Nitrogen and oxygen dual-doped activated carbon as electrode material for high performance supercapacitors prepared by direct carbonization of amaranthus

Materials Chemistry and Physics 231 (2019) 311–321

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Nitrogen and oxygen dual-doped activated carbon as electrode material for high performance supercapacitors prepared by direct carbonization of amaranthus Ao Wang a, b, Kang Sun a, b, Jihui Li a, Wei Xu a, Jianchun Jiang a, b, * a

Key Lab of Biomass Energy and Material, Jiangsu Province, National Engineering Lab. For Biomass Chemical Utilization, Key and Open Lab. of Forest Chemical Engineering, SFA, Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, No. 16, Suojin 5th Village, Nanjing, 210042, China Research Institute of Forestry New Technology, Chinese Academy of Forestry, Xiangshan Road, Beijing, 100091, China

b

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� Amaranthus derived N and O dualdoped activated carbons have been prepared. � Carbons were prepared via one step direct carbonization without activation agents. � The preparation process is simple, not costly and low environmental pollution. � Nitric acid treatment can facilitate super-capacitive performance of carbons. � XC-700-50 exhibits high capacitance, good rate capability and cycling stability. A R T I C L E I N F O

A B S T R A C T

Keywords: Supercapacitor Amaranthus Direct carbonization Nitric acid treatment Capacitance

With supercapacitors application promotion, looking for simple, energy efficient and environment-friendly method for expandable production of carbon materials with uniform heteroatoms doping as high-performance electrode materials for supercapacitors becomes the keystone of current research. Here, we present a simple, not costly, low environmental pollution method for preparation of nitrogen and oxygen dual-doped activated carbons via one step direct carbonization of natural, renewable and abundant amaranthus followed by ultrasonic treatment with nitric acid. Most noteworthy is XC-700-50, which exhibits high conductivity, moderate surface area, reasonable pore size distribution and high O and N doping contents (12.05% and 5.31% respectively). Benefiting from these features, it displays high gravimetric capacitance (326 F g 1 and 294 F g 1 at 0.5 A g 1 and 1 A g 1, respectively), good rate capability, as well as excellent cycling stability (about 97.1% and 94.4% of capacitance retention after 10000 cycles at 2 A g 1 and 10 A g 1, respectively) in the high mass loading electrode (8 mg cm 2), making it a promising electrode material for supercapacitors.

* Corresponding author. Key Lab of Biomass Energy and Material, Jiangsu Province, National Engineering Lab. For Biomass Chemical Utilization, Key and Open Lab. of Forest Chemical Engineering, SFA, Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, No. 16, Suojin 5th Village, Nanjing, 210042, China. E-mail address: [email protected] (J. Jiang). https://doi.org/10.1016/j.matchemphys.2019.04.046 Received 14 July 2018; Received in revised form 26 February 2019; Accepted 16 April 2019 Available online 17 April 2019 0254-0584/© 2019 Published by Elsevier B.V.

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Materials Chemistry and Physics 231 (2019) 311–321

1. Introduction

heavily reliant on chemical activation process with alkali or other chemical regents which is complicated for practical application, as well as environmentally harmful [8]. (2) the post-doping treatment of carbon is difficult to form uniform heteroatoms doping [31,33]. Hence, looking for simple, energy efficient and low environmental pollution method for expandable production of ACs with uniform heteroatoms doping is still the focus of current research [34,35]. Amaranthus is a common vegetable in china, India, and many other tropical countries. It is unique as it contains around 14% protein which comes mainly from lysine rich amino acids [36]. Since lysine is rich in nitrogen and oxygen elements, it is reasonable to use amaranthus as carbon sources to prepare N and O self-doped AC for SC electrode ma­ terials. Besides, amaranthus is also rich in calcium, potassium and iron elements, which served as catalyzer and activation agents, would pro­ mote the formation of porous in the process of pyrolysis. Gao et al. [36] have prepared N-doped carbon derived from amaranthus waste through a facile synthesis approach without any activation agents. And the performance of these carbons used for oxygen reduction reaction was also studied. However, as far as we know, there are no reports about the energy storage performances of amaranthus derived carbons for SC. In this study, a series of nitrogen and oxygen dual-doped ACs derived from amaranthus have been prepared by a simple and not costly method. The impacts of annealing temperature and nitric acid treatment on the chemical compositions, existence form of hetero atoms, conductivity, textural and morphology properties, as well as electrochemical energy storage properties of the prepared ACs were studied. In addition, the impact of mass loadings on capacitance of ACs were also discussed.

