Cellulose-Solvent-Assisted, One-Step Pyrolysis to Fabricate

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Cellulose Solvent Assisted, One-step Pyrolysis to Fabricate Heteroatomsdoped Porous Carbons for Electrode Materials of Supercapacitors Peidong Fan, Jia Ren, Kanglei Pang, Yu Cheng, Xiao Wu, zhiguo Zhang, Junkai Ren, Wei Huang, and Rui Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00589 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Sustainable Chemistry & Engineering

Cellulose Solvent Assisted, One-step Pyrolysis to Fabricate Heteroatoms-doped Porous Carbons for Electrode Materials of Supercapacitors Peidong Fan,a1 Jia Ren,ac1 Kanglei Pang,ac Yu Cheng,a Xiao Wu,ab Zhiguo Zhang,a Junkai Ren,a Wei Huang,*ab Rui Song*a a

School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing, 100049, China b Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China c Sino-Danish College (SDC), University of Chinese Academy of Sciences, Beijing, 100190, China *Email: [email protected]; [email protected] These authors contributed equally to this paper.

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Abstract A low-priced, eco-friendly and sustainable strategy to fabricate materials of superior electrochemical property is of paramount importance for the development of supercapacitors (SCs). Herein, heteroatoms-doped porous carbons (HPCs) were prepared from waste newspapers (WNPs) with the assistance of cellulose solvent, i.e. aqueous solution consisting of NaOH, urea and thiourea (NUT, for short), and subsequently the resultant HPCs were applied as materials for SCs electrodes. Innovatively, the NUT solution act simultaneously as solvent for WNPs, activating agent and co-dopants (N, S, and O) in the subsequent preparation of HPCs materials. Interestingly, the porous structure and doping content of HPCs are tunable by changing the related feed ratio between NaOH and WNPs (WNaOH/WWNPs, 0.5, 1.0, 1.5), along with the pyrolysis temperature (600, 700, 800 °C). The appropriate porosity along with the surface doped by heteroatoms (N, S, and O) of the obtained HPCs result in superior electrochemical performance endowing them with potential for high performance SCs. The CS3-T800 exhibits a maximum specific capacitance of 308 F g-1 (1 A g-1), excellent cycle capability (94.0% of capacitance retention after 5000 charge/discharge cycles) in a three-electrode system. Moreover, the CS3-T800-based symmetric SCs exhibit a energy density of 10.48 Wh kg-1, and power density of 250 W kg-1, all making it a promising electrode material for SCs. The enhancement mechanism for the porous carbon electrode reveals that rationally designed mesopores and micropores are more critical in porous electrode performance. This work hereby provides an excellent paradigm to produce highly porous and conductive carbons from biomass like WNPs for various energy storage applications. Keywords: cellulose solvent; waste newspapers; heteroatoms-doped porous carbons; supercapacitors

Introduction With the mounting demand for portable devices and hybrid electrical vehicle all over the world, supercapacitors (SCs) have received tremendous interests owning to the fast charge/discharge rates, high power density, and excellent cycle performance.1-4 However, the energy density of commercial SCs is currently an order of magnitude lower than that of batteries, which severely impeded their further applications.5,6 Therefore, it is of paramount importance to boost the powder density of SCs to satisfy 2


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the impendent demands of uninterruptible power supplies. SCs, on the other hand, can be categorized into electric double-layer capacitors (EDLCs) that are based on electrostatic charge diffusion and accumulation in the double layer formed at the surface of inert electrodes,7 and pseudocapacitors that are primarily dominated by the rapid and reversible faradaic reactions occur on the electrode surface.8 A relatively high capacitance loading is one of the greatest advantages for pseudocapacitors, however, there are still various barriers on the way to the practical applications due to the poor rate performance and stability. On the contrary, EDLCs have remarkable rate performance and stability, but their shortcomings are discontented capacitance and powder density.9 Recently, heteroatoms-doped porous carbons (HPCs) have been emerged as a promoting materials which can fills the gap between pseudocapacitors and EDLCs.10-12 Many studies have indicated that excellent electrical conductivity, large specific surface area (SSA) of HPCs will considerably contribute to the electrical double-layer capacitance.13-18 Meanwhile the heteroatoms functional groups in carbon framework can enhance the pseudocapacitance of carbon electrodes,19 in that the difference in electronegativity between the heteroatoms and carbon atoms brings a more polarized surface, and thus promoting the wettability of the carbon networks. Therefore, a high transfer rate of electrolytic ions can be ensured in microporous and/or smaller mesoporous, which will improve the availability of surface area and thus enhance supercapacitance performance.20,21 All these characteristics endow HPCs with better electrochemical performances as compared with conventional commercial electrode materials.4,14 Despite of these tremendous advances, some issues are still heavily limiting HPCs practical application, including complicate synthetic process, poorly controlled pore structure as well as the prohibitive cost of precursors like synthetic polymers,3,22 ionic liquids (ILs),23,24 nanocarbons (e.g. graphene25 and carbon nanotube26 etc.). Taken together, therefore, efficacious manipulation (e.g. doping, carbonization, activation, etc.), fabrication simplicity, cost-efficiency and availability of reactants are the imperative issues for extensive application of SCs. 3



