herein, the positive contribution of sulfur species on the pseudo-capacitance should be considered for AHC-4. There are few reports about S-doped carbon materials used as electrodes of EDLCs, and the behavior of sulfur species seems to be unclear [49,50]. Recently, a possible electrochemical performance mechanism of sulfur species is proposed by Zhao and his co-workers [51]. It is suggested that the introduction of electron-rich sulfur dopants

Table 3 Specific capacitance at 0.25 A g−1 and resistance of the activated carbons. Samples

Cm (F g−1 )

Cs (␮F cm−2 )

Rs ()

Rct ()

HC-1 AHC-1 AHC-2 AHC-3 AHC-4

58 228 199 235 264

53 26.7 18.0 26.2 31.1

0.54 0.54 0.54 0.51 0.52

0.56 0.78 0.61 0.56 0.58

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Fig. 5. (a) Ragone plots of the prepared carbons, (b) galvanostatic cycling performance of AHC-1 at 1 A g−1 .

provides more polarized surface as well as reversible pseudo-sites and thus results in superior performance (Eqs. (5) and (6)). Combined the experimental results in this paper, it is believable that the S-doping plays a positive role in the whole capacitance. SO2 SO

+ 2e + H2 O ⇔ + e + H2 O ⇔

SO S(OH)

+ 2OH−

(5)

+ OH−

(6)

The gravimetric specific capacitances (Cm ) of the prepared carbons at current density ranging from 0.25 to 6 A g−1 are determined by galvanostatic charge/discharge test (Fig. 4d). The specific capacitance of the prepared carbons decreases with the current density increases, because the ions of electrolyte have not enough time to reach all the pore surface of the electrode materials at high current density. Although AHC-2 possesses the highest surface area, this carbon shows lower capacitance than other activated carbons. This fact may be caused by two possible reasons. Firstly, AHC-2 possesses much lower percentage of (C O + C OH) and (N-5 + N-6) species which are generally considered to be electrochemical active species induced pseudo-capacitance (Table 2) [34,47,52]. Secondly, AHC-2 possesses much less ultra-micropore (<1 nm) (Table S1) which plays a positive role in the capacitive performance because the ions are much closer to the carbon surface in the very small pores [48]. It is suggested that the lower performance of AHC-2 is caused by synergistic effect of lower percentage of electroactive heteroatom species and less ultra-micropores. AHC-1 shows a high specific capacitance of 228 F g−1 at a low current density of 0.25 A g−1 , while this carbon shows the highest specific capacitance of 166 F g−1 at 6 A g−1 among the investigated carbons. AHC-1 shows similar capacitance to AHC-4 while the latter carbon possesses large amount of S-doping. Although the S-doping may play a positive role in the whole capacitance, it should be noted that the S-doping of AHC-4 is low, and the electroactive oxidized sulfur species is less (Eqs. (5) and (6) and Table 2). Thus the pseudocapacitance is mainly attributed to the nitrogen and oxygen species. Compared to AHC-1, AHC-4 carbon possesses higher N-doping and S-doping, but much lower O-doping. Based on the overall consideration of heteroatom-doping, it is reasonable that AHC-1 gives capacitive performance closed to AHC-4. The rate ratio of capacitance in the investigated current densities is about 73%, 74%, 66% and 56% for AHC-1, AHC-2, AHC-3 and AHC-4, respectively. This fact indicates the best capacitive power performance of AHC-1 among the carbons prepared in this work. The N, S-doped activated carbons (AHC-3 and AHC-4) show lower rate capability than the mono N-doped activated carbons without S-doping (AHC-1 and AHC-2). Sulfur is a large atom with an atomic radius of 100 pm compared to nitrogen (65 pm) and carbon (70 pm). Thus, it is likely that the doping of bigger sulfur atoms will induce more strain and defect sites in the carbon materials, which may leads to the lower rate capability.