The ever-growing energy demand of our society together with the scarcity of fossil fuels and increasing environmental pollution has stimulated a worldwide development of sustainable and renewable en­ ergy resources [1–3]. Relevant sustainable energy storage systems are essential for the rise of sustainable and renewable energy resources [3, 4]. Among various energy storage technologies, supercapacitors (SCs) have drawn tremendous attention due to their high-power density, long life cycle, fast charge/discharge rates and low cost [5–7]. Although SCs have been widely used in a variety of applications, many issues including their energy density, durability, environmental safety and economic efficiency are still being intensively studied for further improvement [8–10]. In general, the above performances (energy den­ sity, capacitance and charge storage) of SCs are strongly depended upon the electrode materials used [11]. Therefore, further developing new materials with improved performance for SCs electrodes becomes the keystone and hotspot of current research [12]. Among the electrode materials exploited, carbon materials have received extensively attention because of their good electronic conduc­ tivity, excellent stability, adjustable porosity/morphology and non-toxicity [13,14]. Although many forms of carbon such as graphene/graphdiyne [15,16], carbon nanofibrous [17] and carbon nanotube [18] have been developed, activated carbon (AC) is still attractive because of its moderate cost, large surface area, well-established electrochemical properties and industrial large-scale production [19–22]. Today, virtually all commercial SC devices use ACs as the electrode materials [23]. As mentioned above, challenges come from scarcity of fossil fuels and environmental pollution, which leads to the requirement of sustainable carbon-rich precursors for ACs. Thus, use of renewable biomass for ACs features the concepts of green chemistry. In addition, ACs derived from renewable biomass have drawn intense attention also because of their tunable pore/surface properties, easy processability and relatively low cost [24,25]. In general, preparation of biomass derived ACs for SC is on basis of combination of carbonization with subsequent activation method [26]. For example, Lust et al. [27–29] prepared a series of activated carbon electrodes derived from sugars and glucose by carbonization and subse­ quent activation. The prepared carbon materials showed excellent elec­ trical double layer energy storage performances. Besides, it is reported that incorporation of heteroatoms can enhance chemical and physical features of ACs, such as surface hydrophilicity, electric conductivity and surface affinity towards aqueous electrolytes, which finally resulting in higher specific capacitance and rate capability [30–32]. Thus, hetero­ atoms (N, O, B, S or P etc.) are always introduced into carbon framework to improve energy storage performance of ACs. For example, Gao et al. [2] fabricated nitrogen and oxygen dual-doped carbon derived from catkins through activation of the resources by ZnCl2. NODC-800 exhibits a high capacity (~251 F g 1 at 0.5 A g 1) with nearly 100% retention rate after 1000 cycles as supercapacitors electrodes. Li et al. [19] prepared nitrogen-doped nanoporous carbon via thermal annealing corncobs and KOH mixture under NH3 atmosphere. The prepared AC shows high spe­ cific capacity of 129 mAh g 1 (185 F g 1) in organic electrolyte at 0.4 A g 1, as well as excellent rate capability and cycling stability. However, challenges still remain: (1) preparation of ACs for SC is always

2. Material and methods 2.1. Raw materials and chemicals Amaranthus were purchased from the Suojin food market of Nanjing. HNO3 (65 wt%) was purchased from Sinopharm Chemical Reagent Co., Ltd, and was used without further purification. 2.2. AC materials preparation The amaranthus-derived ACs were fabricated by direct pyrolysis of the amaranthus under nitrogen atmosphere and ultrasonic washing with acid solutions (Scheme 1). Amaranthus were thoroughly washed with distill water, and then dried at 100 � C for 24 h. Dried amaranthus were grinded to about 50 screen mesh. After then, the amaranthus powder was heated up to different temperature (600 � C, 700 � C, 800 � C, 900 � C) under N2 atmosphere for 3 h at a heating rate of 5 � C/min. After cooling to room temperature, the carbonized product was sonicated in HNO3 of different concentrations for 30 min at room temperature. Subsequently, the solution was filtrated and the residue carbon was thoroughly washed with distill water until the pH of filtrate is neutral. Final ACs were denoted as XC-T-X, where T represent the annealing temperature, and X represent the concentrations (mass percent) of HNO3. For comparison, carbonization products treated with 4.0 M HCl instead of HNO3 were labeled as XC-T, where T stands for the annealing temperature.

Scheme 1. Schematic of the preparation of amaranthus-derived ACs. 312

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2.3. Preparation of working electrode

Elementar. Thermogravimetric (TG) analysis was carried out on a TG209F1 thermogravimetric analyzer (Netzsch, Germany). Measure­ ments were made by heating from 40 to 900 � C at a heating rate of 10 � C min 1 under N2 atmosphere. Scanning electron microscope (SEM) measurements were performed with Hitachi S3400N. High resolution scanning electron microscope (HRSEM) measurements were performed with Zeiss Merlin Compact. Transmission electron microscopy (TEM) were performed with JEOL 2010F TEM operated at 200 kV. Powder Xray diffraction (XRD) was performed on Rigaku MiniFlex 600 I diffrac­ tometer with Cu Kα radiation (λ ¼ 0.15406 nm). N2 adsorption/ desorption isotherms was measured at 77 K with a Micromeritics ASAP 2050 system. X-ray photoelectron spectra (XPS) were recording on a Thermo VG Scientific ESCALAB 250 spectrometer with an Al Kα radi­ ator. The binding energy was calibrated by means of the C 1s peak en­ ergy of 284.6 eV. Raman spectrum was conducted on a confocal laser microRaman spectrometer (LABRAM-HR, JY Co.). The electrical con­ ductivity of ACs were measured by four-probe conductivity measure­ ments on ST-2722 semiconductor resistivity of the powder tester (Suzhou Jingge ElectronicCo., Ltd., China) under a pressure of 10 MPa.