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Waste newspapers (WNPs) is a representative biomass containing low-valued cellulosic material. Nowadays, most of WNPs is simply viewed as one of the low-end alternatives for recycling resources, which contribute severely to greenhouse gas emissions on subsequent dispose process.27 Therefore, utilizing WNPs to fabricate HPCs is undoubtedly significant in view of the environment and economics. Meanwhile, WNPs contain abundant content of CaCO3,28,29 which can produce positive effects on the subsequent activation processes.30,31 Unfortunately, the stiff three dimensional (3D) structure and rare heteroatoms contents of WNPs have complicated

the

pretreatment

and

thus

heavily

limit

the

increase

in

pseudocapacitance.32 Lina Zhang and her group first reported that NaOH/urea and NaOH/thiourea aqueous solutions could be used to directly and quickly dissolve cellulose at low temperatures (-5– -12 oC).33,34 Moreover, Gu et al. identified that the aqueous solution mix of NaOH, urea and thiourea can dissolve cellulose more powerfully.35 Both these works absolutely are valuable in that the dissolution process of cellulose will destroy the 3D structure of WNPs thus generate homogeneous solution, which is conducive to the subsequent utilization of WNPs. More important, the mix solution including NaOH, urea and thiourea of cellulose could serve both as an activating agent and N, S co-dopant in converting WNPs into functional HPCs materials. Herein, inspired by the abovementioned works, we first introduce the ternary cellulose solvent, NUT solution, comprising NaOH, urea and thiourea for the one-step pyrolysis method to fabricate the HPCs with excellent electrochemical energy storage property. Specifically speaking, the NUT solution can simultaneously serve as solvent, activating agent, and co-dopants, which offers a universal route to synthesize value-added HPCs materials from WNPs for energy storage systems. Being served as the electrode materials of SCs, the as-prepared HPCs display superior capacitance, good conductivity, excellent rate capability, long cycling stability, and large power densities. Based on these superiorities, accordingly, the low-cost and massive WNPs present a foreseeable potential for scalable and benign recycling prospects. 4


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Furthermore, we envisage that the cellulose solvent would be extendable to more biomass containing cellulose to fabricate HPCs for the extensive applications including energy storage, environment, electronics, and so forth.

Material AND METHODS Chemical. All chemicals including sodium hydroxide (NaOH), thiourea (CH4N2S), and urea (CH4NO) are provided by Aladdin Corp. Ltd., Shanghai, China. All chemicals are of guaranteed reagent levels. WNPs were collected from the routine offices. Deionized water (18.2 MΩ) was used throughout the experiments. Synthesis of CSx-Tys. Firstly, to investigate the effects of NaOH content, various weights (0.5 g, 1.0 g, 1.5 g) of NaOH were dispersed into 9.0 g of mixed aqueous solution (thiourea/urea/H2O at the ratio of 7.8: 7.8: 84.4 by weight) to be used as cellulose solvent after pre-cooled at -12 oC. As shown in Figure 1, 1.0 g of WNPs pieces were dispersed into each solution and stirred at ambient temperature, r.t., for 30 min. Then, the mixtures were transform to a centrifuge tube and then plunged into liquid nitrogen to be frozen completely. Next, the frozen sample was subjected to freeze drying until the sample was completely dry. The three different precursors obtained were coded as CSx (x = 1, 2, 3, corresponding to the weight ratios between NaOH and WNPs, WNaOH/WWNPs, of 0.5, 1.0, 1.5, respectively). The CSx were treated at 250 °C for 30 min and then heated in 800 °C for 3 h in N2 atmosphere in a tubular furnace. Among them, the CS3 was selected to undergo pyrolysis at another two different temperature (600 oC and 700 oC) to explore the effect of pyrolysis temperature. Upon cooling down to r.t., the black product was added into a 50 mL flask with 40 ml hydrochloric acid (1 M) and further stirred for 1 h, then thoroughly wash with deionized water until a neutral pH value was reached. The porous carbon samples were obtained after being dried at 120 °C for 6 h in vacuum and denoted as CSx-Ty (y = 600, 700, 800, corresponding to the pyrolysis temperature of 600°C, 700°C, and 800°C, respectively).

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Figure 1. Schematic illustration of the preparation of CSx-Tys. Characterization. A scanning electron microscopy (SEM) (SU8000, Hitachi, Japan) and transmission electron microcopy (TEM, HT-7700, Hitachi, Japan) were used to analyze the morphologies of the as-prepared materials. A Rigaku Smartlab X-ray diffraction (XRD) equipment was applied to collect the patterns of the obtained samples using Cu Kα (λ = 0.1544 nm) radiation. Thermo-gravimetric analysis (TGA) was performed on a SDT-Q600 device over the temperature range from 50 to 800 oC at a heating rate of 10 oC min-1 under flowing N2 (100 mL min-1). The Fourier transform infrared (FTIR) and Raman spectra were collected with a Vertex 70 spectrophotometer (Bruker Optik GmbH, Germany) and a Renishaw Invia Raman spectrometer with a 20 mW Ar+ laser source of 532 nm, respectively. X-ray photoelectron spectroscopy (XPS) was performed on the Thermo Scientific ESCA Lab 250 Xi using 200 W monochromatic Al Kα radiation. The surface area and pore size distribution (PSD) were characterized by the N2 adsorption isotherms at 77 K using a Micrometrics ASAP 2460 instrument. The SSA was calculated using the multi-point Brunauer-Emmett-Teller (BET) method. Electrochemical measurements. Both the typical three- and two-electrode systems were performedto test the electrochemical performance of CSx-Tys, and all relevant measurements were carried out on a CHI 660D system (Chenhua Instrumental Co, Ltd., Shanghai). 6


