Promising Nitrogen-Rich Porous Carbons Derived ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Promising Nitrogen-Rich Porous Carbons Derived from One-Step Calcium Chloride Activation of Biomass-Based Waste for High Performance Supercapacitors Jingjiang Liu,† Yuanfu Deng,*,†,‡ Xuehui Li,*,†,§ and Lefu Wang† †

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China The Key Laboratory of Fuel Cell Technology of Guangdong Province, Guangzhou 510640, China § Pulp & Paper Engineering State Key Laboratory of China, Guangzhou 510640, China ‡

S Supporting Information *

ABSTRACT: It has long been demonstrated that KOH and ZnCl2 can be used as efficient chemical activation agents to prepare porous carbons. Herein, we develop a green activation method, that is, onestep calcium chloride (CaCl2) activation sugar cane bagasse with urea, for the preparation of nitrogen-rich porous carbons (NPCs). The nitrogen contents, specific surface areas, pore sizes, and specific capacitances of the obtained NPCs can be effectively tuned by adjusting the ratio of carbon precursor (sugar cane bagasse), nitrogen source (urea), and activation agent (CaCl2). The synthesized threedimensional oriented and interlinked porous nitrogen-rich carbons (3D-NPCs) contain not only abundant porosities which can impose an advantage for ion buffering and accommodation, but also high nitrogen content in the carbons which can obviously increase the pseudocapacitance. Therefore, for the typical sample, obtained from pyrolysis of the mixture of sugar cane bagasse, urea, and CaCl2 in a mass ratio of 1:2:2 at 800 °C for 2 h under N2 atmosphere, shows a high specific capacitance, excellent rate capability (with 323 and 213 F g−1 at the discharge/charge current densities of 1 and 30 A g−1, respectively), and outstanding cycle performance (a negligible capacitance loss after 10 000 cycles at 5 A g−1). KEYWORDS: Nitrogen-rich porous carbons, CaCl2 activation, Biomass waste, Porous structure, Supercapacitors

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effect has received extensive attention.10−20 It has also been demonstrated that nitrogen functionalities seem to be the alternative method for increased extra capacitance, and enhance surface wettability and electrical conductivity of the active materials.12−16 Considering the potential scale of supercapacitors application and the scarcity of fossil energy, utilizing renewable biomass and/or biomass wastes to produce carbon-based materials with special nanoarchitectures for energy application would be considered to be more worthwhile and has attracted increasing attention.21−23 Porous carbons have been prepared by the H3PO4/KOH/NaOH/ZnCl2 chemical activation of a wide range of agricultural wastes, such as banana fibers,24 corn grains,25 coconut shell,26 tamarind fruit shells,27 plant leaves,28 lignin,29 and regenerated silk protein.30 Sugar cane bagasse, a waste agriculture rooting from sugar industry, is an abundant and free renewable agricultural residue lignocellulose. It has also been reported that this biomass waste can serve as an efficient precursor for the preparation of porous carbons by NaOH,

INTRODUCTION Activated carbons (ACs) are regarded as the first candidate electrode materials for electric double layer capacitor (EDLC) due to their large surface area, a relevant cost level, long cycle life, and high specific conductivities.1,2 However, the low energy density and the complicated microporous structures and disordered texture of ACs, which result in a long diffusion distance and a high ion-transfer resistance, are not able to easily meet the growing needs of the high performance demand of the supercapacitors at high current densities. A lot of strategies are under development for optimizing the energy density of capacitors while without scarifying the high power density and/ or cycle life.3,4 Materials with pseudocapacitance are key candidates in this field. For example, transition metal oxides/ hydroxides and conducting polymers represent the two main examples for the improvement of pseudocapacitance for supercapacitors.5−9 However, some important drawbacks, such as low electrical conductivity/high price of the transition metal oxides and poor cycle stability of the conducting polymers, still hinder their wide application.7,8 In this respect, one channel for introducing heteroatoms (B, N, O, P, S, and so on) into porous carbon frameworks to form novel heteroatomdoped carbon materials by enhancing the pseudocapacitance © XXXX American Chemical Society

Received: August 26, 2015 Revised: October 26, 2015

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DOI: 10.1021/acssuschemeng.5b00926 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic of the procedures of N-riched porous carbons.

