From Dead Pine Needles to O, N Codoped Activated Carbons by a

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From dead pine needles to O, N co-doped activated carbons by an one-step carbonization for high rate performance supercapacitors Changyu Leng, Kang Sun, Jihui Li, and Jianchun Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02481 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Sustainable Chemistry & Engineering

From dead pine needles to O, N co-doped activated carbons by an one-step carbonization for high rate performance supercapacitors Changyu Lenga, Kang Suna*, Jihui Lia, Jianchun Jianga a: Institute of Chemical Industry of Forest Products, CAF, National Engineering Lab for Biomass Chemical Utilization,

Key and Open Lab on Forest Chemical Engineering, SFA, Nanjing 210042, China.

*: Corresponding authors: Kang Sun [email protected], Suojin Village 16, Nanjing, China.

Changyu Leng, [email protected], Suojin Village 16, Nanjing, China.

Jihui Li, [email protected], Suojin Village 16, Nanjing, China.

Jianchun Jiang , [email protected], Suojin Village 16, Nanjing, China.

ABSTRACT Dead pine needle derived O, N co-doped activated carbons were prepared by an easy one-step carbonization without adding any chemical reagents. The as-obtained PN-X samples had a high rate performance as electrodes for supercapacitors. It was due to the unique structure of cells in dead pine needles, abundant contents of metal elements (K, Na, Ca, Mg) and a large number of heteroatom contents (O, N). The role of different metal atoms and air for the formation of pores, and the mechanism of one-step carbonization were proposed. Micro/Meso/Macropores are beneficial to ion adsorption and transportation, and the doping O and N can improve physical and chemical properties of the carbon surface and increase electrochemical active sites to generate additional pseudocapacitance. All chemical and physical properties of PN-X samples were provided in detail. Especially, the PN-1000 showed a high specific

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capacitance of 223 F g-1 (178 F cm-3) at 0.5 A g-1 while it still had 150 F g-1 (120 F cm-3) even at 100 A g-1. The results suggested the unique PN-1000 to be a promising electrode material for supercapacitors in many practical applications. Keywords: dead pine needle; one-step carbonization; O, N co-doped; activated carbon; high rate performance; supercapacitor. Introduction With the increasing demands and challenges of energy storage, there is an urgent need for developing high-performance energy storage devices and advanced functional materials. Compared with Li-ions batteries, supercapacitors have a high power density, rapid charge-discharge rate and long cycle life, which have been widely applied in many practical areas, such as electric vehicles, electronic devices, subway system and so on. In the past decades, activated carbons were engineering as electrode materials for supercapacitors.[1-5] Especially, biomass-derived activated carbons (ACs) have attracted lots of attention because of: (a) low cost of raw materials and processes, (b) high chemical stability and thermal stability, which effect the cycling life, (c) high specific surface area, which can generate a high specific capacitance, (d) excellent electrical conductivity, which is crucial to the power density and the rate capability, (e) controlled porosity and abundant pores whose size can be adapted to electrolyte ions.[6-10] In recent years, it is confirmed that ACs should have large volume of micropores for energy storage, mesopores as ion transport channels for fast delivery ions and macropores for ion storage.[11-15] Although biomass-derived ACs as electrode materials for supercapacitors exhibit a high specific capacitance, few of them show an

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excellent rate performance, which is one of the most important characteristics in practical application.[16-20] The traditional methods of preparing ACs are mainly chemical activation, physical activation and chemical-physical activation.[21-23] Some ACs have a high specific surface area, large pore volume and great electrochemical performance. However, they have lots of disadvantages, for example, the chemical activations usually need a large number of chemical reagents while alkali and acid can eat away the inside of the container at a high temperature. The excess agents can not be recycled easily and pollute the environment. Some dead leaves were used to prepare ACs by traditional activation with abundant alkali or acid, which were nothing novel.[24-28] If the unique cell nanostructure of biomass materials can be used to prepare ACs without any agents, only by an easy one-step carbonization, it will have a promising application and the above problems can also be solved. In other words, biomass waste has been a problem in many countries recently. It is a rich source of carbon, heteroatoms or metal elements. However, most of biomass waste is just burnt or buried to produce ash, greenhouse gases or heavy metal circulation. It is meaningful that they can be used to prepare carbons as value-added products, such as materials for supercapacitor electrodes.[29-32] In this paper, dead pine needles are used as precursor to prepare activated carbons by an easy carbonization without any chemical reagents adding during the whole process and the as-obtained ACs also have a high specific surface area and large pore volume, especially, high rate performance as electrode materials for supercapacitors. It is due to the unique cell