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In the three-electrode system, platinum electrode and saturated calomel electrode, SCE, were selected as the counter electrode and the reference electrode, respectively. The working electrode was prepared by mixed 5 wt% PTFE binder, 10 wt% acetylene black and 85 wt% activated materials fully and grounding in a mortar. Then, the slurry was continuously spread on current collectors (1 cm × 2 cm nickel foam), and dried at 80 oC for 24 h. The total mass loading of each electrode was about 4 mg. After the fabricated electrode was pressed at 10 MPa for 1min, the working electrode was obtained. Cyclic voltammetry (CV) curves and galvanostatical charge/discharge (GCD) measurements were recorded in the potential range from -1.0 V to 0 V vs SCE with 6 M KOH as electrolyte. The electrochemical impedance spectroscopy (EIS) measurement was carried out crossing the frequency range from 0.01 Hz to 0.1 MHz. In the two-electrode system, the electrochemical performances of the samples were evaluated in the form of CR2025 coin cells. The test electrodes were two similar electrodes which prepared by using the above mentioned method. The separator was Cellgard 2320 and the electrolyte was 6 M KOH. CV curves and GCD measurements of the cells were recorded in the potential range from 0 V to 1.0 V.

RESULTS AND DISCUSSION Material characterization. SEM and TEM observations were employed to visualize the morphology of WNPs and CSx-Tys (Figure 2, Figure S2-S4). As shown in Figure 2a, there are no obvious defects or pores on the surface of WNPs. Meanwhile, the magnified SEM images of CSx-Tys (Figure 2b-2f3, Figure S2a-2d) clearly reveal the open and interconnected pores forming in a 3D macroporous frameworks. Interestingly, the size of porous network can be tuned by changing the WNaOH/WWNPs or pyrolysis temperatures, i.e. the structure of obtained materials will translate from nanolayer to three-dimensional macroporous frameworks, with pyrolysis temperature elevating from 600 °C to 800 °C or the WNaOH/WWNPs from 0.5 to 1.5. Specifically and more clearly, the SEM images in high magnification of fabricated CS3-T800 (Figure 2f1-f2) demonstrate a network comprising of intensively 7



interwoven mesopores and macropores with pore sizes ranging 10 nm to 100 nm. Besides, the CS3-T800 is largely composed of a quasi-amorphous frameworks with a marginal degree of graphitization, encompassing few graphitic carbon layers at the edges and a large number of micropores (Figure 2h1-2h2, Figure S4). Noticeably, some partially aligned graphitic layers are existed in the amorphous porous structure of the CS3-T800 (Figure 2h2), implying a good conductivity in favor of the transportation of electrons.10,28 In addition, the EDX maps in Figure 2g indicate that the elements C, N, O, and S are uniformly distributed throughout the CS3-T800.

Figure 2. SEM images of (a) WNPs, (b) CS1-T800, (c) CS2-T800, (d) CS3-T600, (e) CS3-T700, (f1-f3) CS3-T800, (g) the relevant EDX images of C, O, S and N elements ofCS3-T800, and (h1-h2) TEM and HRTEM images of the CS3-T800. The N2 adsorption/desorption and PSD measurements are carried out to 8


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investigate the textural properties. All CSx-Tys present typical type IV N2 adsorption/desorption isotherm with a type-H4 hysteresis loop according to IUPAC classification, indicating the presence of micropores and mesopores (Figure 3a).37,38 In detail, the isotherms exhibit superior adsorption property in the low relative pressure (P/P0 < 0.01), implying that CSx-Tys are rich in micropores. Moreover, the appearance of significant hysteresis characteristics caused by capillary condensation at relatively higher pressure (P/P0 = 0.4–1.0) signifies the existence of abundant mesopores, while a slight upward tendency at the high relative pressure (P/P0 = 0.90– 1.0) corresponds to the presence of macropores.9,39,40 Therefore, as well indicated in the above TEM observations, CSx-Tys present a radically developed hierarchical porosity network of copious micropores, and mesopores along with sparse macropores. The porosity is commonly attributed to the mechanism of NaOH activation, which is based on the following reaction: 6NaOH + 2C = 2Na + 2Na2CO3 + 3H2.37 Normally, WNPs contain abundant contents of CaCO3 that will decompose at a high temperature (CaCO3 = CO2 + CaO),28,29 which can account for the obvious

weight losses ranging from 650 oC to 700 oC of WNPs in the TGA experiment (Figure S1). Subsequently, CaO tend to connect each other and may occupy certain space between carbon layers.41 After washing with hydrochloric acid and deionized water (see Experimental part), the metallic oxide particles will be removed and the therein generated pores will lead to improved SSA.30