Table 1. Detailed Experimental Parameters for the Preparation of Carbon-Based Materials

a

sample

mass ratio bagasse:CaCl2: urea

bagasse (g)

CaCl2 (g)

urea (g)

yield based on bagasse (%)

S100 S102 S120 S121 S122 ZnCl2−S122a KOH−S122b

1:0:0 1:0:2 1:2:0 1:2:1 1:2:2 1:2:2 1:2:2

1.0 1.0 1.0 1.0 1.0 1.0 1.0

0 0 2.0 2.0 2.0 2.0 (ZnCl2) 2.0 (KOH)

0 2.0 0 1.0 2.0 2.0 2.0

21.9 33.2 24.6 26.8 30.1 27.0 14.0

ZnCl2 as activation agent. bKOH as activation agent.

ZnCl2, and H3PO4 activation and calcination.31−33 Some of the as-prepared porous carbons show acceptable capacitance for supercapacitors. More recently, hierarchical porous carbon aerogels were also prepared from freeze-drying cellulose aerogels using sugar cane bagasse as the raw material followed by KOH activation,34 and a medium specific capacitance of 142.1 F g−1 was obtained at a discharge current density of 0.5 A g−1. The use of biomass or biomass wastes as precursors for the preparation of porous carbons for supercapacitors is a sustainable route. However, most of the as-prepared carbons display medium specific capacitance because of little nitrogen content, which will hinder their application for high energy density supercapcitors. In this regard, the mass production of nitrogen-rich porous carbons (NPCs), originating from the pyrolysis of the biomass wastes under different experimental conditions, is a very attractive direction to improve their specific capacitance. Unfortunately, the relative study on the use of sugar cane bagasse as precursor for the preparation of Ndoped carbon material has not been found, although many literatures reported the preparation of porous N-doped carbons for the improvement of the pseudocapacitance for supercapacitors, by introducing nitrogen on carbon frameworks through the post-treatment with ammonia gas,35,36 or by employing N-containing materials as an additional precursor,37−39 or by a direct pyrolysis of the N-enriched biomass under different synthetic conditions.40−45 It has been implied that the use of NaOH/KOH/ZnCl2 as activation reagents will increase the surface area and improve pore structure, but will also significantly decrease the N-content of the as-prepared carbon-based materials.43,45 In addition, the use of those activation regents may result in more or less severe disadvantages, such as the strong corrosion of the instrument and equipment as well as high toxicity and cost, which will restrict widespread and practical use or increase the mass production cost. Therefore, to derive carbons from the low price and sustainable biomass/biomass waste with a high N-

content, high specific surface area and high performance for supercapacitors via a green and low cost approach is known to be a significant and urgent challenge. Chemical activation of biomass wastes by CaCl2, instead of NaOH/KOH/ZnCl2, to prepare porous carbons for adsorption of metal ions, organic dyes, and gas was investigated.46−48 However, the investigation on the preparation of porous Ndoped carbons using the CaCl2 chemical activation has not received much interest to date. Considering the low cost, easy availability, and environmental friendliness of the CaCl2, it is thought that the preparation of N-enriched porous carbons from the CaCl2 chemical activation of biomass wastes (such as sugar cane bagasse) under suitable pyrolysis temperature will be a cheap, renewable, and environmentally friendly strategy. Herein, we demonstrate a facile and highly efficient route to fabricate N-doped carbons with oriented and interlinked porosity, which are derived from a one-step CaCl2 activation of sugar cane bagasse precursor in the presence of urea. The synergistic roles of the CaCl2 and urea on the porous structure and nitrogen content of the as-prepared N-doped carbons are discussed in detail. Also, the possible creation mechanism of the pore structure and performances of the as-prepared samples for supercapacitors are also investigated.