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nanostructure of dead pine needles, abundant content of metal elements (K, Na, Ca, Mg) and a large number of heteroatom content (O, N). The role of different metal atoms and air for the formation of pores, and the mechanism of one-step carbonization are proposed in detail. This paper aims to propose an idea that biomass waste derived porous carbon materials should be designed from the nanostructure of cells in raw materials. The metal elements and heteroatoms of the biomass should also be considered and used, which can replace chemical reagents and avoid environment pollution to realize the green chemistry. Experimental Materials The dead pine needles (Cedrus) were collected in northern China in autumn. All other chemicals were purchased from Aladdin and used without further treatment. Preparation of dead pine needle-derived activated carbons The dead and dry pine needles were collected in great quantity, then cleaned, washed and dried at 110°C in an oven for 12 h. The dry pine needles were cut into pieces and kept in a completely dry atmosphere. During the one-step carbonization process, 10 g dead pine needle pieces were heated in a crucible from room temperature to 600, 800, 1000, 1200°C with heating rate of 10 ℃ min-1 and kept at the final temperature for 1 h in air. The as-obtained samples were denoted as PN-X (X=600, 800, 1000, 1200). Scheme 1 showed the nanostructures of dead pine needles and PN-1000. As shown in Scheme 1b, the longitudinal section of dead pine needles showed the porous rodlike structure, which was derived from empty plant cell cavities. The cross section of

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dead pine needles in Scheme 1c exhibited honeycomb-type pores. It can be clearly seen in Scheme 1e, f, g that the carbon framework of PN-1000 reserved the structures of cell cavities in dead pine needles to a great extent. The internal profile of PN-1000 in Scheme 1e showed the wavy carbon framework with porous nanostructures of dead pine needle cells after carbonization. The cross section of PN-1000 in Scheme 1f also showed abundant pores, which confirmed the wavy nanostructures in the internal profile of PN-1000 from another angle. The external surface of PN-1000 in Scheme 1g exhibited carbon layers with interconnected pores. So the empty cell cavities of dead pine needles were considered and used to prepare carbon materials with unique porous carbon frameworks.

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Scheme 1. (a) the dead pine needles; (b) the longitudinal section of the dead pine needles; (c) the cross section of the dead pine needles; (d) the PN-1000; (e) the internal profile of PN-1000 (f) the cross section of PN-1000; (g) the external surface of PN-1000; (b’), (c’), (e’), (f’) and (g’) was the magnified image of the red cycle part of (b), (c), (e), (f) and (g).

Structure characterization Thermogravimetric Analysis (TGA) was performed to find the pyrolysis temperature and demonstrated the possible reaction of metal ions during the carbonization process. The elemental contents of green and dead pine needles were obtained from elemental analyzer(vario EL cube CHNOS, Germany). The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the variation tendency of surface morphology and porous nanostructures. The property of activated

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carbon structure was investigated by Raman spectra and X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) measurements were also carried out to analyze the surface atomic content. N2 adsorption (77K) was used to analyze the specific surface area and pore size distribution of the porous carbon samples. The specific surface area (SSA) was calculated from the N2 isotherm with p/p0 in the range 0.1-0.25 by applying BET method. The pore size distribution (PSD) were also calculated by the DFT equation using N2 adsorption data. The total pore volume was confirmed by the amount of nitrogen adsorbed (p/p0=0.9). The average pore sizes of micropores and mesopores were estimated by the DFT method with the p/p0 values between 10-7-0.1 and 0.1-0.99, respectively. Cyclic voltammetry (CV), galvanostatic charge-discharge curve (GCD) and electrochemical impedance spectroscopy (EIS) were carried out on the electrochemical workstation (Bio-Logic, France). Electrochemical measurements The typical three-electrode system and two-electrode system were both used to text the electrochemical performance of all samples. All electrochemical measurements were carried out on an electrochemical station (VMP3, Bio-Logic). The working electrode was prepared by coating 5 wt.% polytetrafluoroethylene, 10 wt.% acetylene black and 85 wt.% PN-X activated carbons fully and grounding in a mortar. Then the slurry was pressed on a stainless steel to form the working electrode in which active material is contained. The mixture was pressed on a stainless steel plate under 10 M Pa for 1 min. The thickness of electrode material was 100 µm, the electrode area was 1 cm2. The coin-type supercapacitor (two-electrode system) was assembled by using