9


3

1600

CS3-T800 CS3-T700 CS3-T600

1200 800 400

2500

1.0

mesopores micropores

3 -1

c

0.2 0.4 0.6 0.8 Relative Pressure (P/Po)

2000 1500 1000 500 0

CS1-T800CS2-T800CS3-T800CS3-T700CS3-T600

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CS3-T800 CS3-T700 CS3-T600

CS1-T800 CS2-T800

b

0.10 0.12

0.08 0.06

0.08

0.04

0.04

0.02

0.00

1

2

3

4

5

0.00

Specific Pore Volume (cm g )

0 0.0

0.12

-1

CS1-T800 CS2-T800

3

a

dV/dW Pore Volume (cm g nm)

3000

-1

Quantity Adsorbed (cm g STP)

2000

2 -1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Specific Surface Area (m g )


20 2.5

40 60 80 Pore Width (nm)

d

2.0

100

mesopores micropores

1.5 1.0 0.5 0.0

CS1-T800CS2-T800CS3-T800CS3-T700CS3-T600

Figure 3. (a) N2 adsorption/desorption isotherm, (b) pore size distribution, (c) BET surface area distribution, and (d) pore volume distribution of the CSx-Tys. Figure 3b shows the pore size distribution of CSx-Tys, exhibiting that the pores in CS3-T600 is mainly micropores, while the CS3-T700 and CS3-T800 possess micropores- and mesopores-mixing innerstructure with the pore size ranging 0.5 nm– 5 nm. In this case, the SSA, pore volume of micropores and mesopores were respectively calculated by t-plot method, excluding the rare amount of macropores.39,40 Noticeably, the adsorption capacities increase with the elevating pyrolysis temperature. Concretely, with the pyrolysis temperature increasing from 600 °C to 800 °C, both SSA and pore volume increase dramatically from CS3-T600 (~775 m2 g-1, ~0.48 cm3 g-1), CS3-T700 (~2082 m2 g-1, ~1.5 cm3 g-1), to CS3-T800 (~2812 m2 g-1, ~2.29 cm3 g-1) (Figure 3c-3d). Meanwhile, the contribution fractions of the mesopores for total SSA and pore volume increase from CS3-T600 (~0.14, ~0.32), CS3-T700 (~0.20, ~0.51) to CS3-T800 (~0.31, ~0.53), which is beneficial for the transport of electrolyte ions to achieve an excellent rate performance of SCs electrodes.41,42 Therefore, these results suggest that higher pyrolysis temperature will 10


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not only favor the generation of pores, but will produce more mesopores.9,39 Besides, one point should be noted is that higher pyrolysis temperature is the key to govern the pore structure. For CSx-Tys prepared with lower WNaOH/WWNPs, such as CS1-T800 CS2-T800, they can also achieve appreciable porosity, i.e. the obtained SSAs and pore volumes are ~764 m2 g-1 and ~0.56 cm3 g-1 (CS1-T800), ~1244 m2 g-1 and ~0.95 cm3 g-1 (CS2-T800), respectively. It has been widely demonstrated that mesopores and macropores would supply transport and diffusion channels for electrolyte ion, while the micropores tend to offer more space for ion storage energy.42-45 Therefore, the resultant CSx-Tys are potential to be novel electrode materials in SCs to realize a high specific capacitance and rate capability. XRD and Raman spectrum are performed to assess the micro-structure of CSx-Tys. The broad but weak diffraction peak at 22o–24o and an inconspicuous peak at ~43o are indicative of the (002) and (100) planes of graphite, respectively (Figure 4a). Hence, the XRD results indicate that CSx-Tys assume an amorphous and disordered structure.40,46 And the high intensity in the low angle region should be root in the existence of abundant micropores in the resultant samples,5 as proved by the aforementioned results of N2 adsorption/desorption test.

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CS1-T800 CS2-T800 CS3-T800 CS3-T700 CS3-T600

(100)

10 1.2 1.0

c

20

30 40 2Theta (deg.)

ID/IG

50

60

1000

280 240

ID/IG

200

0.6

)

160

-1

0.4

1200

120


1400 1600 1800 -1 Ramanshift (cm )

CS1-T800 CS2-T800

0.2 CS1-T800CS2-T800CS3-T800CS3-T700CS3-T600

G

2000

d

D band

0.8

D

Intendisty (a.u.)