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EXPERIMENTAL SECTION

Materials. Sugar cane bagasse was received by Jinling Sugar Co. Ltd. (nitrogen content 0.44%, carbon content 44.38%, and hydrogen content 7.26% by the elemental analysis). It was smashed by grinder with average particle size of around 200 meshes before use. Calcium chloride was purchased from Sinopharm Chemical Reagent Co. Ltd. Urea was acquired from Fucheng Chemical Reagent Co. Ltd. (Tianjin, China). All chemical reagents were of analytical grade. Synthesis of Three-Dimensional Nitrogen-Rich Porous Carbons (3D-NPCs). One-pot synthetic step: bagasse (1.0 g), calcium chloride (2.0 g), and urea (2.0 g) (with the mass ratio of 1:2:2) were mixed in 50 mL of deionized water, and the mixture was impregnated at 80 °C for 4 h. Then, the mixed precursor was loaded B


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Figure 2. (a, b) FE-SEM images of S120 sample, (c, d) FE-SEM images of S121 sample, and (e, f) FE-SEM images of S122 sample. into an alumina boat and heated to 800 °C at 5 °C min−1, keeping at this temperature for 2 h. After calcination and cooling down to room temperature, the obtained sample was first immersed into the HCl (2 mol L−1) with stirring for 1 h, and completely washed by distilled water several times until the pH of ∼7. Finally, the sample described above was dried under vacuum at 50 °C for 12 h and named as S122. Typical synthesis procedures were shown in Figure 1. In addition, the S122-700 and S122-900 samples were further prepared by the same procedures as the S122 sample except the mixed precursor was calcined at 700 and 900 °C, respectively. For comparison, we prepared S100 by the same procedures as those for S122 except no urea and CaCl2 were added into the precursor, the S102 sample by the same procedures as those for S122 except no CaCl2 was added into the precursor, the S120 sample by the same procedures as those for S122 except no urea was added into the precursor, and the S121 sample by the same procedures as S122 with the mass ratio of bagasse:calcium chloride:urea = 1:2:1. Detailed preparation parameters were shown in Table 1. In addition, the KOH−S122 and ZnCl2−S122 samples, obtained from chemical activation of bagasse and urea using KOH and ZnCl2 activation agents, respectively, were also fabricated (the detailed parameters and the yield of the as-prepared samples were also shown in Table 1). Materials Characterizations. The microstructures of all resulting samples were studied with field emission scanning electron microscopy

(FE-SEM, Ultra Plus, Carl Zeiss, Germany) and transmission electron microscopy (TEM, JEM-2100, Japan). The Raman spectra were obtained using a Bio-Rad FTS6000 Raman spectrophotometer with a 532 nm blue laser beam. Nitrogen absorption/desorpotion measurements were performed at −196 °C using an ASAP 2020 system (Micrometitics). All samples were degassed at 200 °C for 2 h prior to sorption measurements. The elemental microanalysis (C, H, and N) and atom binding states were characterized by elemental analyzer (Vario EL) and X-ray photoelectron spectroscopy (XPS, Escalab 210, Germany), respectively. X-ray diffraction (XRD) patterns of samples were examined on a diffractomter (D/Max-2400, Rigaku, Japan) advance instrument using Cu Kα radiation (λ = 1.5418 Å) at 40 kV, 100 mA. The 2θ range used in the measurements was from 5° to 80°. Electrochemical Measurements. In a three-electrode system, the tested sample was loaded onto a normal to the glassy carbon electrode (5 mm diameter). Generally, accurately weighed (4.0 mg) sample was ultrasonically dispersed in 0.400 mL Nafion solution (0.25%, DuPont, USA). An 8.0 μL portion of the above suspension was dropped onto the working electrode surface and dried at room temperature (about 0.4 mg cm−2). All of the electrodes were immersed in 6 mol L−1 KOH electrolyte.49 The electrochemical performances were investigated by cyclic voltammetry (CV) and galvanostatic charge/discharge measurements from CHI 660E electrochemical workstation. The charge− C


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Figure 3. (a) TEM and (b) HR-TEM images of the S120 sample; (c) TEM and (d) HR-TEM images of the S121 sample; (e) TEM and (f) HR-TEM images of the S122 sample. discharge cycle stability was employed on a LAND CT2001A (Wuhan Land Instrument Company, China). During the two-electrode test, the electrodes were made by mixing S122 (80 wt %), polyvinylidene fluoride (PVDF, 10 wt %), and commercial carbon black (10 wt %). The mixture was coated on nickel foam and then pressed at 15 MPa followed by drying at 393 K for 12 h. A button-type supercapacitor was assembled with two similar carbon electrodes (about 2.6 mg cm−2) separated by a polypropylene membrane in 6 M KOH electrolyte. The charge−discharge performance was measured using a CHI 760E electrochemical workstation (CH Instrument, Shanghai, China). The specific capacitance of the S122 material (C, in F g−1) was calculated on the basis of the discharge curve according to eq 1.50,51

C=

ΔV

1 CV 2 2 × 4 × 3.6

(2)

P=

E Δtd

(3)

Here C is the specific capacitance of a single electrode in a two electrode supercapacitor (F g−1), and V is the usable voltage after IR drop (V); Δtd is the discharge time (h).