Fig. 5 shows the Ragone plots of the prepared carbons. Obviously, much higher energy density values can be achieved by the activated carbon. The AHC-1 carbon exhibits a high energy density of about 7 Wh kg−1 at a power density of 45 W kg−1 . This carbon also exhibits high performance at fast charge/discharge rates, and it retains a remarkable energy density of 5 Wh kg−1 at a high power density of 2700 W kg−1 . The cycling life of the AHC-1 carbon is also investigated by using galvanostatic long-term cycling test up to 1000 cycles at 3 A g−1 . At the first 50 cycles, AHC-1 shows an apparent decrease of specific capacitance which may be due to some loss of pseudo-capacitance resulted from the decomposition of some unstable functional groups (e.g. carbonate acid derived from the chemical adsorbed water or pyridinic groups) during charge/discharge cycles. The capacitance of AHC-1 became nearly constant after 650 cycles. This carbon shows an acceptable cycle life with relatively high capacitance value of 154 F g−1 after 1000 cycles, about 86% of initial discharge capacitance. Electrochemical impedance spectroscopy (EIS) test was carried out in a frequency range from 10 mHz to 100 kHz. Fig. 6 shows the Nyquist plot of the AHC-1 and the equivalent circuit for the fitting of the EIS data achieved by ZView software (the inset). The Nyquist plot of AHC-1 could be divided into several distinct parts, an uncompleted semicircle part at high frequency and a linear part at low frequency. At very high frequencies, the imaginary part (Z ) of the impedance is near to zero and the real part of resistance (Z ) measured is the ohmic resistance derived from the electrolyte and the contact between the electrode and the current collector (Rs ). As listed in Table 3, the internal resistance of all the samples is very low (below 0.6 ), indicating good conductivity of the test cell. In the range of medium-high frequencies an uncompleted semicircle loop can be observed, which stand for charge transfer resistance (Rct ) at the interface between the electrolyte and electrode. In the middle frequencies, the inclined portion of the curve (about 45◦ ) is ascribed to the Warburg impedance (W), responding to the frequency dependence of ion diffusion/transport from electrolyte to

Fig. 6. Nyquist plot of AHC-1.

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the micropore surface of carbon electrode. At low frequencies, the imaginary part of the impedance increases, which corresponds to the ideal capacitive behavior of the carbons. Cdl is a double-layer capacitor, and CL is the limit pseudo-capacitor given by the redox reaction of heteroatom-containing groups [53]. As shown in Table 3, the charge transfer resistance (Rct ) of the prepared carbons is much lower than the ordered mesoporous carbons [51,53]. The lower values of Rct may be related to the abundant heteroatom-doping of the prepared carbons. According to XPS results, large amount hydrophilic oxygen and nitrogen species exist on the carbon surface increase the polarity of the carbon surface and facilitated the contact between it and the aqueous electrolyte. AHC-3 reached an optimum value of Rct , which may be due to the synergistic effect of dual N, S-doping. The Rct of AHC-4 is slightly larger than that of AHC-3, although the former carbon contains more nitrogen species than the latter. For AHC-4, larger S-doping will induce more strain and defect sites in the carbon materials, which may leads to the slight increasing of charge transfer resistance for AHC-4. 4. Conclusion Tunable N-doping or dual N, S-doping were successful achieved by firstly using human hair as heteroatom additives via a method of hydrothermal carbonization procedure and sequent KOH activation. The introduced N or S species could be tunable by controlling the dosage of human hair additives. Due to the synergistic contribution of multi N, O, S-doped species, the carbons showed excellent capacitive performance, up to 264 F g−1 for AHC-4 in KOH electrolyte. The prepared carbons also AHC-1 shows an acceptable cycle life with relatively high capacitance value of 154 F g−1 after 1000 cycles. The present work exhibit a good prospect for development of tunable dual heteroatom-doped carbon materials with high surface area and excellent capacitive performance from renewable biomass. Acknowledgments This work was financially supported by Outstanding Young Scientist Foundation of Shandong Province (BS2009NJ014), Natural Science Foundation of China (NSFC 51107076), Key Sci-Tech Development Project of Shandong Reference Province (2009GG10007006), Key Sci-Tech Research Project of Education Department of China (210125). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2013.06.065. References [1] B.E. Conway, V. Birss, J. Wojtowicz, The role and utilization of pseudocapacitance for energy storage by supercapacitors, Journal of Power Sources 66 (1997) 1–14. [2] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nature Materials 7 (2008) 845–854. [3] S.L. Candelaria, Y. Shao, W. Zhou, X. Li, J. Xiao, J.-G. Zhang, et al., Nanostructured carbon for energy storage, Nano Energy 1 (2012) 195–220. [4] G. Yu, X. Xie, L. Pan, Z. Bao, Y. Cui, Hybrid nanostructured materials for highperformance electrochemical capacitors, Nano Energy 2 (2013) 213–234. [5] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chemical Society Reviews 38 (2009) 2520–2531. [6] Q.Q. Zhou, X.Y. Chen, B. Wang, An activation-free protocol for preparing porous carbon from calcium citrate and the capacitive performance, Microporous Mesoporours Materials 158 (2012) 155–161. [7] J. Zhou, X. Yuan, W. Xing, W. Si, S. Zhuo, Capacitive performance of mesoporous carbons derived from the citrates in ionic liquid, Carbon 48 (2010) 2765–2772.

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