(002)

b

Transmittance (%)

Intendisty (a.u.)

a

FWHM (cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500

CS3-T800 CS3-T700 CS3-T600

1000 1500 3000 3500 Wave Number (cm-1)

4000

Figure 4. (a) XRD patterns, (b) Raman spectra, (c) Variation of intensity ratio of Dto G band, and FWHM of D band, and (d) FTIR spectra of the CSx-Tys. Raman spectroscopy is a powerful technique to detect the structure and quality information on the graphitized carbon materials, including defect structures, disorder, defects density, and doping levels. As seen in Figure 4b, two obvious strong bands are located at ~1355 and ~1590 cm-1, assigned to the D-band and the G-band, respectively. More insights are available by analysis the variation of the ratio of D-band and G-band intensities (ID/IG) as well as the full width at the half maximum (FWHM) value of D-band (Figure 4c). The result reveals that the CSx-Tys with higher WNaOH/WWNPs or subject to higher pyrolysis temperature assume lower FWHM values of D-band, which is attribute to the improved graphite degree with higher pyrolysis temperature or WNaOH/WWNPs.13,22 On the other hand, the ID/IG values are roughly proportional to the ratio of disordered and ordered carbon in the sample.1,40 In this case, the ID/IG values of CS3-T600, CS3-T700, and CS3-T800 decrease from ~0.95, ~0.84, to ~0.83 with pyrolysis temperature increasing from 600 oC, 700 oC, to 800 oC. Likewise, the ID/IG values of the samples CS1-T800, CS2-T800, and 12


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CS3-T800 decrease from ~0.96, ~0.88, to ~0.83 with the WNaOH/WWNPs increasing from 0.5, 1.0, to 1.5. For CS3-T800, the lowest FWHM value of D-band along with the lowest ID/IG value suggest the highest graphite degree, which is significantly inductive to the electric conductivity.47 This is in well agreement with the electrochemical impedance following. The FTIR results prove that heteroatoms doping are successfully realized on the porous carbon frameworks of CSx-Tys (Figure 4d). The major characteristic bands at ~1030 cm-1, and ~1397 cm-1 could be assigned to C-O, C-N stretching, and the characteristic peak at 3350 cm-1 and the three peaks at ~1452 cm-1, ~873 cm-1, ~657 cm-1 are attributed to the stretching vibration of O-H band and the bending vibration of N-H band, respectively, which indicative of the copious functional groups including O or N elements.9,48,49 The XPS will further explore the chemical compositions of the CSx-Tys. As indicated in Figure 5a, four characteristic peaks are observed centered at ~164.2, ~284.3, ~400.2, and ~531.6 eV, assignable to the S 2p, C 1s, N 1s, and O 1s, respectively. As listed in Table 1, the CS3-T600 exhibits higher heteroatoms doping contents as compared to CS3-T700 and CS3-T800, in which N, S, and O doping contents (at.%) can reach up to 5.86, 1.08 and 11.81, respectively. However, with the increasing of pyrolysis temperature the contents of N, S, and O obviously decrease, which can be specifically found in CS3-T800, i.e. the N, S and O doping contents (at.%) are only 1.28, 0.53, and 6.13, respectively. This observation could be attributed to heteroatoms functional groups in carbon frameworks are more facile to be attacked by NaOH at higher pyrolysis temperature or more NaOH content.40,50 Table 1. The various heteroatoms content of CSx-Tys

elemental content (at.%) Sample

C

N

O

S

CS1-T800

72.78

11.68

12.45

3.09

CS2-T800

80.91

7.61

10.36

1.12

CS3-T800

92.06

1.28

6.13

0.53

13



CS3-T700

89.25

2.21

7.97

0.57

CS3-T600

81.25

5.86

11.81

1.08

The high resolution O1s spectra of CS3-T800 (Figure 5b) can be deconvoluted into five subpeaks, ascribed to quinone-type oxygen (O1, ~530.2 eV), carbonyl (O2, ~531.6 eV), C-OH phenol

(O3, ~533.3 eV), ester groups (O4, ~533.6 eV),

carboxylic or chemisorbed oxygen (O5, ~534.3 eV), respectively.9 Noticeably, carbonyl group and quinone-type oxygen are the two largest components in CS3-T800, which are exactly more electrochemically active.51-53 Meanwhile, XPS scans of N1s (Figure 5c) reveal that four types of Ncontaining forms are existed in the CS3-T800 including pyridinic-N (N-6, ~398.2 eV), pyrrolic-N (N-5, ~399.7 eV), graphitic-N (N-Q, ~400.7 eV), and oxidized-N (N-X, ~402.8 eV).40 Specifically, the N-5, N-6 and N-Q are more important for CS3-T800, in that the N-6 would enhance electron transfer to the carbon matrix, and the N-5 assumes good electron-donor characteristics and higher charge mobility, which contributes to effectively improve the electrochemical performance,9,39 and the N-Q could boost the electronic conductivity of carbon networks.54,55 Moreover, the N-6 and N-5 positioned at the graphite edges would participate in redox reactions bringing about pseudocapacitance and therefore improve the capacitance performance.50,56 C1s

Intendisty (a.u.)

S2p

S2s

N1s

O1s

b

O1s

CS1-T800 CS2-T800 CS3-T800 CS3-T700

Content O1 5.3% O2 35.9% O3 27.0% O4 24.0% O5 7.8%

Intendisty (a.u.)

a

CS3-T600 200

400 600 800 1000 1200 Binding Energy (eV)

c

N1s N-6 N-5 N-Q N-X

Content 7.9% 59.6% 28.6% 3.9%

528

530 532 534 536 Binding Energy (eV)

538

S2p

d Intendisty (a.u.)