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RESULTS AND DISCUSSION To demonstrate the effect of the chemical activation reagent of CaCl2 and the nitrogen source of urea on the specific surface area, pore structure, and nitrogen content of the obtained samples, five resulting samples, that is, S100, S102, S120, S121, and S122, were prepared and investigated. Figure 2 showed the typical FE-SEM images of the S120 (a, b), S121 (c, d), and S122 (e, f) samples. In order to elucidate the role of CaCl2 in the pyrolysis process, the FE-SEM images of the other two contrasting samples are shown in Figures S1a,b and S2a,b in SI. In comparison with the inactivated sample (S100) or only N-doped sample (S102), the CaCl2 activated

2I m Δt

E=

(1)

Here I is the discharge current (A), (ΔV)/(Δt) (V s−1) is the slope obtained by fitting a straight line to the discharge voltage, and m is the mass (g) of active material in a single electrode. The energy density (E, in W h kg−1) and average power density (P, in W kg−1) were calculated according to eqs 2 and 3. D


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Table 2. Elemental Analysis, XPS Analysis, BET Surface Area, and Pore Size Parameters of the S100, S102, S120, S121, and S122 Samples elemental analysis

XPS

sample

C%

N%

H%

C%

N%

O%

SBETa (m2 g−1)

Smicb (m2 g−1)

Vtotalc (cm3 g−1)

Dd (nm)

S100 S102 S120 S121 S122

79.88 75.22 83.84 72.19 69.20

1.70 4.32 1.02 6.70 8.92

0.90 1.45 1.19 2.92 1.30

91.18 86.62 91.70 87.50 82.42

1.49 3.90 0.92 5.59 8.34

7.33 9.48 7.38 6.91 9.24

145.40 760.49 584.98 945.76 805.58

145.40 734.59 370.73 778.15 632.54

0.09 0.51 1.07 1.39 0.68

2.40 2.68 7.31 5.89 3.38

a

Specific surface area determined according to BET (Brunauer−Emmett−Teller) method. bMicropore surface area from T-plot method. cTotal pore volume. dAdsorption average pore diameter.

Figure 4. (a) N2 adsorption/desorption isotherms, (b) XRD patterns, (c) Raman spectra, and (d) XPS results of the as-prepared carbon-based materials.

initially stored in the interconnected ion-buffering reservoirs. Especially for the S121 and S122 samples with N-doping, which can obviously improve surface wettability and hydrophilicity. Meanwhile, the carbon walls around them are covered by electrolyte, providing a quick supply and short diffusion distance and resulting in good electrochemical performance as demonstrated in the followed section. By comparison, the S100 and S102 samples (as shown in Figure S1c,d as well as Figure S2c,d) both show compact accumulation of carbon layers. The pore size distribution of the five obtained samples was further verified by the nitrogen adsorption/desorption isotherms measurements. Detailed Brunauer−Emmett−Teller (BET) specific surface areas and pore size distributions of the five samples are shown in Table 2 and Figure S3, respectively. In addition, the percentage of mesopores of the as-prepared samples was listed in Table S1. Figure 4a shows the isotherms of the five as-prepared carbon-based materials. The isotherm of the S121 and S122 exhibits an obvious capillary condensation step and a hysteresis loop in the middle and high pressure range, combining characteristics of type II and type IV isotherms52

samples (S120, S121, and S122) indeed show a very different morphology. Specifically, it can be obviously be seen that both of the S100 and S102 samples show irregular lumps and a compact network structure. After chemical activated by CaCl2 (S120, S121, and S122), the carbon particles are oxidized and expanded to develop a rough and fluffy structure. Furthermore, these three samples show an interconnected meso- and macroporous network structure rather than the compact samples of S100 and S102. The inner structure of the CaCl2 activated samples can be further viewed by TEM and HR-TEM images, as shown in Figure 3. The TEM images of the S120 sample (Figure 3a,b) show that some carbon layers are interconnected to form obvious mesoporous and marcoporous structure. After addition of urea into the precursor, the S121 (Figure 3c,d) still retains the interconnected porous network with hierarchical pore structure. When the urea was increased too much, the S122 sample (Figure 3e,f) displays very thin layered carbon morphology, without obvious macropores. The meso- and/or macroporous structure of the three samples will form interconnected ion-buffering reservoirs when they are immersed in the electrolyte; thus, plenty of electrolytes are E


