Three-dimensional hierarchical porous carbon with high oxygen

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Three-dimensional hierarchical porous carbon with high oxygen content derived from organic waste liquid with superior electric double layer performance ZHI-QIANG HAO, Jingpei Cao, Ya-Li Dang, Yan Wu, Xiao-Yan Zhao, and Xian-Yong Wei ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05423 • Publication Date (Web): 13 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Sustainable Chemistry & Engineering

Three-dimensional hierarchical porous carbon with high oxygen content derived from organic waste liquid with superior electric double layer performance Zhi-Qiang Hao, Jing-Pei Cao*, Ya-Li Dang, Yan Wu, Xiao-Yan Zhao, Xian-Yong Wei Key Laboratory of Coal Processing and Efﬁcient Utilization (Ministry of Education), China University of Mining & Technology, Xuzhou 221116, Jiangsu, P. R. China * Corresponding author Tel./fax: +86-516-83591059 E-mail address: [email protected] (J.-P. Cao)

ACS Paragon Plus Environment

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ABSTRACT The organic waste liquid produced in the production of vitamin C is an acidic pollutant which is difficult for effective treatment and utilization, and lignin is an abundant biopolymer with abundant oxygen-containing groups. In this study, organic waste liquid and lignin are used as precursors for preparing oxygen-rich hierarchical porous carbon with three-dimensional structure by synergistic reaction and activation. The obtained hierarchical porous carbons have a high specific surface area (3111.6 m2 g-1) and high oxygen content (13.8 wt%). As the electrode material for electric double layer capacitor, hierarchical porous carbon exhibits high specific capacitances (428 F g-1 at 0.04 A g-1) and excellent capacitance retention of 96% after 12000 cycles under two-electrode system. Meanwhile, its energy density is up to 12 Wh kg-1. The excellent electrochemical performance can be attributable to the rational pore structure and rich oxygen functional groups of hierarchical porous carbon, so it is a promising carbon-based electrode material for electric double layer capacitor. Thereby, this paper provides a practical and green strategy for the large-scale production of porous carbon and the value-added utilization of organic waste liquid and lignin. Keywords: Organic waste liquid; Three-dimensional structure; Synergistic reaction; Oxygen-rich hierarchical porous carbon; Electric double layer capacitor.


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INTRODUCTION With the explosion of population, the demand for resources is increasing sharply. Unfortunately, most of non-renewable resources on the earth (i.e., coal, oil and natural gas) have been reduced gradually owing to overexploitation, and it inevitably produces waste with high pollution during the production process.1 Hence, the value-added and efficient utilization of energy and biomass waste is urgent. Electric double layer capacitor (EDLC), as a promising energy storage device, possesses the advantages of long cycle life, fast charge/discharge rate and high power density,2-4 and can effectively solve the problems mentioned above. Porous carbon materials, such as activated carbon,5 porous carbon sphere6 and activated carbon nanofiber,7 are desirable carbon-based electrode materials for EDLC because of their high specific surface area (SSA), excellent conductivity, superior structural stability and low cost. However, apart from relatively poor energy density, the rate capacity of carbon-based electrode materials is universally undesirable due to their unsuitable pore structure.8,9 Hierarchical porous carbon (HPC), which has rational proportions of micro/meso/macrospores, can achieve superb rate capacity based on its special pore size distribution and developed porosity, so that electrolyte ions can be transferred rapidly in pore channel of HPC.10 Besides, micro/meso/macrospores play different roles in charge/discharge. Micropore as the main component of SSA can offer abundant active sites for the ions adsorption to increase specific capacitance.11 Mesopore serves as the fast transmission channel of ions,12 and macropore is equivalent to the “buffer zone” of electrolyte, ensuring ions can enter the pore channel promptly.13 Li et al.14 adopted resol and formaldehyde as precursors to prepare HPCs, which has a good rate capability of 83.5% at 10 A g-1. Huang et al.13 obtained HPC using animal bone as raw material, and its specific capacitance could reach 130 F g-1 even at 100 A g-1. Furthermore, heteroatoms doping is