Working electrodes were prepared by pressing a mixture of 80% ACs, 10% acetylene black and 10% polytetrafluoroethylene (PTFE) onto nickel foam at 15 MPa, followed by drying at 60 � C in a vacuum over­ night. The thickness of electrode material on working electrodes was about 120 μm, and the electrode area was 1 cm2. Unless otherwise statement, the active materials (ACs) loaded on the electrode is 8.0 mg for each electrode (Do not include the additive carbon black and PTFE). 2.4. Electrochemical measurements The ACs electrochemical properties were evaluated in three or two electrodes system via VMP3 instrument (Bio-Logic, France). In the threeelectrode system, ACs electrode served as the working electrode, Pt wire was used as the counter electrode, and Hg/HgO was used as a reference electrode. In the two-electrode system, both the positive and negative were the above-mentioned working electrode. 6 M KOH aqueous were used as the electrolytes. Cyclic voltammetry (CV) was performed in the potential range of 0.8 V to 0 V vs. Hg/HgO, and the scan rate was in the range of 5–100 mV s 1. Galvanostatic charge/discharge (GCD) tests were performed at current densities of 0.5–10 A g 1 over a voltage range of 0.8–0 V vs. Hg/HgO. Electrochemical impedance spectroscopy (EIS) measurements were performed at frequencies from 100 kHz to 10 mHz. Specific capacitance values were evaluated from the galvanostatic discharge curves by taking account the mass loadings of ACs (Unless otherwise stated, the mass loadings of the ACs are 8 mg cm 2) using the following equation [37,38]: C¼

I⋅Δt m⋅ΔV

3. Results and discussion 3.1. Composition and thermogravimetry analysis of raw material In this work, amaranthus were used as the raw materials for ACs preparation. It is well known that the properties of raw materials are closely related to the structure and performance of ACs [41]. Thus, the elemental compositions of amaranthus were first investigated by elemental analysis and ICP. As shown in Table 1, the N and O contents of amaranthus are in high level, which may point to high N and O contents of ACs derived from amaranthus. Besides, it can be seen that K, Ca and Fe contents of amaranthus are also very high. Normally, in the preparation of ACs, these metals especially K are always used as the activator (pore forming regent) or catalyzer, which helps the formation of pores [42]. This implies a self-activation may occur when amaranthus is pyrolyzed at high temperature, as well as more pores and high specific surface area of the ACs. Thus, it is reasonable to assume that O, N co-doped ACs with developed aperture can be formed by pyrolysis of amaranthus. The TG analysis curves of amaranthus are displayed in Fig. 1. Seen from the figure, the mass loss of amaranthus can be divided into three stages with temperature increasing. When the temperature is lower than ~250 � C (Stage I), the mass loss can be mainly attributed to the vola­ tilization of water. After entering stage II (250–600 � C), organic struc­ ture breaks and recombination, as well as volatilization of small molecules produced by decomposition become the main cause of mass loss. Besides, pores begin to form in this stage. When the temperature is higher than 600 � C (Stage III), the mass loss becomes slow. In this stage, aromatic rings condense into carbon. In addition, self-activated pore forming and graphitization are also occurred in this stage. In sight of this, the annealing temperature set in our experimental protocol is in the range of 600 � C–900 � C.

(1)

where C is the specific capacitance, I is the response current density, Δt is the discharge time, m is the total mass of the ACs (for three-electrode system, it refers to the mass of ACs on working electrode; for twoelectrode system, it refers to the mass of ACs on both electrodes), ΔV is the potential range within the Δt. Coulombic efficiency (ηt) of the SCs were calculated from chargedischarge curves according to the following equations [39]:

ηt ¼

td � 100% tc

(2)

where ηt is the coulombic efficiency, td is the discharge time, tc is the charge time. Energy and power densities (Ragone plots) were evaluated from the galvanostatic discharge curves of two-electrode system according to the following equations [40]: 1 E ¼ CV 2 2

(3)

E Δt

(4)

P¼

3.2. Characterization results of amaranthus-derived AC materials

where E is the energy density, P is the power density, C is the specific capacitance of the total symmetrical system (measured in two-electrode system), V is the potential range during charge-discharge measurement, Δt is the discharge time. The coulombic efficiency, specific capacitances, EIS and cycle sta­ bility were measured in three-electrode system; the energy density and power density were measured in two-electrode system.

Elemental analysis of the prepared ACs were carried out to investi­ gate the composition and the percentage of N and O incorporation. Seen from the Table 2, all ACs derived from amaranthus are doped with N and O, and the percentage of them decreased with the annealing Table 1 The elemental compositions of amaranthus obtained from elemental analysis and ICP.

2.5. Characterization Metal contents of materials were determined by inductively coupled plasma-optical emission spectrometer (ICP, OPTIMA7000, PE corpora­ tion, America). Elemental analyses were taken with Vario MICRO, 313

Sample

Wt% (C)

Wt% (H)

Wt% (N)

Wt% (O)

Wt% (Ca)

Wt% (Fe)

Wt% (K)