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two similar electrodes separated by a piece of polypropylene membrane in 1 M H2SO4 aqueous electrolyte. A platinum foil and Ag/AgCl were used as the counter and reference electrodes in three-electrode system, respectively. The voltage window for aqueous electrolyte was set to be 0-0.9 V. And the galvanostatic charge-discharge curves were performed under a current density range from 0.1 to 10 A g-1. The scan rate was in a range 5-100 mV s-1. Electrochemical impedance spectroscop were collected with frequencies ranging from 100 kHz to 10 mHz. Result and discussion Table 1 Elemental composition of green and dead pine needles Sample

K (%)

Na (%)

Ca (%)

Mg (%)

C (%)

N (%)

Ca/C ratio

Green pine needle dead pine needle

0.37 0.7

0.1 0.1

0.84 1.2

0.13 0.15

50.1 51.8

2.03 1.33

1.6 2.3

The metal elements of green and dead pine needle were confirmed with the elemental analysis as shown in Table 1. From the above results, the K, Na, Ca, Mg contents of dead pine needles are higher than green pine needles due to the loss of some organic matter and moisture. It was one of the reasons that dead pine needles were chosen as precursor to prepare carbon materials in this paper. Many reports clarified that Ca salts were mixed in the samples can create macropores and improve the specific surface area.[33-34] And Na, K salts are strong activated reagents to prepare ACs with micropores and mesopores. Ca and Mg were known to be good porogens and can hold up some space in the carbon frameworks while Na, K play the role of forming pores and getting through channels during the carbonization process. The role of different metal atoms during the formation of pores were discussed and combined with the thermogravimetric analysis (TGA) and N2 adsorption.

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Fig. 1. Thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) analysis (left and right y axes, respectively) under an air atmosphere, heating rate: 10°C min-1.

As shown in Fig. 1, there were several stages of thermogravimetric analysis process of dead pine needles, the first stage (up to 200°C) was associated with the desorption of absorbed water. The second stage (250°C-375°C) had the most obvious weight loss, which corresponded to CO2 and H2O evolution. It was due to the decarboxylation and dehydration of the materials, respectively. The next step, the range of 400-550°C, corresponded to the degradation of dead pine needles. And the rapid elimination of C-H, C-O, C-C bonds released gaseous volatiles, such as CO, CO2 and CH4. Dead pine needles also had some oweight losses of 600-900°C, which demonstrated the decomposition of carbon frameworks. The sodium/potassium (Na/K) compounds generated during heat treatment had a crucial role in the formation of the micro/ mesopores network and the development of channels.

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Scheme 2. Synthesis scheme for the preparation of ACs from dead pine needles in air. (1) Pyrolysis of the organic moiety and formation of Na2CO3 /K2CO3; (2) Decomposition of Na2CO3/K2CO3 with the formation of Na2O/K2O (gasification and chemical activation reactions), calcium/ magnesium (Ca/Mg) carbonate was formed firstly; (3) Reduction of Na2O/K2O with formation of metallic Na/K (chemical activation and Na/K intercalation processes), CaO/MgO can combine with each other and hold up some space between graphene layers; (4) Removal of Na/K/Ca/Mg inorganic compounds (washing).