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an effective method to improve EDLC energy density. This is because effective SSA can be obtained by enhancing the wettability of porous carbon material, due to the presence of surface functional groups.15 Meanwhile, pseudocapacitance is also engendered by faradaic reactions of heteroatoms.16 Hence, the capacitance and energy density of EDLC can be enhanced by heteroatoms doping. Although oxygen doping is a relatively simple doping method, it can significantly improve electric double-layer capacitance. Kim et al.17 proved the porous silicon carbide flakes oxidized by H2O2 possessed superior electrochemical performance in comparison with pure porous silicon carbide flakes. Vitamin C is the most needed vitamin for human and is also one of the most widely used vitamins. The annual demand for vitamin C is about 200 thousand tons. Thereby, numerous organic waste liquid (OWL) with strong acidity would be produced during the industrial production of vitamin C. Moreover, the main components of OWL are light vitamin C and some organic acids (i.e., gulonic acid, formic acid and nucleic acid), which are difficult to be disposed and used directly. In addition to incineration, OWL is usually used to produce low-valued products, including dicarboxyl, abstergent and soil improver. Therefore, the value-added and green utilization of OWL will achieve good economic and social benefits. Furthermore, lignin is the second most abundant organism in the world after cellulose,18 and has anticipated features such as high carbon content, superior stability and ample functional groups.19 Alkaline lignin is regarded as a waste in general.20 Additionally, many research groups have selected lignin as the precursor to prepare porous carbons by various preparation methods as electrode materials for EDLC. Klose et al.21 reported that porous carbon prepared by softwood lignin shows high SSA of 1800 m2 g-1 and large specific capacitance of 200 F g-1 at 10 A g-1 in an ionic liquid electrolyte. Simultaneously, lignin possesses rich oxygen-containing groups and three-dimensional molecular structure (Fig. S1).22 OWL as a waste


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also contains massive carbohydrates, so that it is possible to prepare HPC with high oxygen content by the crosslinking of OWL and lignin. However, until now and to our knowledge, there are no researchers prepare HPC as electrode material by the synergistic reaction of OWL and lignin, based on the special structure of lignin. In this paper, OWL and lignin were chosen as carbon sources to prepare HPC by carbonization and KOH activation, and the prepared HPCs were applied to EDLC. Besides, the influence of preparation conditions on the pore structure and electrochemical performance of HPC was investigated. Biomass wastes (OWL and lignin) are used to prepare value-added electrode material for EDLC, which is always very attractive and interesting. This study not only achieves the green application of wastes, but also achieves the efficient and sustainable utilization of resources. EXPERIMENTAL Materials OWL produced in the industrial process of vitamin C was pre-treated by filtration for removing its insoluble substance. Alkaline lignin is purchased from TCI Co., Ltd., and other experimental reagents are analytical grade without any treatment. Besides, the organic waste residue (OWR) was prepared by drying OWL. Preparation of HPCs The as-treated OWL and lignin were mixed with different weight ratios, and were stirred violently to ensure that lignin is distributed uniformly in OWL. The suspension was then dried in air dry oven at 100 oC for 12 h to obtain a solid mixture (SM), which was then ground to powder (< 200 meshes). The preparation method of HPCs mainly includes the following process. Step 1, the SM was carbonized at different carbonization temperatures for 2 h under Ar atmosphere. Step 2, to prepare HPC, the carbonized SM (CSM) was activated by KOH for 2 h



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under Ar atmosphere, and activation temperature and KOH/char weight ratio (mKOH/mchar) were investigated. Additionally, both carbonization and activation have fixed heating rate of 10 oC min-1. For the refinement of HPC, the as-obtained crude products were washed by 2 M HCl and adequate deionized water until pH = 7. After dried in vacuum at 200 oC for 3 h, the as-prepared HPCs were denoted as M-HPCx-y-z, where M = 0.5, 1, 2 and 3, standing for lignin quality added to 5 g of OWL; x = 500, 600, 700 and 800, standing for carbonization temperature; y = 500, 600, 700 and 800, standing for activation temperature; z = 3, 4, 5 and 6, standing for KOH/char weight ratio. For comparison, pure lignin and OWR were also individually carbonized (600 oC) and activated (600 oC)

for 2 h with KOH/char weight ratio of 5/1, and were designated as ALG600-600-5 and

AOWR600-600-5, respectively. Besides, the graphical schematic for the preparation of HPCs is illustrated in Fig. 1. Material Characterization All samples which need to be characterized were pulverized and dried at 100 oC for 12 h. In addition, the characterization methods include ultimate analysis, thermogravimetric (TG) and derivative thermogravimetry (DTG) analyses, field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometer, Raman spectrum, X-ray photoelectron spectroscopy (XPS) and N2 adsorption-desorption experiment. The instrument models are added to the Supplementary material. Among which, OWR, lignin and SM were measured by TG analysis, and the detailed temperature program was also listed in Supplementary material. Electrochemical measurements In this paper, two-electrode system (button-type cell) with 6 M KOH solution as electrolyte was introduced for the EDLC electrochemical measurements, including galvanostatic charge-discharge