Amaranthus

37.60

3.9

5.31

32.25

2.10

0.27

10.48

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different peaks, including ketone O1 (531.3 � 0.2 ev), hydroxyl O2 (532.4 � 0.3 ev) and carboxyl O3 (533.9.0 � 0.5 ev) [47]. Seen from Table 2, the relative contents of O1 and O3 decreased, while O2 increased as the increasing of temperature. This phenomenon can be attributed to the order of their stability, which is O2>O1>O3. At higher temperature, O1 and O3 decompose firstly, which leads to relatively low contents of them. Besides, there was no new O1s peak appeared after treatment with nitric acid, while the relatively contents of O1, O2 and O3 changed significantly, as reflected in the increase of O1 and O3, and decrease of O2. Since hydroxyl and alkyl groups can be oxidized to ke­ tone and carboxyl in nitric acid solutions, thus this again can be attributed to the oxidation of nitric acid [43,46]. In a word, the elemental and XPS analysis results confirm that ACs with high level of N and O could be prepared through direct pyrosis of amaranthus without activation. And, the increasement of N and O contents of ACs can be obtained by nitric acid treatment rather than hydrochloric acid. In addition, high level of O and N contents of XC-70050 also implies its high capacitance. The porosity and surface area are important features of carbon ma­ terials in context to their super-capacitive performances. Thus, these properties of the prepared carbons were studied by N2 adsorption/ desorption isothermal analysis. As shown in Fig. 3A and Fig. S3A, all the carbons showed typical Type-IV isotherms, indicating the presence of both micropores and mesopores [47]. This result is further demonstrated in Fig. 3B and 3SB, which are the pore size distribution of XC-T and XC-T-X carbons. The detail textural structure of prepared carbons is summarized in Table 3. It can be seen that, the surface area of carbons decreased as the annealing temperature increasing, which is opposite to the results of the effect of temperature on contents of heteroatom doping. While, if on considering both the factors of surface area and heteroatom doping, it can be found that annealing at 700 � C can obtain the high contents of heteroatoms essential for electrocatalytic activity with the least loss of valuable surface area. Thus, it can be inferred that XC-700 may have the highest capacitance among the XC-T carbons. As for the effect of nitric acid, it can be seen that the surface area of the carbons decreased after treatment with nitric acid (Table 3). While, this decrease is not regular with the increase of the nitric acid concen­ trations. The reasons for the above results are complicated, may be the combine effects of pores enlargement and heteroatoms doping [44]. Generally, nitric acid treatment can enlarge the pores, and this is usually detrimental to surface area. However, for the pores undetectable (<0.8 nm), the enlargement of their size will make them detectable, which finally lead to the increase of surface area. Besides, heteroatoms doping may also increase the surface area of the carbons. As seen in Table 3, XC-700-50 shows the highest surface area and the widest pore width among the XC-T-X carbons. Besides, as can be seen in Fig. 3B, XC-700-50 contains the most mesoporous and microporous. Combing the heteroatoms contents results mentioned above, it is reasonable to assume that XC-700-50 will obtain the best electro­ chemical properties such as the highest capacitance and rate capability. Fig. 4A is the XRD pattern of XC-700-50. It shows broad peaks centered at 24.2� and 42.8� , corresponding to (002) and (100) lattice

Fig. 1. TG analysis of amaranthus under N2 atmosphere.

temperature increasing. Beyond that, it also can be seen that compared to the samples treatment with hydrochloric acid, the N and O contents of ACs treatment with nitric acid are higher. This can be attributed to the oxidation of nitric acid, which introduced oxygen functional groups such as carboxyl group, ketone group and nitro group into the ACs [43,44]. Besides elemental analysis, XPS analysis was also carried out to study the surface chemical composition of ACs, as well as the chemical status of N and O in the doped carbons. Fig. 2A is the full XPS spectra of ACs derived from amaranthus, from which C1s, N1s and O1s can be observed. Table S1 displays the chemical compositions of XC-T and XCT-X obtained from XPS. As expected, the N and O contents of XC-T decreased with the annealing temperature increasing. And, with the increase of nitric acid concentration, more O and N were introduced into the carbon. It can be seen that these results are consistent with the elemental analysis results mentioned above. The peak corresponding to N1s of XC-T can be dived into three different binding configurations, including pyridinic N1 (398.5 � 0.2 ev), pyrrolic N2 (400.0 � 0.2 ev), quaternary/graphitic N3 (401.0 � 0.2 ev) (Fig. 2B, Fig. S1) [45]. Different from the total N contents, the relative contents of the three different N statues change variously with the temperature increasing. As shown in Table 2, the N1 contents decreased, while N3 contents increased with the temperature increasing, suggesting the low stability of N1 and the formation of graphitic N3 at high temperature. The N2 contents firstly increases when the temperature increases from 600 � C to 800 � C, and then decreases when the temperature reaches 900 � C. This result may be due to that the N2 peak is relatively stable when the temperature is below 800 � C yet begin to decompose at higher temper­ ature. Interestingly, after treatment with nitric acid, a new peak (N4) at about 405.0 � 0.5 ev appears, which can be classified as nitro type N (Fig. 2B, Figs. S1D–S1F) [43,46]. With the concentrations increase of nitric acid, the N1, N2 and N3 peaks basically remain, suggesting little influence of nitric acid on them. In contrast, the relatively contents of N4 increased with the nitric acid concentration increasing. As illustrated in Fig. 2C and Fig. S2, the O1s peak can be deconvoluted into three

Table 2 Summary of chemical composition of ACs obtained from elemental analysis and XPS. Samples

Ca (Wt%)

Ha (Wt%)

Na (Wt%)

Oa (Wt%)

N1b (%)

N2b (%)

N3b (%)

N4b (%)

O1b (%)

O2b (%)

O3b (%)

XC-900 XC-800 XC-700 XC-600 XC-700-65 XC-700-50 XC-700-30 XC-700-10

90.07 81.53 75.81 72.31 70.53 71.98 73.72 74.66

0 0 0.36 0.41 3.22 2.42 1.90 1.81

0.76 3.43 5.07 7.54 5.33 5.31 5.20 5.11

2.80 7.77 8.53 9.02 12.25 12.05 10.74 9.25

4.5 18.1 23.9 30.3 23.3 24.4 25.1 26.0

44.7 47.4 43.7 42.9 35.0 35.5 35.6 36.8

50.8 34.5 32.4 26.8 30.8 29.6 30.8 31.5

10.9 10.4 8.5 5.7

18.9 24.1 24.4 37.5 31.5 30.0 27.2 25.5

69.0 66.6 58.5 38.2 39.1 45.8 52.7 56.5

12.1 13.3 17.1 24.3 29.4 24.2 20.1 18.0

a b

Data obtained from elemental analysis. Data obtained from XPS. 314

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Fig. 3. (A) N2 adsorption-desorption isotherms of XC-700 and XC-T-X carbon materials (B) Pore size distribution calculated using DFT method slit pore model, differential pore volume and pore width for XC-700 and XC-T-X carbon materials. The inset is the local enlarged figure of the original picture at the pore width in the range of 0–4 nm. Table 3 Textural properties of XC-T and XC-T-X carbon materials.