As shown in Scheme 2, first, sodium/potassium carbonates were formed (< 650°C), then decomposed at higher temperature (Na2CO3/K2CO3→CO2+Na2O/K2O).[35-36] The CO2 reacted with carbon (CO2+C→CO) to generate micropores. At the same time, metallic sodium/potassium was produced via the reduction of Na2O/K2O in carbon framework. (Na2O/K2O+C→Na/K+CO).[37-38] Sodium/potassium played an important role in formation of additional porosity, since their vapors can intercalate between the carbon layers. And calcium/ magnesium (Ca/Mg) carbonate was formed firstly, then decomposed at a high temperature (CaCO3/MgCO3→CO2+CaO/MgO).[39-40] Calcium/ magnesium oxides had a higher chemical inertness than sodium/potassium oxides. So CaO/MgO can combine with each other and hold up some space between graphene layers, generating pores with the specific pore size distribution, which also exhibited 10


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in the size distribution plot of ACs and TEM images. The metallic oxide particles can be removed by washing with dilute hydrochloric acid and deionized water. Scheme 2 showed the mechanism of the easy one-step carbonization and the role of different metal ions during the formation of pores. The air can provide CO2, O2 and H2O during the whole carbonization process. CO2 can combine with metal elements to form Na2CO3/ K2CO3/ CaCO3/ MgCO3.[41-42] H2O (water vapor) can catalyze metal carbonates to promote the decomposition of active sites. CO2 and O2 can also react with active sites of frameworks to create and enlarge pores, sweep the remaining impurities and interconnect the pores and channels. (CO2 +C→CO/ O2+C→CO2/ H2O+C→CO+H2).[43] However, excess air and long-time carbonization under a high temperature can burn the carbon materials completely. All PN-X samples were confirmed to be prepared by an one-step carbonization for 1 h after a large number of experiments. And the PN-1000 had an ideal and pure structure with the suitable pore size distribution and interconnected pores, which were considered as desirable candidate for improving both specific capacitance and rate performance.[44-46]

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Fig. 2. (a) N2 adsorption-desorption isotherms, (b) The corresponding pore size distributions of the PN 600-1200 samples.(X axis: 0-10 nm). Table 2 Characteristics of pores in PN-X samples Samples

SSA (m2 g-1)

Pore Volume (cm3 g-1)

Average pore size (nm)

PN-600

461

0.25

2.18

PN-800

763

0.42

2.35

PN-1000

783

0.49

2.55

PN-1200

620

0.40

2.59

The porous texture of PN-600, 800, 1000 and 1200 were summarized in Fig. 2 and Table 2, according to IUPAC classification, the isotherms of PN-600, 800, 1000, 1200 were all type IV adsorption-desorption isotherms with obvious hysteresis loops in the desorption branch at the relative pressure of the range from 0.4 to 1.0, which indicated that there were not only micropores, but also a large amount of mesopores and macropores existed in the hierarchically porous carbon materials. The micropores can generate specific capacitance, mesopores can provide the short diffusion pathway for the fast ions transfer and macropores can storage ions. The hysteresis loops got gradually larger, which meant that the average pore sizes of PN-X samples were larger with temperature increasing. It can be seen in Table 2 that the average pore sizes of PN-600, 800, 1000, 1200 were 2.18, 2.35, 2.55 and 2.59 nm, respectively. It can be seen in Table 2 that the specific surface area and total pore volume first

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increased from 600-1000 °C and then decreased slightly at 1200 °C. The above results indicated that the sodium/potassium atoms expanded the pores and got through the channels continuously with the rise of temperature. It was also the reason that the proportion of micropores was smaller while the proportion of small mesopores was larger. The total pore volumes of PN-X samples were small, which resulted in a high powder density to obtain an ultrahigh volumetric capacitance. At a temperature around 800 °C, the metallic sodium/potassium and air generated pores and channels to form interconnected network porous carbon materials.[47] Meanwhile, part of calcium/ magnesium oxides can combine with each other to hold up more space. The excessive shrinkage of pores and the agglomeration of calcium/ magnesium oxides leaded to the collapse of carbon framework, which destroyed SSA and PV irreversibly at 1200 °C. The results can also be proved by data in Table 2.

Fig.3. (a) XRD pattern and (b) Raman spectrum of PN samples.