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reaction with Ca2+ and carbonate was dissolved, resulting in the porous structure in the N-rich carbons. The structure of the obtained carbon-based materials was further studied by powder X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), and the results are shown in Figure 4b,d, respectively. It can be seen that the XRD patterns of S100 and S102 samples both exhibit a sharp and narrow diffraction peak around 2θ = 25.8° corresponding to the (002) reflection of the graphite carbon and a broad peak around 2θ = 22.6° corresponding to the (002) peak of the disordered carbon. After chemical activation by CaCl2, the (002) peak around 2θ = 25.8° of the S120, S121, and S122 samples completely disappears, indicating a less ordered graphitic structure possibly due to strong chemical activation by CaCl2 during the thermal treatment. However, two new broad peaks around 2θ = 21.5° and 11.6° were observed in the three samples; the peak at 2θ = 21.5° can be interpreted in terms of short-range order of the carbon nanosheets along the stacking direction, and considerable conjugated sp2 carbon network has been restored.49 The weak peak at 2θ = 11.5° is similar to the intertube space of multiwalled carbon nanotubes, which originates from the relatively uniform spacing of the channel at the folding axis.58 Those results also demonstrate that the samples obtained from the chemical activation by CaCl2 display some oriented and interlinked porosities. This interconnected micro-, meso-, and macropores are important for the application of high power density supercapacitors, due to it being able to provide ion channels to facilitate ion transportation and accommodation. The less ordered graphitic structure is also in agreement with the Raman results. In the Raman spectra as shown in Figure 4c, the two prominent peaks located at ∼1350 and ∼1590 cm−1 correspond to the D-band (C−C, disordered graphite structure) and G-band (sp2-hybridized carbon) of carbon materials.59,60 The ID/IG of the S122 sample (1.51) is higher than that of the S121 (1.34), S120 (1.25), S102 (1.22), and S100 (1.16), suggesting the most defect sites and edges in the S122 sample. In fact, the S122 shifts to higher wave numbers as compared to other samples. Obviously, the D peaks of the obtained samples upshift when the ratios of urea to sugar cane bagasse are increased. In particular, the D peaks for the S120, S121, and S121 are about 1315, 1321, and 1325 cm−1, respectively (Figure S7). This effect is mainly related to the increase of nitrogen doping content with the enhancement of urea during the preparation process. The higher nitrogen content leads to the higher wavenumber of the D peak.61 For the two S100 and S102 samples, they also exhibit the similar result. In addition, the slight shift of the G-band could be attributed to the structural defects of 3D-NPCs by nitrogen doping.61 XPS (Figure 4d) is used to identify the N and O contents of the samples. For the S102, S121, and S122 samples, there are three obvious peaks at around 284.8, 399.9, and 533.2 eV that corresponded to C 1s, N 1s, and O 1s peaks, respectively. For the S120 and S100 samples, there are only two obvious peaks at around 284.5 and 533.2 eV that corresponded to C 1s and O 1s peaks, respectively, although very little N content was detected by the element analysis (Table 2). It is also interesting that the N content of the S122 sample, obtained from both technologies (8.92% based on EA and 8.34% based on XPS, respectively), is higher than that of the S102, KOH−S122, and ZnCl2−S122 samples, with the respective 4.32%, 1.68%, and 7.39% values based on the EA results, although the same ratio of sugar cane bagasse to urea was used. This result reveals that combination