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(GCD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and cycle life tests. Meanwhile, the preparation of electrodes, methods of electrochemical measurements, models of electrochemical instrument and equations of computation are all listed in Supplementary material in detail. RESULTS AND DISCUSSION Physiochemical characterization After filtration, the solution concentration of OWL is about 44.0%. Moreover, the appearance of CSM is bulgy (Figs. S2c and S2d), which is different from carbonized OWR (COWR, compact type, Fig. S2a) and carbonized lignin (CLG, pulverous type, Fig. S2b). It should be caused by the synergistic reaction and crosslinking between OWL and lignin under high temperature. Besides, their micro-morphologies also have significant differences (Fig. 2). COWL and CLG are irregular particles and rods, respectively, and has no pores (Figs. S2e and S2f), but the CSM possesses some macropores and no rods (Figs. 2a and S2g). It suggests that OWL and lignin are integrated into a whole by thermal treatment. Fig. 2a also shows many pores are interconnected in the interior of CSM, indicating the existence of three-dimensional structure in the interior of CSM,23 which contains tiny graphite domains (Fig. 2b).24 Furthermore, the internal pores of CSM were rooted in the evaporation of volatiles from precursors under high temperature, and lignin plays the role of three-dimensional frame during carbonization. The prepared HPCs have hierarchical porous structure (i.e., macropore, mesopore and micropores) after the activation of CSM, as shown in Figs. 2c and 2d), caused by the activation of excessive KOH (KOH/char weight ratio of 5/1). Meanwhile, the interior structure of 1-HPC600-600-5 is also a three-dimensional pore structure (Figs. 2c



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and 2d), which can be clearly observed from its surface cracks. In addition, the prepared HPC indeed has well-developed porosity in its interior and amorphous structure, according to Figs. 2e and 2f. The special morphology of HPCs might be since CSM is etched by KOH during activation, so that its original structure is destroyed to form the three-dimensional porous structure in the inner of carbonized samples. As shown in Fig. 3a, the Raman spectra of CSM and 1-HPC600-600-5 include D-peak (ca. 1340 cm-1) and G-peak (ca. 1580 cm-1), which should originate from the disordered structure of carbon material and the graphite lattice vibration,25 respectively. Their peak intensity ratio (ID/IG = R) is relevant to the graphitic degree of carbon material, and the R value of CSM (0.75) is relatively low in comparison with HPC (0.82). This is because the graphite structure of CSM is damaged after KOH activation, so that the defect degree of 1-HPC600-600-5 increases. Meanwhile, the 2D peak appearing at ca. 2790 cm-1 represents the packing of different graphene layers.9 Besides, the results of XRD spectra (Fig. S3) also demonstrate that the crystallinity of CSM is higher than 1-HPC600-600-5 due to its relatively strong peak intensity, and 1-HPC600-600-5 has well-developed micropores according to its high intense scattering peak at low diffraction angle.26 The theoretical TG/DTG curves of SM are calculated via the TG analyses of lignin and OWR (Fig. S4), and the experimental and theoretical TG/DTG curves have obvious discrepancy (Fig. 3b). For the experimental TG/DTG curves of SM, there is a weightless peak from 110 to 210 oC, which is owing to the release of the combined water and small molecules volatiles in OWR. More volatiles are lost with the increase in temperature. Moreover, the experimental weight loss (60.6%) is lower than theoretical value (71.2%) under 900

oC.

Additionally, the main differences between

experimental and theoretical curves appear in the temperature stage of 210-900 oC, and the