Fig. 2. (A) Full scan XPS of all prepared ACs (B and C) N1s and O1s XPS spectrum of XC-700 and XC-700-50.

planes of typical turbostratic carbon, which is a variant of h-graphite [48]. Except XC-600, the rest as-prepared carbons all show the similar peaks as the XC-700-50 (Fig. S4), which suggests the presence of graphite carbon of them. It is well known that the graphitization of carbon is close related to the its conductivity, which can influence the rate stability and cycle life of materials for SCs. While, since the differ­ ences of XRD patterns of the prepared carbons are not huge, it is hard to compare the graphitization degree of them. Raman spectra is often used to study the graphitization degree/dis­ order degree of carbon materials. Fig. 4B and Fig. S5 are the Raman spectra of prepared ACs. As can be seen in the Fig. 4B, it exhibits two

Samples

SBET (m2 g 1)

Total pore volumes (cm3 g 1)

Micropore volumes (cm3 g 1)

Average pore width (nm)

XC-600 XC-700 XC-800 XC-900 XC-70010 XC-70030 XC-70050 XC-70065

805.2058 1172.7876 1200.0434 1222.6917 1014.6727

0.5669 0.7493 0.7638 0.8092 0.6742

0.3598 0.4111 0.4197 0.4290 0.3972

2.116 2.693 2.728 2.811 2.711

1028.0282

0.7024

0.3925

2.733

1076.0288

0.7614

0.4095

2.830

810.7252

0.5647

0.2466

2.786

peaks at about 1360 cm 1 and 1580 cm 1, which characterized as “D” and “G” peaks respectively. In general, “D” peak relates to graphite microcrystals with distorted structure, “G” peak relates to graphite structure, and their integrated intensity ratio (IG/ID) is commonly used 315

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oxidation level and less contents of oxygen-containing functional groups, which finally result in smaller FWHM value of G band. With the increase of nitric acid concentrations, the oxidation level and contents of oxygen-containing functional groups will also increase, and thus result in the increase of FWHM value of G band. The electrical conductivity of the prepared carbons is displayed in Table S2. As expected, the results are coincided with the Raman spectra data. It is worth to mention that XC-700 and XC-700-50 have the con­ ductivity of 1.776 S cm 1 and 2.778 S cm 1 respectively, which are fairly good value. Finally, the morphology of XC-700-50 was also investigated by SEM, HRSEM and TEM. As shown in Fig. 5A, there is no obvious morphology feature of XC-700-50 at first sight. Maybe it can only say XC-700-50 is composed of irregular carbon sheets and granules with some degree of uniformity. However, when observe the morphology of XC-700-50 at high magnifications (Fig. 5B and C), it is interesting to found that XC700-50 is mainly composed of randomly stacked irregular flat carbon nanosheets with the thickness in the range of 30–100 nm. Stacked car­ bon nanosheets produce many macropores and mesopores, which will be served as ion buffer and channel during the electrochemical energy storage process. Besides, thin carbon nanosheets also shorten the diffusion distance of electrolyte ions, which may result in high rate capability of XC-700-50. Fig. 5B shows the typical image of XC-700-50. The result further verifies the stacked architectures of XC-700-50. It is a little disappointing, but within understanding. ACs are normally composed of stacks of disordered graphite-like carbons, which always lead to high surface area and porosity [36]. Taking into account all the results mentioned above, we can come to the conclusions that XC-700-50 which has high contents of N and O, high surface area and porosity, excellent electrical conductivity, will exhibit superior perfor­ mance when used as a supercapacitor electrode material. 3.3. Electrochemical properties study of prepared amaranthus-derived AC materials As mentioned above, the prepared ACs have their unique advantages such as high surface area and porosity, excellent electrical conductivity, high contents of heteroatoms (N and O) doping, which makes them promising candidates as electrode materials for SCs. Thus, the electro­ chemical performances of the amaranthus-derived ACs were studied in a standard three-electrode system in 6 M KOH solutions by CV, GCD and EIS in a potential range of 0.8 V-0 V [47]. Fig. 6A is the overlaid CV curves of the prepared amaranthus-derived ACs at a scan rate of 10 mV s 1. Clearly, all prepared ACs show irregu­ larly rectangle shape, which are typical for CVs of porous carbon elec­ trode materials. However, if on observation, it can be seen that some of the ACs display broad humps in the experimental potential range, while XC-800 and XC-900 display more regularly rectangular shape without or with negligible humps, suggesting a feature of pure electric double layer capacitance. As mentioned above, the main difference between XC-800 and XC-900 and other prepared ACs is their oxygen and nitrogen con­ tents. The oxygen and nitrogen contents of XC-800 and XC-900 are very low, while the other ACs have relatively high contents of heteroatoms. Thus, it is reasonable to come to the conclusion that the broad humps are caused by redox reactions of the nitrogen and oxygen functional groups on the carbon frameworks [52]. Besides, it also can be seen from Fig. 6A that XC-700-50 has the largest area among the studied ACs. Since the specific capacitance is proportion to the area of the CV curve, this im­ plies the highest specific capacitance of XC-700-50 under the experi­ mental conditions. Fig. 6B and Fig. S6 show the impact of varying scan rates on the voltammetric current response at prepared ACs. As shown in Fig. 6B, the CV curves of XC-700-50 still maintains the rectangular shape even at high scan rates of 100 mV s 1, indicating high rate capability. In contrast, the CV curves shapes of the other ACs change significantly with the increasing of the scan rates (Fig. S6). This can be mainly attributed to the irrational pore size distribution which is not suitable for fast ion