0

Intendisty (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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S1 S2 S3 S4 S5

Content 14.8% 9.0% 19.5% 28.5% 28.2%

14


396

398

400 402 Binding Energy (eV)

404

162

164

166 168 170 172 Binding Energy (eV)

174

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Figure 5. (a) The XPS spectra of CSx-Tys and high resolution (b) O1s, (c) N1s, and (d) S2p spectra and related fitting peaks of CS3-T800. The high-resolution XPS scans of S2p can be deconvoluted into five subpeaks (Figure 5d). The two peaks, ~163.6 eV and ~164.8 eV, representing S 2p3/2 (S1) and S 2p1/2 (S2) of the C-S bond of the thiophene-S, respectively. And three peaks at ~167.7 eV (S3), ~168.6 eV (S4), and ~169.3 eV (S5) should be assigned to varied oxidized sulfur groups of C-SOx-C bond (x, 2–4).39,53,57 All these attest that the S element are successfully incorporated into CS3-T800. Moreover, more electrons would be located on the surface of CS3-T800, owing to the synergistic interaction of conjugated carbon accompany with the electron-rich S element.48 Besides, a series of redoxfaradic reactions would possibly occur on the S-doped carbon, leading to the superior electrochemical performance. Collectively, the content of N, O and S species for CSx-Tys are summarized in Table S1, and the proposed redoxfaradic reactions of N-, O- and S-containing surface functional groups in CS3-T800 are described in Figure S5. Electrochemical performances. CV and GCD are efficient methods to evaluate the electrochemical performance of the fabricated SCs with the as-synthesized CSx-Tys as electrodes. (All calculation equations (1-8) of electrochemistry are shown in the SI section.) Herein, the tests were conducted by using 6 M KOH as aqueous electrolyte in a three electrode system. As expected, all samples show quasi-rectangular shapes due to the excellent electrochemical behavior of the carbons frameworks (Figure 6a). Meanwhile, a wide reversible hump in CV curves of CSx-Tys suggest the presence of pseudocapacitance.9 It should be noted that the CS3-T800 electrode assumes a much larger integrated area than other electrodes, validating that the electric double-layer capacitance can be enhanced by increasing the 15



SSA.1 Besides, the wide reversible hump in CV curves of CS3-T800 implies the presence of pseudocapacitance.22 The enhanced energy storage capability could be reasonably ascribed to the synergetic effect from the porous structure with rational pore size and its distribution, and the effective heteroatams doping, especially N and S binary doping. As shown in Figure 6b, the GCD profiles of CSx-Tys exhibit a quasi-isosceles triangle shape at the current density of 1 A g-1, suggesting CSx-Tys assume excellent double-layer capacitive performance and decent electrochemical reversibility.58 Further, the specific capacitance can be determined from the discharge curves according to the equation Cs = It/mV (equation 1, SI part). Apart from CS3-T800, the pyrolysis temperatures, the SSAs, and electrochemical characteristics of some biomass-derived porous carbon electrode materials are given in Table S2. Thanks to the high SSA, well-distributed porous structure, and the effective heteroatoms doping, the CS3-T800 delivers the largest gravimetric capacitance (Cs) values (~308 F g-1, 1 A g-1), relative to ~135, ~200, ~254, ~161 F g-1 for CS1-T800, CS2-T800, CS3-700, CS3-T600, respectively. The superiority of CS3-T800 as the electrode is further illustrated. The CV curves of CS3-T800 at varying scan rates exhibit a quasi-rectangular shape with scan rates ranging from 2 mV s-1 to 50 mV s-1, implying the fast charging/discharging capability and a decent double-layer capacitance especially at the high scan rate; while a serious distortion suggest more presence of pseudocapacitance when scan rate reach to 200 mV s-1 (Figure 6c). Correspondingly, the GCD curves with increasing current densities (1 to 20 A g-1) show approximately triangular shape, suggesting low internal resistances, being attributable to low mass transfer resistances owing to the high N-S-O –doping degree, extremely high SSA and hierarchical pore structure manifested in pore size and its distribution, and good conductivity of the CS3-T800 electrode (Figure 6d). Additionally, the CV curves at varying scan rates and GCD curves of other CSx-Tys with varying current densities are depicted in Figure S6 and Figure S7, suggesting the similar electrochemical characteristics of CS3-T800. In view of the practical case, rate capability and cycling stability are two 16


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important considerations for SCs electrodes. The rate performance is evaluated by the GCD measurement under enhanced current load. As Figure 6e indicated, the specific capacitances of CS3-T800 electrode are determined to be ~308, ~298, ~245, ~212, ~201 F g-1 at current densities of 1, 2, 5, 10, 20 A g-1, respectively, obviously the highest among all the samples. Moreover, CS3-T800 exhibit the best rate capability about 65.3% capacitance retention at 20 A g-1, comparing with the capacitance retentions ranging from 50.9% to 60.1% of other CSx-Tys.