The XRD pattern of PN samples were shown in Fig. 3a. There were no sharp and high peaks due to amorphous state of the PN-600. While it can be clearly found that the PN-800, PN-1000 and PN-1200 exhibited two XRD peaks at 2θ=22-24° and 42-44°,

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which corresponded to the diffraction of (002) and (100) crystal plane of the graphitic sheets, respectively.[48] It indicated that dead pine needles can form graphitic layers more easily under a low heating temperature than other biomass materials. The (002) diffraction peak of PN-1000 was higher than that of PN-600 and PN-800, which indicated that the graphitization degrees of samples were slightly enhanced with activation temperature rising. However, the low and broad intensity of (002) diffraction peak and weak (100) diffraction peak suggested that the PN samples contained highly amorphous, disordered nanostructure and rarely quantity of ordered graphitic lattices. The Raman spectrum of the PN samples was shown in Fig. 3b, a G-band at 1590 cm-1 and D-band peak at 1330 cm-1 were existed. The dangling bonds of the vibrations of carbon atoms were usually related with peak at 1330 cm-1 (D-band) for the in-plane terminations of disordered graphite and the vibration in all sp2 bonded carbon atoms was closed associated to the peak at 1590 cm-1 (G-band).[49] The G peak of PN-1000 was slightly higher and narrower than D peak of PN-1000, it indicated that the degree of crystallinity of PN-1000 was higher than PN-600 and PN-800. However, the strong D-band peak of PN-1000 demonstrated that it had a low degree of crystallinity since it still contained a obvious quantity of disordered defects and sections.

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Fig. 4. The SEM and TEM images of the pine needle-derived activated carbons. (a)The harsh and rough surface of PN-600. (b) The smooth surface of PN-800 with less non-carbonated particles and organic impurities. (c) The porous nanostructures of the PN-1000. (d) The smoothest surface and graphitic layers of PN-1200. And the (a’), (b’), (c’) and (d’) were magnified images of the red cycle part of (a), (b), (c) and (d), respectively. (e) The TEM image of PN-600. (f) The TEM image of PN-800. (g) The TEM image of PN-1000. (h) The TEM image of PN-1200.

It can be seen in Fig. 4a that there were a lot of organic impurities and non-carbonated particles on the rough surface of PN-600. With the rise of temperature, the surface of PN-800 in Fig. 4b was smoother than that of PN-600, so PN-800 had less non-carbonated particles and impurities. Fig. 4c obviously indicated that the surface of PN-1000 had abundant defects and pores. The surface of PN-1200 was the most smooth and it exhibited the graphite structure in Fig. 4d. It can be seen in Fig. 4e, f, g that there were more and more pores in the carbon frameworks of PN samples with the temperature rising. The degree of graphitization and interconnected pores of carbon materials were two crucial factors as electrodes for high rate supercapacitors.

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ACs with a high degree of graphitization had a great electrical conductivity, which can benefit the charge transfer.[50-51] It can be also proved by the rectangular CV curves of PN-1000 and PN-1200. The interconnected pores and channels can provide the rapid transport of electrolyte ions at the high current density. As shown in Fig.4g, the pores of PN-1000 were interconnected. In Fig. 4h, there were ordered graphitic lattices of PN-1000, 1200 in Fig. 4h and abundant amorphous carbons of PN-600 and PN-800, which can match the results of XRD and Raman spectra. Table 3 XPS surface elemental composition of PN samples Sample

Carbon at% (1s)

Oxygen at% (1s)

Nitrogen at% (1s)

PN-600 PN-800 PN-1000 PN-1200

81.41 84.88 86.59 89.60

16.78 14.18 12.52 9.95

1.81 0.94 0.89 0.45

Fig. 5. Characterizations of PN-1000. (a) XPS survey spectrum of PN-1000. (b) High-resolution XPS analysis of the C1s peaks of PN-1000. (c) High-resolution XPS analysis of the O1s peaks of PN-1000. (d) High-resolution XPS analysis of the N1s peaks of PN-1000.