Fig. 4. (A) XRD pattern over the 2θ ranges of 10–80� and (B) Raman spectra of XC-700-50.

to characterize the different types of carbon-based materials [49]. Higher fractions of the graphitic region produce larger IG/ID ratios. As can be seen in Table S2, the IG/ID ratios of XC-T carbons increased with the annealing temperature increasing, which indicates high temperature is beneficial to the improvement of graphitization degree. Interesting, with the nitric acid concentrations increase, the IG/ID ratios also have a slight improvement. This result can be attributed to that the graphitized carbon is more stable than disorder carbon during nitric acid treatment under ultrasonic. Disorder carbon decomposed and became less during nitrolysis, while graphitized carbon basically remained, which finally led to the increase of IG/ID of XC-T-X carbon samples. Besides, sp2 crystallite size of the ACs were also calculated according to the equation mentioned before by A. C. Ferrari et al. [50]. As shown in Table S2, the sp2 crystallite size of ACs increase with the increase of IG/ID ratios. In addition, the full width half maximum (FWHM) of G band give us some interesting results (Table S2). With the increase of annealing tempera­ ture, the FWHM values of G band decrease slightly, which is contrary to IG/ID ratios results. However, with the nitric acid concentrations in­ crease, the FWHM values of G band increase gradually, which is consistent with IG/ID ratios results. S. J. Kim et al. [51] noted that the FWHM value of G band will increase with respect to the oxidation level. Inspired by this, this interesting result may be interpreted from the point of oxidation level and contents of oxygen-containing functional groups. Under inert atmosphere, higher annealing temperature will result in less 316

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Fig. 5. SEM and HRSEM (A–C) and TEM (D) images of XC-700-50 at different magnifications.

Fig. 6. (A) CV curves of all prepared ACs at the scan rate of 10 mV s 1 (B) CV curves of XC-700-50 under different scan rates (C) GCD curves of all prepared ACs at the current density of 1 A g 1 (D) GCD curves of XC-700-50 at different current densities (E) Specific capacitances of all prepared ACs at different current densities (F) Comparative Nyquist plots of prepared ACs in 6 M KOH solutions at an applied potential of 0 V with sinusoidal signal of 5 mV between 10 mHz and 100 kHz. The inset is the Randles equivalent circuit model representing the circuit elements in the Nyquist spectra.

transport, as well as unstable surface oxygen or nitrogen functional groups with irreversible redox reactions. It is well known that GCD test is an accurate and reliable method for evaluating the specific capacitance of electrodes [53]. The specific capacitance of ACs electrodes can be calculated by the equation (1) mentioned above. In this paper, the GCD tests are carried out at current densities from 0.5 A g 1 to 20 A g 1 on a three-electrode cell system. Fig. 6C shows typical GCD measurements obtained for all of prepared ACs at current density of 1 A g 1 with the same mass loading per surface