0 CS1-T800 CS2-T800 CS3-T800 CS3-T700 CS3-T600

-4 -8 -1.0

-1.0 100 200 300 400 500 600 700 Time (s) -1

d

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2A g

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2mv s 5mv s-1

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-1

10mv s

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e

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0.0

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5 10 15 20 -1 Current Density (A g )

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100 200 300 400 500 600 Time (s) CS1-T800 CS2-T800 CS3-T800 CS3-T700 CS3-T600

f

)

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)

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-0.6

c

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Potential (V vs SCE)

4


b

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Specific Capacitance (F g

-1

Current Density (A g )

8

Specific Capacitance (F g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400 350 300 250 200 150 100 0

1000

2000 3000 4000 Cycle numbers

5000

Figure 6. (a) CV curves at scan rate of 10 mV s-1 of CSx-Tys. (b) GCD profiles at current density of 1 A g-1 of CSx-Tys. (c) CV curves of CS3-T800 at different scan rates, (d) The GCD profiles at different current densities of CS3-T800. (e) rate 17



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performance, and (f) cycle stability after 5000 cycles of CSx-Tys. The synergy between hierarchical porous structure and N-S-O tri-doping is believed to endow the obtained materials with excellent capacitance and rate performance. Firstly, the micropores with appropriate pore size would supply adequate sites for charge storage. Second, the mesopores will favor ion penetration and transfer into the micropores. Third, the macropores should play as linkage between the electrolyte and the inter-channels of the electrode material. Finally, the N-S-O

tri-doping

on

the

carbon

frameworks

would

offer

significant

pseudocapacitance, and meanwhile the functional groups derived could also boost the wettability of the carbon-analogous electrode, obviously beneficial to improve the capacitance behavior.28 In addition, CS3-T800 also exhibits the best cycling stability which displays a negligible capacitance degradation and retains a high retention of ~94.0% after 5,000 cycles at 20 A g-1, depicting superior long-term cycling stability (Figure 6f). In comparison, the other CSx-Tys based SCs electrodes manifest less than ~92.0% specific capacitance retentions after 5,000 cycles. The possible reasons may include the larger pore size facilitating the ion transport, the trimmed contribution of pseudo-capacitance which rate capability is generally inferior to double layer capacitance,14 and the reduced diffusion- and pseudo charge-transfer resistances.40 Electrochemical impedance spectroscopy (EIS) enables to obtain deep insights about the internal resistance and resistance between the electrode and electrolyte. As the Nyquist plots of CSx-Tys shown in Figure 7a, the nearly vertical line in low frequency range exhibits the good capacitive properties of the electrode materials. Aside from this, the charge transfer resistance derived from the semicircle in high frequency corresponds to the electrochemical reaction impedance of the electrode materials, and a smaller circular signifies a rapid charge transfer rate.59,60 Obviously, all CSx-Tys exhibit lower charge transfer resistances, and CS3-T800 assumes the lowest value. 18



a

25

15 1.2 Z'' (ohm)

10 5 0

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20 30 Z' (ohm)

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Normalized C'

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Phase angle

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10 CS1-T800 CS2-T800 CS3-T800 CS3-T700 CS3-T600

d

0.8 0.6 0.4 0.2 0.0


10

0.01


10

Figure 7. The EIS results of the CSx-Tys in a three-electrode system: (a) Nyquist plots, the inset is the enlarged area in high frequency range. (b) Bode curves. (c) Normalized real part capacitance, C', vs frequency. (d) Normalized imaginary part capacitance, C'', vs frequency. From the Bode phase diagrams in Figure 7b, the phase angles of CS3-T800, CS3-T700, CS2-T800 are determined to be 85.9o, 83.9o, 82.5o, respectively, which are quite approaching that of an ideal capacitor 90o. Further, the evolutions of normalized C' and normalized C'' vs frequency are revealed in Figure 7c and Figure 7d, respectively. The normalized C'' decreases with frequency increasing. And in low frequency range, the C' of CS3-T800, CS3-T700, and CS2-T800 are approaching to plateaus, implying their decent frequency responsibility; while the CS1-T800 and CS3-T600 all display a sharp decline, implying the poor frequency response (Figure 7c). On the other hand, the peak value in the normalized C'' indicates that the component changes into purely capacitive intrinsic (Figure 7d),61 and accordingly, the 19



relative frequency (f0) and relaxing constant (τ0 =1/2πf0) are derived, where τ0 is the duration of the electrode subject to g charging and discharging reversibly.62,63 As indicated, the f0 of the CS3-T800, CS3-T700 and CS2-T800 are respectively 0.57 Hz, 0.50 Hz and 0.27 Hz, higher than CS3-T600 (0.13 Hz), and CS1-T800 (0.09 Hz). Thus, the calculated τ0 of CS1-T800, CS3-T600, CS2-T800, CS3-T700, CS3-T800 are 1.77 s, 1.22 s, 0.59 s, 0.32 s, and 0.28 s, respectively. Apparently, the CS2-T800, CS3-T700, and CS3-T800 would be more favor to ion diffusion, particularly, at higher charging/discharging rates. In view of the prominent electrochemical performance, CS3-T800 is further fabricated into a two-electrode system to probe the supercapacitor properties. As shown in Figure 8a, the CV curves at different scan rates retain a rectangular-like shape with unclear hump as the scan rates increase from 2 mV s-1 to 200 mV s-1, presenting a combined feature of electric double layer capacitance and minor part of pseudo-capacitive property.22 In particular, a specific capacitance of CS3-T800 in 2-electrode is determined to be ~302 F g-1 at 1 A g-1; and the specific capacitance became 196 F g-1 when current density increases to 20 A g-1, demonstrating the excellent rate capacity. The typical triangular and liner feature as shown from the GCD curves with current densities increasing from 1 to 20 A g-1 (Figure 8b), further verify the above result that pseudocapacitance occupies a minor fraction.9 Cycling stability is obviously a vital consideration for the practicability. Impressively, the CS3-T800 electrode presents an excellent cycling stability at current density of 20 A g-1, and the specific capacitance can still maintain ~95.0% of the initial capacity after 5,000 charge/discharge cycles (Figure 8c). Moreover, the GCD curves are highly overlapped after 5,000 cycle operations, accordingly demonstrates that the impedance is almost unaffected (inset in Figure 8c), and the CS3-T800 electrode assumes excellent electrochemical capacitance companied with excellent long-term cycle stability. The energy and power densities derived from the GCD profiles are assessed with a cell potential from -1 to 0 V in 6 M KOH aqueous electrolyte. As shown in Ragone plot, the symmetric device assumes a maximum energy density of ~10.48 Wh kg-1 at a 20