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The elemental analysis exhibited the content of C, O and N in PN-1000 by XPS measurements in Fig. 5. The content of oxygen and nitrogen in the PN-1000 can influence the electrochemical performance directly. Fig. 5a presented the XPS survey spectrum of PN-1000, revealing C1s, O1s and N1s peaks, which proved that the O and N atoms have been doped into the carbon frameworks of PN-1000 successfully. The results of C1s, O1s and N 1s were exhibited in Fig. 5b, c and d in detail, respectively. As shown in Fig. 5b, the C 1 s spectrum was decomposed into four peaks at 284.8, 285.6, 287.1 and 289.36 eV, which were C-C, C-O, C=O and O-C=O, respectively. Meanwhile, Fig. 5d showed four types of nitrogen, which were located at 403.1, 401.1, 399.8, 398.4 eV, corresponding to the pyridinic-N-oxide (N-oxide), quaternary-N (N-Q), pyrrolic-N (N-5) and pyridinic-N (N-6) groups, respectively. Additionally, the contents of oxygen and nitrogen in PN-1000 calculated from XPS were 12.52 and 0.89 at% as shown in Table 3. And it can be clearly seen that the contents of O and N groups decreased with the rise of temperature. The N and O can improve the stability and wettability of the carbon material surfaces. It was believed that the O-C and N-C can provide a lot of active sites to generate pseudocapacitance and improve the rate performance.[52] However, it was important to find a balance between pores and the contents of heteroatoms. The interconnected pores and channels were also key factors for excellent electrochemical performance. It can be proved by the electrochemical performance of PN-1000 in Fig. 6.

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Fig.6. Electrochemical performance characteristics of PN samples measured in 1 M H2SO4. (a) Galvanostatic charge-discharge curves at high current density of 0.5 A g-1. (b) CV curves at the scan rate of 5 mV s-1. (c) Rate performance of PN-600, 800 and 1200. (d) Rate performance of PN-1000. (e) Cycling stability of PN-1000 at 10 A g-1. (f) Nyquist plots of PN samples.

The electrochemical performance of PN samples for supercapacitor electrodes was characterized by galvanostatic charge-discharge curve (GCD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in Fig. 6. The specific capacitance, CV curves and EIS was tested by three-electrode system while cycling stability were tested by two-electrode system. As shown in Fig. 6a, all PN samples 18


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were charged and discharged smoothly with the typical triangle curves at the current density of 0.5 A g-1. Although PN-600 had the highest contents of nitrogen and oxygen groups, it had the lowest specific capacitance among all PN samples. Because specific surface area, pores and channels were the most important factors for carbon materials, which can generate double electrical layers during the charging-discharing process. It can be seen in Fig. 6a that the PN-1000 had a longer discharge time and smaller IR drop than the other samples. It was due to the highest efficiency specific surface area, the largest pore volume and suitable pore size distribution of PN-1000. And it also had 12.52 at% oxygen and 0.89 at% nitrogen groups, which can provide a considerable pseudocapacitance. At the current density of 0.5 A g-1, the specific capacitance of PN-600, 800, 1000 and PN-1200 were 101 F g-1, 214 F g-1, 223 F g-1, 128 F g-1, respectively. The powder density of PN-1000 can achieve 0.80 cm3 g-1, which can generate an ultrahigh volumetric specific capacitance of 178 F cm-3 The electrochemical performance of PN samples as electrodes for supercapacitors was characterized by cyclic voltammetry (CV) measurements at 5 mV s-1 in Fig. 6b. The CV curves of PN-1000 and PN-1200 showed almost symmetrical rectangular shapes, indicating an excellent electrochemical behavior. With the rise of temperature, there were more and more graphitic lattices generated in the carbon frameworks of PN samples, which can be proved by XRD pattern, Raman spectrum in Fig.3 and SEM, TEM images in Fig.4. And the graphitic carbon layers in carbon frameworks can improve the electrical conductivity of porous carbon materials and the chemical and physical stability of carbon nanostructure during a rapid charging-discharging process.