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experimental weight loss rate is obviously slow compared to theoretical weight loss rate. This should be caused by the exchange of hydrogen or free radicals between lignin and OWL.27 As shown in Fig. 4, it is remarkable that the functional groups of CSM are different from those of COWR and CLG based on the FTIR spectra of COWR, CLG and CSM. In general, the side chains in lignin are mainly composed of methoxyl, phenolic hydroxyl and aldehyde;19 besides, OWL contains the abundance of oxygen-containing groups, such as hydroxyl, carboxyl and carbonyl according to its FTIR spectrum (Fig. 4). Additionally, the condensation polymerization of small organic molecules also occurs to form biomass polymers, which might be owing to the oxidation and the acidity of OWL. By adequately stirring, lignin was evenly dispersed in OWL, which is beneficial for the reaction between lignin and OWL. Meanwhile, lignin can not only reduce the decomposition of polymer in OWR,28 but also efficiently react with OWR, leading to the formation of three-dimensional structure of CSM owing to the three-dimensional frame of lignin. Furthermore, based on the Fig. 4, the functional groups of CSM are composed of -OH at 3430 cm-1,29 -CH in arenes/epoxide at 3250-3000 cm-1,30 C=O at 1625 cm-1,31 -OH/C=C in arenes at 1410 cm-1 and aryl ether at 1200-1005 cm-1.32 Compared with COWR and CLG, the -OH content of CSM is relatively low based on its weaker peak intensity, which is owing to the dehydration of partial adjacent hydroxyls on lignin and OWR. Moreover, the carboxyl groups on COWR and CLG are converted into C=O after co-thermal treatment, suggesting the possible existence of decarboxylation between OWR and lignin. Hence, it demonstrates that crosslinking reaction occurs between OWR and lignin by dehydration and decarboxylation during carbonization, resulting in the differences in functional group species of CSM, COWR and CLG. Besides, the XPS spectra of CSM, COWR and CLG are shown in Fig. S5 to further confirm the functional group changes, and the test results are similar to FTIR results. The oxygen-containing groups of CSM are mainly in the



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form of carbonyl, ether and ester (Fig. S5b), which are distinctly different from COWR and CLG (Figs. S5d and S5f). Meanwhile, the possible and simplified crosslinking process is shown in Fig. S6. The N2 adsorption-desorption isotherms and DFT pore size distributions of 1-HPC600-600-5, ALG600-600-5 and AOWR600-600-5 are displayed in Figs. 5a and 5b, respectively. Based on IUPAC, the isotherms of 1-HPC600-600-5 and AOWR600-600-5 possess both the features of type I and IV isotherm, and the isotherm of ALG600-600-5 is a typical type I isotherm.33 Furthermore, the volume absorbed shows a dramatic rise when relative pressure (P/P0) increases from 0 to 0.2, suggesting the presence of abundant micropores. These isotherms show a slight upward trend from P/P0 = 0.2 to 1.0 and have different hysteresis loops existing between 0.4 and 1.0, which are caused by mesopores.34 Besides, the hysteresis loops of 1-HPC600-600-5 and AOWR600-600-5 are type H4, because they have relatively large mesopores embedded in a matrix with interconnected and small channels.35 Meanwhile, the SSA of 1-HPC600-600-5 is 2753.9 m2 g-1 which is higher than those of ALG600-600-5 (2031.3 m2 g-1) and AOWR600-600-5 (2493.6 m2 g-1), indicating the activation advantages of CSM in comparison with CLG and COWR. The DFT pore size distributions of 1-HPC600-600-5, ALG600-600-5 and AOWR600-600-5 are dominated by micropores, and mainly concentrated in 1-2 nm (Fig. 5b), which is propitious to the transfer of electrolyte ions.36 Moreover, the mesoporous volume of 1-HPC600-600-5 is larger than ALG600-600-5 and AOWR600-600-5, which is beneficial to the formation of electric double layer under high current density. The detailed pore structure information is listed in Table 1. Furthermore, the pore size distributions of other M-HPCs also have developed pore structures which consist


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of many micropores and few mesopores (Fig. S7), and the SSA peaks at 3111.6 m2 g-1 (Table S1). Table 1 Textural parameters of samples. SBETa

Smicb

Sexternalc

Vtotd

Vmicb

De

Yaf

Cgg

(m2 g-1)

(m2 g-1)

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

(nm)

(%)

(F g-1)

1-HPC600-600-5

2753.9

2543.9

210.0

1.43

1.12

2.05

42

428

AOWR600-600-5

2493.6

2306.3

187.3

1.25

1.01

2.00

38

293

ALG600-600-5

2031.3

1916.8

114.5

0.92

0.80

1.82

48

309

Sample

a

SBET: Specific surface area from multiple BET method.

b

Smic, Vmic: T-method micropore surface area and t-method micropore volume.

c

Sexternal: Difference of SBET and Smic.

d

Vtot: Total pore volume at P/P0 = 0.99.

e

D: Average pore diameter.

f

Ya: The yield of activation.

gC : g

The specific capacitance at 0.04 A g-1.