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powder density of 250 W kg-1, after normalized by the total mass of the active materials. Meanwhile the energy density remains as high as 7.42 Wh kg-1 at high power density of 10 kW kg-1 (Figure 8d). This value overwhelms those of existing porous carbon-based and carbon nanomaterials-based SCs devices using the same aqueous electrolyte, typically in the range of 4–8 Wh kg-1.5,64-67 Taken together, large SSA, suitable pore structure, moderate heteroatoms doping, high degree of graphitization and ensuant minute internal resistance of CSx-Tys based electrodes account for such high energy and power densities. Exemplarily, a tandem device filled with 6 M KOH electrolyte is able to light 18 red light-emitting diodes (LED, 2.0 V) compounding in parallel after charging for only 10 s, depicting the high energy and power densities of CS3-T800 (inset in Figure 8d).

Figure 8. (a) CV curves at different scan rates, (b) GCD profiles at various current densities, (c) the cycle stability after 5000 cycles at 20 A g-1 (inset depicts the initial and 5000th GCD curves at current density of 20 A g-1), (d) Ragone plot of specific energy density vs power density (the inset shows a photograph of 18 red LED 21



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powered by two tandem SCs.) of the CS3-T800 in a two-electrode system. Furthermore, the obtained HPCs can also serve as anodes materials for lithium ion batteries. To investigate the performance as lithium-ion battery anodes, a half-cell configuration in 1 M LiPF6 electrolyte was fabricated, and the electrochemical performance of CS3-T800 electrodes was evaluated by using CV and GCD techniques (Figure S8). The remarkable lithium storage behavior of the resulting porous carbons might be accounted for its hierarchical porous nanostructure with high surface area, which can curtail the transport length for lithium ions as well as supply a large electrode/electrolyte interface, and then facilitate the charge-transferring reactions.54 Moreover, heteroatoms doping on the partial graphite carbon are chemically active and thus further boost the electrical conductivity and reactivity by offering more available active sites for Li+ adsorption and enhance Li+ storage ability.

CONCLUSION In this study, a simple but effective all-in-one strategy was presented to produce HPCs for SCs electrode, that is, one-step pyrolysis to simultaneously accomplish carbonization, activation, and heteroatoms-doped process with the assistance of the cellulose solvent, i.e. aqueous solution including NaOH, urea and thiourea. The pore structure and doping content can be modulated by adjusting WNaOH/WWNPs and pyrolysis temperature. As indicated that the higher pyrolysis temperature and WNaOH/WWNPs not only can result in materials with ideal pore structure, but also contribute to enhance the electrical conductivity of materials. As a consequence, the sample CS3-T800, obtained from highest WNaOH/WWNPs (1.5) and highest pyrolysis temperature of 800 oC, assumes maximum specific capacitance of 308 F g-1 (1 A g-1), excellent cycle capability (94.0% of capacitance retention rate after 5000 galvanostatic cycles at 20 A g-1) in the three-electrode system. Furthermore, the as-assembled symmetric supercapacitor by using CS3-T800 as the electrodes displays a high energy density of 10.48 Wh kg-1 and power density of 250 W kg-1

along with

an excellent cycling stability. Specifically, the fabricated specific capacitance can still maintain ~95.0% of the initial capacity after 5,000 cyclic operations (at current 22


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density of 20 A g-1). It is anticipated that the easy, cost-effectively and sustainable fabrication, combined with the appealing electrochemical properties of this WNPs-derived HPCs bestow great potential for application in energy storage systems. ASSOCIATED CONTENT

*S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The calculation equations of electrochemistry; TGA; SEM images; TEM images; CV curves; GCD curves; LIB cell test (PDF)

ACKNOWLEDGEMENTS This work is financially supported by the National Science Foundation of China (21072221, 21172252).

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Cellulose Solvent Assisted, One-step Pyrolysis to Fabricate Heteroatoms-doped Porous Carbons for Electrode Materials of Supercapacitors.

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Cellulose-Solvent-Assisted, One-Step Pyrolysis to Fabricate

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