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So it may be the reasons that PN-1000 and PN-1200 had the best CV curves. It can be also seen in Fig. 6b that PN samples had peaks on the curves around 0.4 V. Because there were nitrogen and oxygen doped in carbon frameworks, which can generate the pseudocapacitance. Fig. 6c was the rate performance of PN-600, 800 and 1200 as electrodes for supercapacitors in 1 M H2SO4 electrolyte. The specific capacitance of the PN-1000 electrode showed the slowest fading rate. The PN-1000 showed a specific capacitance of 223 F g-1 at 0.5 A g-1 while it still had 150 F g-1 even at 100 A g-1 in Fig.6d. It was due to abundant pores and the interconnected channels, which can reduce the resistance in pores and slow the fading capacitance at high scanning rates. Compared with PN-1000, the other three samples showed inferior rate performance under high current density. Especially, the PN-600 exhibited a rapid decrease, which was due to few pores and channels under the low carbonization temperature. And the PN-1000 exhibited a capacitance retention of 98% at a high current density of 10 A g-1 after 10000 cycles in Fig. 6e. The above results proved that PN-1000 had a high-rate performance as electrodes for supercapacitors. Fig. 6f showed the Nyquist plots for all PN samples. Compared with the PN-600, 800, and 1200, the PN-1000 exhibited a almost vertical line in the low frequency region range from 100 kHz to 0.01 Hz, indicating a good capacitive performance. The PN-1000 plot also showed a straighter slope and short transition process in the plot, indicating a much lower charge transfer resistance. Furthermore, from the high frequency region (inset in Fig. 6f), the PN-1000 had shorter Warburg domain in the

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plot and smaller semicircle, which proved evidently better ion transfer efficiency and smaller contact resistance between electrolyte solution and electrode material than the other three samples. Above all, the PN-1000 had the lowest equivalent series resistance (ESR), which was due to good intrinsic electronic properties of samples, low ions transfer resistance in the materials and low contact resistance between electrode and current collector. CONCLUSION In this paper, dead pine needles were used as precursor to prepare activated carbons with the high rate performance as supercapacitor electrodes. The PN samples were prepared by an easy one-step carbonization without adding any reagents. The unique cell structure, abundant metal atoms (K, Na, Ca, Mg), heteroatoms (O, N) of dead pine needles and the role of air were considered, which can replace chemical reagents, improve chemical and physical properties of ACs and provide a high rate performance during the charging-discharging process. The whole process was green chemistry, safe and sustainable. The mechanism of one-step carbonization was proposed in detail that Ca and Mg can hold up some space in the carbon frameworks while Na and K played the role of forming pores and getting through channels. The air can create and enlarge pores, sweep the remaining impurities and interconnect the pores and channels. The interconnected pores and great surface properties of PN-1000 resulted in a high rate performance as electrodes for supercapacitors. It showed a specific capacitance of 223 F g-1 (178 F cm-3) at 0.5 A g-1 while it still had 150 F g-1 (120 F cm-3) even at 100 A

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g-1. So the PN-1000 was a promising carbon material for the high-rate supercapacitors in many practical applications. ACKNOWLEDEMENTS This research is financially supported by the projects in forestry public benefit research sector (201404610). REFERENCE [1] Zuo W, Li R, Zhou C, et al. Battery-Supercapacitor Hybrid Devices: Recent Progress and Future Prospects. ADV SCI, 2017. (DOI: 10.1002/advs.201600539) [2] Lu X, Yu M, Wang G, et al. Flexible solid-state supercapacitors: design, fabrication and applications. ENERG ENVIRON SCI, 2014, 7(7):2160-2181. (DOI: 10.1039/C4EE00960F) [3] Wang S, Wu Z S, Zheng S, et al. Scalable Fabrication of Photochemically Reduced Graphene-Based Monolithic Micro-Supercapacitors with Superior Energy and Power Densities. ACS NANO, 2017, 11(4):4283. (DOI: 10.1021/acsnano.7b01390) [4] Dong L, Xu C, Li Y, et al. Flexible electrodes and supercapacitors for wearable energy storage: a review by category. J MATER CHEM A, 2016, 4(13):4659-4685. (DOI: 10.1039/C5TA10582J) [5] Sun Y, Yan X. Recent Advances in Dual‐Functional Devices Integrating Solar Cells and Supercapacitors. Solar Rrl, 2017, 1(3-4). (DOI: 10.1002/solr.201700002) [6] Hou J, Cao C, Ma X, et al. From rice bran to high energy density supercapacitors:

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Synopsis The PN-1000 with the high rate performance is prepared by an one-step carbonization, which is sustainable, green chemistry, low cost and high value-add.