XPS was employed to determine the surface compositions and chemical states of 1-HPC600-600-5. According to the XPS results, the contents of C and O are 88.7% and 10.9% respectively, which is similar to the ultimate analysis of 1-HPC600-600-5 (Table 2). As listed in Table S2, the oxygen content of 1-HPC600-600-5 is comparable with other high oxygen-content carbon-based electrode materials which are prepared by oxidation treatment or oxygen-rich precursors. Meanwhile, its deconvoluted C 1s spectrum include five peaks (Fig. 5c), which are attributed to the presence of C=C/C-C (284.8 eV), C-O in epoxy/alkoxy



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(286.1 eV), C=O in carbonyl (287.2 eV), C=O in ester/carboxylic (288.4 eV) and -COOH (289.7 eV).37,38 The fitted peaks of high-resolution O 1s spectrum are composed of C=O (531.8

eV),

O=C-O

(533.5

eV),

adsorbed

H2O/O2

(535.7

eV)

and

-OH

in

cyclohexanol/phenol (537.8 eV),39,40 as shown in Fig. 5d. Hence, 1-HPC600-600-5 possesses rich oxygen and many kinds of oxygen-containing functional groups (Table 2), which can improve its electrochemical performance by increasing wettability and generating pseudocapacitance.41 Table 2 Ultimate and XPS analyses of 1-HPC600-600-5. XPS analysisc (%)

Ultimate analysis (wt%, daf) Sample C 1-HPC600-600-5 84.7

H

N

Oa

Sb

C 1s

O 1s

N 1s

0.2

1.0

13.8

0.3

88.7

10.9

0.4

a

Calculated by difference.

b

Total sulfur in dried basis.

c Atomic

ratio.

Electrochemical properties All electrochemical measurements were tested by two-electrode system with 6 M KOH solution as electrolyte. Besides, the specific capacitance of M-HPCx-y-z could be calculated by their GCD curve slopes according to Eq. S1 and Fig. S8, and 1-HPC600-600-5 has the highest specific capacitance of 428 F g-1 at 0.04 A g-1 (Table S1). Therefore, the ideal preparation conditions of HPCs as EDLC electrode material are lignin/OWL weight ratio of 1/5, carbonization temperature of 600 oC, activation temperature of 600 oC and KOH/char weight ratio of 5/1. Nevertheless, the specific capacitance of 1-HPC600-500-3 is significantly lower than other samples and its GCD curves deviate ACS Paragon Plus Environment

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from the straight line seriously, caused by its high internal resistance. This is since its relatively low activation temperature, resulting in the small SSA and poor porosity. On the contrary, the HPCs with well-developed porosity have the relatively desired electric double layer performance according to Figs. S7 and S8. AOWR600-600-5 and ALG600-600-5 were also evaluated by GCD, CV and EIS measurements for highlighting the excellent electrochemical performance of 1-HPC600-600-5. As exhibited in Fig. 6a, the GCD curves of AOWR600-600-5, ALG600-600-5 and 1-HPC600-600-5 are approximate isosceles triangle, showing their superior EDLC performance. The discharge time of 1-HPC600-600-5 is longer than those of AOWR600-600-5 and ALG600-600-5, so that its specific capacitance of 428 F g-1 is the highest compared with AOWR600-600-5 (293 F g-1) and ALG600-600-5 (309 F g-1) at 0.04 A g-1. Fig. 6b also indicates similar results based on the quasi-rectangular CV curves of samples and the largest integral area of 1-HPC600-600-5. This is because that the SSA of 1-HPC600-600-5 (2753.9 m2 g-1) is higher than those of AOWR600-600-5 (2493.6 m2 g-1) and ALG600-600-5 (2031.3 m2 g-1). 1-HPC600-600-5 also contains various species of oxygen-containing functional groups, such as C=O and -OH, which are beneficial to the increase in specific capacitance. Meanwhile, the specific capacitances of AOWR600-600-5, ALG600-600-5 and 1-HPC600-600-5 appear different degrees of decline with the increase in current density (Fig. 6c), owing to the micropores kinetical limitation. In other words, electrolyte ions cannot enter ultramicropores and form electric double layer rapidly under excessive current density.4,42 The attenuation of specific capacitance can be predicted, as AOWR600-600-5, ALG600-600-5 and 1-HPC600-600-5 are mainly composed of micropores (Figs. 5a and 5b). The specific capacitance of 1-HPC600-600-5 is still 288 F g-1 even at 10 A g-1 indicating a good rate performance, which is because the pore size distribution of 1-HPC600-600-5 is rational and concentrated in large micropores (1-2 nm) and small mesopores (2-4 nm). Besides, the specific capacitance of 1-HPC600-600-5 is also