(dark cyan and magenta triangle in line in Figure 4a). The S121 and S122 have a BET surface area of 945.76 and 805.58 m 2 g−1, total pore volume of 1.39 and 0.68 cm3 g−1, H−K (original) micropore volume of 0.41 and 0.34 cm3 g−1, and an average pore diameter of 5.89 and 3.38 nm, respectively. From comparisons of these adsorption isotherms of the five samples, it is clear that samples of S102 and S122 display similar adsorption capacities at low relative pressure, indicating their comparable amount of micropores. The S102 sample has a negligible adsorption capacity at the middle and high pressure range, which suggests the S102 sample is merely microporous. The S120 sample exhibits a similar isotherm as the S121 and S122 samples. However, the adsorption capacities are much lower at low relative pressure and higher at middle-to-high relative pressure than that of the S122 sample, respectively, which suggests that the S120 sample displays more obvious mesoporous and macroporous character. The S100 sample shows a small specific surface area (145.40 m2 g−1) and very small pore volume (0.09 cm3 g−1), which is much smaller than those observed in the S102, S120, S121, and S122 samples. These results suggest that both of CaCl2 and urea can act as efficient chemical activation reagents for improving the porosity and specific surface area of the carbon materials. Furthermore, the meso- and macropores of the carbon-based materials are mainly caused by the activation of CaCl2. It is found that the percentage mesopores of the S120 sample reached 82%. This value is slightly smaller than that of the porous carbon obtained from ZnCl2 activation of rice husk53 and starch-derived mesoporous carbons prepared by the template method.54 However, the SBET, Vt, and the percentage of mesopores are considerably higher than those of the reported porous carbon prepared by the activation of coal tar pitch with surfactant (CTAB) and CaCO3, respectively.55 In addition, it is noted that the ZnCl2−S122 sample only shows microporous structure (as shown in Figure S4). The KOH− S122 sample shows a high specific surface area of 2210 m2 g−1; however, it displays a negligible adsorption capacity at the middle and high pressure range in the isotherm (Figure S5). These two typical examples further demonstrated that CaCl2 is a very efficient activation agent for making meso- and macropores in our experimental condition. In order to explore the creation mechanism of porous structure of the as-prepared nitrogen-rich porous carbons, we did some additional experiments. In fact, the creation mechanism may be complicated. On the basis of the experimental results (as shown in Figure S6), the creation mechanism of porous structure may be attributed the following factors: (1) The urea in the whole system plays the dual role of a nitrogen source and an expanding agent,56 and the hydrolysis of some urea in aqueous solution at 80 °C results in ammonium and carbonate ions.57 (2) Some of the urea and ammonium ions may be associated with the hydroxyl of the sugar cane bagasse, and simultaneously, some of the ammonium ion and CaCl2 species are able to seep into the skeleton of the biomass waste. After that, the primary crystal structure of the sugar cane bagasse was changed (the red line in Figure S6), and the CaCl2 may be highly dispersed into the biomass framework, resulting in no obvious XRD peaks in the mixed precursor (the red line in Figure S6). (3) After calcination at high temperature, the NH3 caused by in situ decomposition of ammonium ion may react with carbon, and the highly dispersed CaCl2 was gathered together to form the crystallinity of CaCl2·2H2O (JCPDS 700385, the blue line in Figure S6). (4) After being washed by water and HCl, the CaCl2 and a little of CaCO3 caused by F


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Figure 5. (a) CV curves of different samples at 50 mV s−1 in 6.0 M KOH aqueous electrolytes. (b) Galvanostatic charge/discharge curves of different samples at discharge/charge current density of 1 A g−1 in 6 M KOH aqueous solution. (c) CV curves of the S121 electrode at various scan rates. (d) CV curves of the S122 electrode at various scan rates. (e) Galvanostatic charge/discharge curves of the S121 electrode at various current densities. (f) Galvanostatic charge/discharge curves of the S122 electrode at various current densities. (g) The specific capacitances of the S121 and S122 as a function of the discharge/charge current densities. (h) Cycling stability of the S121 and S122 electrodes at 5 A g−1. All data were obtained from the three-electrodes tests.

deconvolution of high resolution N 1s core level peaks, as shown in Figures S8, S9, and S10, respectively. The N 1s core level is fitted using Casa XPS software by four peaks corresponding pyridinic N (N-6 at 398.3 ± 0.2 eV), pyrrolic or pyridonic N (N-5 at 400.2 ± 0.2 eV), quaternary N (N-Q at 401.2 ± 0.2 eV), and oxidized N (N-X at 403.0 ± 0.2 eV).41,62 The percentage of each component is shown in Table S2. It is

of CaCl2 and urea as an activator and a nitrogen source is very crucial for the preparation of N-rich carbon material. This result is also demonstrated by the fact of much high N content (6.70% based on EA and 5.59% based on XPS, respectively) in the S121 sample, although less urea was used to nitrogen source during the preparation procedure. The surface N functionalities of the S102, S121, and S122 samples are identified by the G