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For Table of Contents Use Only

Table 1. Elemental composition of green and dead pine needles K Na Ca Mg C N Sample Ca/C ratio (%) (%) (%) (%) (%) (%) Green pine needle 0.37 0.1 0.84 0.13 50.1 2.03 1.6 dead pine needle 0.7 0.1 1.2 0.15 51.8 1.33 2.3

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Samples PN-600 PN-800 PN-1000 PN-1200

Table 2. Characteristics of pores in PN-X samples Pore Volume (cm3 Average pore size (nm) SSA (m2 g-1) g-1) 461 0.25 2.18 763 0.42 2.35 783 0.49 2.55 620 0.40 2.59

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Table 3. XPS surface elemental compositon of PN samples Sample Carbon at% (1s) Oxygen at% (1s) Nitrogen at% (1s) PN-600 81.41 16.78 1.81 PN-800 84.88 14.18 0.94 PN-1000 86.59 12.52 0.89 PN-1200 89.60 9.95 0.45

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Scheme 1. (a) the dead pine needles; (b) the longitudinal section of the dead pine needles; (c) the cross section of the dead pine needles; (d) the PN-1000; (e) the internal profile of PN-1000 (f) the cross section of PN-1000; (g) the external surface of PN-1000; (b’), (c’), (e’), (f’) and (g’) was the magnified image of the red cycle part of (b), (c), (e), (f) and (g).

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Fig. 1. Thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) analysis (left and right y axes, respectively) under an air atmosphere, heating rate: 10°C min-1.

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Scheme 2. Synthesis scheme for the preparation of ACs from dead pine needles in air. (1) Pyrolysis of the organic moiety and formation of Na2CO3 /K2CO3; (2) Decomposition of Na2CO3/K2CO3 with the formation of Na2O/K2O (gasification and chemical activation reactions), calcium/ magnesium (Ca/Mg) carbonate was formed firstly; (3) Reduction of Na2O/K2O with formation of metallic Na/K (chemical activation and Na/K intercalation processes), CaO/MgO can combine with each other and hold up some space between graphene layers; (4) Removal of Na/K/Ca/Mg inorganic compounds (washing).

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Fig. 2. (a) N2 adsorption-desorption isotherms, (b) The corresponding pore size distributions of the PN 600-1200 samples.(X axis: 0-10 nm).

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Fig.3. (a) XRD pattern and (b) Raman spectrum of PN samples.

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Fig. 4. The SEM and TEM images of the pine needle-derived activated carbons. (a)The harsh and rough surface of PN-600. (b) The smooth surface of PN-800 with less non-carbonated particles and organic impurities. (c) The porous nanostructures of the PN-1000. (d) The smoothest surface and graphitic layers of PN-1200. And the (a’), (b’), (c’) and (d’) were magnified images of the red cycle part of (a), (b), (c) and (d), respectively. (e) The TEM image of PN-600. (f) The TEM image of PN-800. (g) The TEM image of PN-1000. (h) The TEM image of PN-1200.

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Fig. 5. Characteriazations of PN-1000. (a) XPS survey spectrum of PN-1000. (b) High-resolution XPS analysis of the C1s peaks of PN-1000. (c) High-resolution XPS analysis of the O1s peaks of PN-1000. (d) High-resolution XPS analysis of the N1s peaks of PN-1000. 41



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Fig.6. Electrochemical performance characteristics of PN samples measured in 1 M H2SO4. (a) Galvanostatic charge-discharge curves at high current density of 0.5 A g-1. (b) CV curves at the scan rate of 5 mV s-1. (c) Rate performance of PN-600, 800 and 1200. (d) Rate performance of PN-1000. (e) Cycling stability of PN-1000 at 10 A g-1. (f) Nyquist plots of PN samples.

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From Dead Pine Needles to O, N Codoped Activated Carbons by a

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