area. As seen, all ACs prepared exhibit symmetrical triangle shape with a slight distortion. The triangle shape is characteristic of double layer capacitance, while the distortion may result from the Faradaic effect of doped heteroatoms [37,54]. The shape distortion of XC-600 and XC-700-65 are more obvious, which implies more Faradaic reactions may happen during the GCD tests. Moreover, from Fig. 6C it is clearly seen that the capacitance increases as the following (180 F g 1) < XC-800 orders: XC-600 (157 F g 1) < XC-900 (188 F g 1) < XC-700 (217 F g 1)
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(242 F g 1) < XC-700-30 (258 F g 1) < XC-700-50 (294 F g 1). This result can be contributed to the combined influences of specific surface area, pore size distribution, heteroatom doping contents and conduc­ tivity. It is worth mentioning that the doping of heteroatoms such as N and O has at least the following benefits for the improvement of capacitance [30,47,55,56]. On the one hand, some N and O functional groups such as pyridinic and pyrrolic nitrogen groups, carboxyl/hy­ droxyl, phenol and ketone oxygen groups have electrochemical activity, which can direct produce pseudo capacitance. Meanwhile, some func­ tional groups such as quaternary nitrogen and nitro type nitrogen may not have electrochemical activity, but they display positive charge and can improve electron transfer through the carbon, which is also benefit for the enhancement of capacitance [47]. Besides, doping N and O on the carbon surface can improve the wettability of AC electrodes by aqueous electrolytes, which accelerates the ions diffusion from electrolytes to the pores surface of electrode, and further enhances the capacitance per­ formances of SC. The nitrogen and oxygen contents of XC-600 are high, yet its specific area and conductivity are very low, thus it shows the lowest specific capacitance among the XC-T ACs. XC-800 and XC-900 have high specific area and conductivity, as well as hierarchical pores structure, benefit for their capacitance, but they have the lowest contents of nitrogen and oxygen functional groups among the prepared XC-T ACs. Therefore, they only have the medium capacitance. By comparison, XC-700 has high specific surface area, medium conductivity and nitrogen and oxygen contents, suitable pores structure, therefore it displays relatively higher capacitance. As for the nitric acid treated ACs (XC-T-X), they all display higher capacitance than XC-700, as we have expected. As shown above, after treatment with nitric acid, the pore size distribution was optimized, the O and N contents and conductivity were enhanced, meanwhile the specific surface area only decreased slightly (Table 2, Table 3 and Table S2). These changes finally lead to the capacitance enhancement of nitric acid treatment carbons. Among them, XC-700-50 have the over­ whelming superiority compared to other samples, which leads to the highest capacitance of it. It is interesting that though XC-700-65 and XC600 have similar specific surface area and heteroatom doping contents, there is a significant difference in their capacitance. One of the reasons for this result may be that XC-700-65 has higher conductivity and larger average pore size which implies it has more suitable pores for double layer charge storage compared to XC-600 (Table 3 and Table S2). Another reason may be that the contributions to pseudo capacitance of different functional groups are not same, XC-700-65 has more ideal N or O functional groups for pseudo capacitance than XC-600 [52]. Fig. 6D and Fig. S7 are the GCD curves of prepared ACs at different current densities. The gravimetric capacitance of the prepared ACs at different current densities can be estimated from the discharge curves. Fig. 6E demonstrates the relationship between the capacitance and current density for different ACs. Unfortunately, but can be expected, the capacitance of all ACs decrease with the increase of the current density. This may be because the ions can not diffuse into all micropores to form electric double layer at high current density [57,58]. Besides, during charge and discharge process at high current density, some Faradic reactions caused by heteroatom functional groups may not able to occur. As can be seen in Fig. 6E, the specific capacitances of XC-700-50 are 326 F g 1, 294 F g 1, 278 F g 1, 228 F g 1, 180 F g 1 and 150 F g 1 at current densities of 0.5 A g 1, 1 A g 1, 2 A g 1, 5 A g 1, 10 A g 1 and 20 A g 1respectively, demonstrating its excellent rate performance and promising as electrode for SCs. The outstanding rate performance of XC-700–50 may be attributed to the existence of hier­ archical pore structure which facilitate rapid ions diffusion, as well as high effective surface area, good conductivity, fast and reversible Faraday reactions caused by heteroatom functional groups. In contrast, the other prepared ACs, especially the XC-700 and XC-700-65 display relatively terrible rate capabilities. The capacitance of them are 254 F g 1 and 266 F g 1 at current density of 0.5 A g 1, while only 5.3% and19.6% of their capacitance were retained at high current density of

20 A g 1. According to their textural and heteroatom doping properties, these results again can be attributed to the bad pore structure and size distribution of them. Coulombic efficiency of prepared ACs were also calculated from the GCD curves (Table S3). Out of our expectation, some ACs like XC-600 and XC-700 show relatively low coulombic efficiency (<90%) at cur­ rent density of 1 A g 1. According to conductivity results mentioned above, the low conductivity and high internal resistance of them may be the reasons for this result [59]. As also can be seen in Table S3, with the annealing temperature increasing the coulombic efficiency of XC-T ACs increase, which again can be attributed to their different conductivity [59]. Besides, with the increasing of current density, the coulombic ef­ ficiency of XC-700-50 also increase. At low current density, side re­ actions and irreversible oxidation-reduction reactions will occur, which will result in the increase of charge time and the reduction of coulombic efficiency. At high current density, the charge-discharge process is mainly affected by the electric double layer, while the pseudo-capacitance is negligible, therefore the coulombic efficiency of XC-700-50 will increase. EIS is another technical for determining the supercapacitive prop­ erties of materials [60]. Though it is not reliable and accurate, it is still important approach to make a through inquiry into the equivalent series resistance (ESR), which plays a very important role in energy delivery at fast discharge rate of AC based electrodes. Fig. 6F is the Nyquist spectra of prepared ACs in the frequency range of 100 kHz to 10 mHz. The inset of Fig. 6F is an equivalent circuit which is represent by a revised Randles circle with a series of resistors and capacitors in series and parallel [61, 62]. The intercepts of Nyquist plot to real axis in the high frequency region demonstrate the value of resistance of electrolyte, electrode and packaging contact, which is represented as Rs. The proximate semicircle offers interfacial charge transfer resistance and double layer capaci­ tance, which is modeled by Rct and Cdl connected parallel to each other. After then, a straight line inclined at an angle to the imaginary axis can be seen from Fig. 6F. Since ideally polarizable capacitance which is denoted as Cl would give rise to a parallel line to the imaginary axis, the inclined line suggests a resistive element is associated with Cl [62]. This resistance can be modeled as a leakage resistance Rl connected parallel to Cl. Finally, the long tail in the low frequency region demonstrate the diffusion resistance of ions into the bulk of electrode, which is repre­ sented as Warburg element W. To further comparison of supercapacitive properties of ACs, the equivalent circuit parameter values of them were obtained by fitting of the Nyquist spectra using the EIS data fitting software ZSimpWin. As shown in Table S3, the Rs values of the all samples are low, demonstrating the low solution resistance. The existed fine distinction may be due to the operational error during the prepa­ ration of working electrode or the different thickness of the samples. Besides, it also can be seen that the Rct values of prepared ACs are agree with the conductivity results displayed in Table S2, which means the higher the conductivity the lower the Rct. As for the W value, the smaller of it implies the lower diffusion resistance and faster ion diffusion pos­ sibility of the material. Take all the resistances into account, it can be found that the obtained values of Rs (0.93 Ω), Rct (0.406 Ω) and W (1.192 S-sec^0.5) of XC-700-50 are responsible for facile access of elec­ trolyte ions to the electrochemical reactions, and suggesting its best supercapacitive properties. Cycle stability is an important factor determining whether the ma­ terial can be used in practical applications. As shown in Fig. 7A, the cycle stability of XC-700-50 is examined by continuous cycling at 2 A g 1 and 10 A g 1. Over 10000 cycles, the capacitance retentions are 97.1% and 94.4% at 2 A g 1 and 10 A g 1, respectively, demonstrating excellent electrochemical cycling stability of XC-700-50. The insets of Fig. 7A are the first and last cycles of charge-discharge curves. It can be seen that both the two curves show symmetric and linear shape. There is only a slight decrease of discharge time after 10000 cycles, further demonstrating the superior reversibility of XC-700-50. Energy density and power density are two practical parameters to 318