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Figure 6. Electrochemical performances of the S122 device measured in a two-electrode system. (a) CV curves of the S122 sample at different scan rates. (b) Galvanostatic charge/discharge curves of the S122 sample at various current densities. (c) Ragone plot of the S122 electrode performed in 6 M KOH aqueous solution. (d) Electrochemical impedance spectra (inset: magnified region).

electrodes at various current densities are shown in Figure 5e,f and Figure S11, respectively. The corresponding specific capacitance was calculated on the basis of the G-C/D method, and the correlation between the specific capacitance and the various current densities for S121 and S122 electrodes is presented in Figure 5g and Figure S11, respectively. It is noted that the S122 sample exhibits the highest capacity among all electrodes at the discharge/charge current densities of 1−30 A g−1. Specifically, the S122-based electrode gives an outstanding specific capacitance as high as 323, 270, 254, 175, and 120 F g−1 at a current density of 1, 5, 10, 50, and 100 A g−1, respectively. Meanwhile, the S121-based electrode displays higher specific capacitance than that of the S122 when the discharge/charge current density reaches 30 A g−1, which demonstrates a very fast and efficient charge and ion transfer of the S121 sample, resulting in its high rate performance. Even if the current density is up to 100 A g−1, the specific capacitance of the S121 sample reaches 190 F g−1. This phenomenon may originate from the high meso- and macroporous volume of the S121 sample. The high specific capacitances of both of S122 and S121 samples are comparable the state-of-art N-doped carbons derived from the KOH chemical activation (Table S3).41−45 This result might be due to the high doping of nitrogen atom which increased the hydrophilicity and polarity of carbon materials and thus induced pseudocapacitive behavior. In addition, the oriented and interlinked mesopores formed in the S121 and S122 sample are vital for furnishing a smooth and convenient ion-transfer pathway and thus enhanced electrolyte accessibility to the microporous area. It is also noted that the S121 sample shows small IR drops when it cycled at high current densities (the detailed IR drops at different current densities of S121 sample were listed in Table S4). The IR drop at high

noted that the percentage of the N at the edge of graphite plane (N-5, N-6, and N-X), which is more active than that located in the middle of the graphite plane (N-Q),41,63,64 is very high content for the S122 (85.6%) and S121 (99.99%). Both of the high nitrogen content and the high percentage of active nitrogen are very important for the application in supercapacitors since the nitrogen on the surface gives a great contribution to pseudocapacitance. Electrochemical performance of the S121 sample and S122 sample is evaluated in a three-electrode system. The other three samples obtained from different experimental conditions have also been tested as comparisons. Figure 5a shows the cyclic voltammograms (CVs) of different electrodes at a scan rate of 50 mV s−1 in 6 M KOH aqueous solution. The CVs of S102, S120, S121, and S122 samples all display rectangular shape, indicating idea capacitive behavior. Clearly, the S122 sample demonstrates the highest specific capacitance among the five samples, due to the linear relations between specific capacitance and the area of CV. It is also noted that more developed humps present in the CV (magenta color in Figure 5a) of the S122 sample suggest a remarkable contribution from pseudocapacitance. Different from the four activated samples, the S100 sample shows a triangle-like CV curve, which is caused by the ionsieving effects.63 This phenomenon may indicate the existence of ultrafine pores which are not accessible in the inner pore. Figure 5b shows the galvanostatic charge/discharge (G-C/D) curves of different samples at current density 1 A g−1 in 6 M KOH aqueous solution. For the S121 and S122 samples, as shown in Figure 5c,d, the rectangle-like shaped CV curve is also observed even as the scan rate is increased to 200 mV s−1, indicating a very fast and efficient electron ion transfer. The (GC/D) curves of the S121 and S122 electrodes and the other three H