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Fig. 7. (A) Variation of specific capacitances and capacitance retention with the number of cycles of XC-700-50 at 2 A g 1 and 10 A g 1. Insets are the first and last cycles of charge-discharge curves of XC-700-50 at 2 A g 1 and 10 A g 1. (B) Ragone plots of XC-700-50.

evaluate the overall energy and power properties of SCs [63]. In this work, they were evaluated from the galvanostatic discharge curves of two-electrode system (Fig. S8) according to equations (3) and (4). As expected, all the specific capacitance based on the total mass of two electrodes are about a quarter of the single electrode’s value (Fig. S8) [63]. The energy density and power density of XC-700-50 were calcu­ lated and summarized in Ragone plots. As shown in Fig. 7B, in aqueous electrolyte system, the XC-700-50 based supercapacitor displayed high energy density of 6.69 Wh kg 1 at a power density of 201.5 W kg 1, and remained 2.78 Wh kg 1 at 7468.7 W kg 1. This result can be attributed to the excellent rate capability of XC-700-50, and certify that the power density could vary in a wide range without obviously compromising the energy density. Further, the impact of mass loading on the capacitance were studied by employing the GCD method. As shown in Fig. 8A, the specific capacitance of XC-700-50 decrease from low mass loading (442 F g 1, 2 mg cm 2) to high mass loading (294 F g 1, 8 mg cm 2). The result may be attributed to the thickness changes caused by different mass loading. When the geometric area of electrode is fixed, the thickness of electrode film increases with the increases of mass loading. Generally, a thick electrode film is detrimental to capacitance behavior [64]. This is embodied in two aspects [65]: on the one hand, as the mass loading increases, more active species such as heteroatom functional groups are buried and can not generate pseudo capacitance; on the other hand, the distribution of electrolyte ions is also hindered within the thick electrode

Fig. 8. (A) Variation of specific capacitance with the mass loadings of XC-70050 obtained from GCD tests at 1 A g 1 (B) The specific capacitances of XC-T samples with/without nitric acid treatment.

film. In the end, we employed the GCD method to obtain an insight into the impact of nitric acid treatment on the capacitance of other XC-T samples. As seen from Fig. 8B, all the specific capacitances of the XC-T samples increase after treatment with 50% nitric acid, which demon­ strating the universality of nitric acid treatment for capacitance enhancement. It is interesting to find that the extent of the capacitance influence of nitric acid treatment varies with the samples. As seen, after nitric acid treatment, the capacitance of XC-600 increase significantly, while in contrast the capacitance of XC-900 only increase slightly. As mentioned before, high temperature is beneficial to graphitization. Since graphitized carbon is stable and more difficult to be modified by nitric acid, may be this is the main reason for the above result. Besides, this result also inspires us that for different samples, the suitable concentration of nitric acid for modification may be different. 4. Conclusion In conclusion, we have prepared nitrogen and oxygen dual-doped ACs via one step direct carbonization of amaranthus, which is renew­ able and abundant, without any activation, followed by ultrasonic treatment with nitric acid to remove metal compounds, modify surface 319

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functional groups and optimize pore structure. The whole preparation process is simple, not costly and low environmental pollution. With the increase of annealing temperature, the conductivity, surface area and graphitization degree of ACs increase while their hetero atoms doping contents decrease. Carbon pyrolyzed at 700 � C (XC-700) shows the best electrochemical energy storage performance among the XC-T samples. The nitric acid treatment can modify the structure and functional properties of ACs, leading to the change of their electrochemical energy storage performances. The electrochemical energy storage performance varies with the nitric acid concentrations. Among the as prepared ACs, XC-700-50 exhibits high surface area and conductivity, reasonable pore size distribution, high O and N doping contents, making it displays high gravimetric capacitance (326 F g 1 and 294 F g 1 at 0.5 A g 1 and 1 A g 1, respectively), good rate capability, as well as excellent cycling stability (about 97.1% and 94.4% of capacitance retention after 10000 cycles at 2 A g 1 and 10 A g 1, respectively), at high mass loading (8 mg cm 2). Benefiting from these features, XC-700-50 is considered as promising electrode material for SCs, may has a great potential for practical application.

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