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porosity, high N content and reasonable specific surface area, the as-prepared S122 sample displays significant improved specific capacitance compared to the S100, S102, S120, and S121 samples, with reversible specific capacitances of 270 F g−1 at a discharge/charge current density of 5 A g−1 and almost 100% capacitive retention after 10 000 cycles. In addition, the S121 sample shows excellent rate performance with reversible specific capacitances of 190 F g−1 at a discharge/charge current density of 100 A g−1. It is suggested that these two N-doped carbons are promising electrode materials for high performance supercapacitors. We believe that this facile and environmentally benign method will be useful for the preparation of N-rich carbons via other biomass/biomass wastes. Also, the obtained N-rich carbons may be suitable for other applications such as lithium ion batteries and catalysis. Further investigation is ongoing in our group.

current density is a common phenomenon for carbon-based materials,20,34,37,43,66 which is related to the electrical conductivity and porous texture (size distribution and shape of the pores) of the electrode.37,63 The IR drop of the S121 sample is about 46 mV (as shown in Table S4) when it was charged at 100 A g−1, which is smaller or comparable to the reported values in previous studies.20,34,43,66 In addition, it is also found that the IR drops of the S121 change slightly when it cycled from 1 to 100 A g−1, explaining the good rate performance of S121. Similar results were also observed in the S122 sample (Table S4). In order to demonstrate that the S122 sample obtained from 800 °C displays the best electrochemical performances, the S122-700 and S122-900 samples were also tested, and the results were listed in Figure S12 and Table S5. It is found that both of the two samples displayed lower specific capacitances (185 and 110 F g−1 for S122-700 and S122-900, respectively, at 50 mV s−1) than that of the S122 sample. The long-term cyclic stability of the S121 and S122 electrodes was investigated using G-C/D measurement at 5 A g−1, and the result is shown in Figure 5h. It displays only a very slight variation of specific capacitance with cycle number. After 5000 cycles of C/D, the two electrodes almost exhibit 100% of the original capacitance, indeed indicating their long-term electrochemical stability. Another S122 electrode cycled at the same current density up to 10 000 cycles (Figure S13) which also showed no obvious capacitance loss, which further demonstrated the cycle stability of this material. In order to further understand the electrochemical performance of the S122 sample, a two-electrode system was also fabricated. The CV curves in Figure 6a have quasirectangular shapes, suggesting an ideal capacitive behavior for a twoelectrode capacitor in 6.0 M KOH electrolyte. Figure 6b shows the charge/discharge curves of the S122 electrode at different charge/discharge current density, and the corresponding specific capacitances were listed in Figure S14. When the current density was increased from 0.25 to 5 A g−1, the specific capacitance of the S122 electrode decreased from 300 to 132 F g−1. Correspondingly, the IR drop increased from 0.3 to 3.2 mV. Figure 6c shows the Ragone plots of the supercapacitor made of the S122 sample. The energy density of the S122 capacitor is 10.41 and 4.58 W h kg−1 at the specific power density of 250.0 and 4892.5 W kg−1, respectively. These values are a little lower than those of the 3D hollow porous grapheneballs-based supercapacitors51 and some higher than the phosphorus-doped graphene,20 N-doped porous carbon nanofibers,39 and hierarchical porous carbon-based supercapacitors.50 Electrochemical impedance spectroscopy (EIS) analysis can be confirmed by the Nyquist plots (Figure 6d) recorded from 0.1 to 100 000 Hz at open circuit potential in 6 M KOH, which are analyzed by the software of ZView 2 on the basis of the electrical equivalent circuit, as shown in the inset modeled equivalent circuit of EIS. R1 stands for ionic resistance of the electrolyte, and R2 is the charge transfer resistance at the active material/current collector interface, which is caused by the Faradaic reaction.67,68 As shown in Figure 6d, the S122 samples have low ionic resistance of 2.74 Ω, and possess small interfacial charge transfer resistance of 1.45 Ω.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00926. Additional data from characterization methods, including SEM, XPS, and adsorption−desorption isotherms (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a linked project of China’s Natural Science Foundation and Qinghai Province (No. U1407124), the National Natural Science Foundation of China (No. 21336002), the Fundamental Research Funds for the Central Universities (2015ZZ046), and the Natural Science Foundation of Guangdong Province (Nos. 2014A030313240, 2015A030311048).

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REFERENCES

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CONCLUSION N-doped carbons with different nitrogen content and mediumrange specific surface areas and interlinked porosities were prepared successfully by a one-step CaCl2 activation of sugar cane bagasse and urea. Through combining the multilevel I


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