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higher than those of AOWR600-600-5 and ALG600-600-5 under different current densities, as 1-HPC600-600-5 possesses three-dimensional hierarchical porous structure, large SSA and rich oxygen-containing groups.43 Fig. 6d illustrates the EIS measurement results of AOWR600-600-5, ALG600-600-5 and 1-HPC600-600-5. These Nyquist plots include semicircles in high-frequency region and vertical lines in low-frequency region, and there is a positive correlation between semicircle diameter and charge transfer resistance (Rct).44-46 Hence, the Rct value of 1-HPC600-600-5 is the smallest, ascribed to its smallest semicircle diameter. The intersection of Nyquist plot and Z’ axis is related to the value of solution resistance (Rs),47 and the Rs values of AOWR600-600-5 (0.52 ), ALG600-600-5 (0.67 ) and 1-HPC600-600-5 (1.27 ) are quite low because of their well-developed pore structure and good wettability. Additionally, the Nyquist plot of 1-HPC600-600-5 is almost perpendicular to Z’ axis in low-frequency region, suggesting its exceptional pore accessibility and capacitive performance.9,48 As Fig. 6e demonstrates, the GCD curve of 1-HPC600-600-5 still has good symmetrical characteristic even under high current density, which can be attributed to its fast charge/discharge performance. However, these GCD curves are curved rather than straight lines, since pseudocapacitance is generated during charge/discharge. In addition, the CV curve of 1-HPC600-600-5 gradually deviates from rectangular shape when the scan rate increased from 10 to 200 mV s-1 (Fig. 6f), but it is still closed to the rectangle, indicating that 1-HPC600-600-5 possesses excellent electrochemical reversibility. The CV curves at different scan rates have a hump-like peak during cathodic sweep (0-0.5 V), which is caused by the reversible oxidation reaction of oxygen-containing groups on the 1-HPC600-600-5 surface.49 It is since the acidity/basicity and electron donating/accepting ability of porous carbons are improved due to the existence of oxygen-containing functional groups, leading to introduce pseudocapacitance. This phenomenon is


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mainly due to the partial polarization of C-C bonds, resulting from the induction of oxygen atoms. Furthermore, the oxygen-induced faradaic redox reactions in charge/discharge process have been reported when KOH solution is used as electrolytes,42,50,51 and the reversible pseudocapacitive reactions of oxygen-containing functional with its acid-base property in the KOH solution are as follows: O -H +

O

+

2H2O + 2e-

+

2OH-

(1)

O -H +

O OH + OH-

C

C

O + H2 O + e -

(2)

O

OH + OH-

O + H2 O + e -

(3)

Besides, the schematic for storage behaviour and surface faradic redox reactions (oxygen-containing groups) of 1-HPC600-600-5 in EDLC system are illustrated in Fig. 7. Moreover, the oxygen-containing functional groups of carbonyl compound (C=O) have an efficient effect on enhancing specific capacitance by the increase in pseudo-capacitance.52-55 It is remarkable that the hierarchically porous structure is beneficial to the fast transport of ions, and pseudocapacitance can be developed by the faradic redox reactions of oxygen-containing groups. Cycle life is an essential estimation indicator for electric double layer capacitance, so that the cycle life test of 1-HPC600-600-5 as electrode material for EDLC was carried out for 12000 cycles at 5 A g-1. As exhibited in Fig. 8a, the capacitance could be retained at 96% after 12000 cycles, and the last 6 cycles can maintain isosceles-triangle-like shape (insert of Fig. 8a), demonstrating superior electrochemical stability and reversibility. The capacitance retention, however, appears an upward



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trend (> 100%) in the primary stage of cycle measurement, as electrolyte ions need enough time to fully enter all pores and also continuously activate carbon-based electrode in the cycle-life test, causing the increase in specific capacitance.56,57 Additionally, its coulombic efficiency is gradually closed to 100% with the increase in cycle numbers. According to Fig. 8b, the energy density and power density of EDLC can reach 12 Wh kg-1 and 9908 W kg-1, respectively. It is significant that the 1-HPC600-600-5 based EDLC possesses desired energy/power density, which have advantages over other porous carbon-based EDLC electrode materials prepared by lignin using various preparation methods.58-63 As listed in Table S3, the specific capacitance of 1-HPC600-600-5 is also superior to other reported carbon-based electrode materials derived from lignin/biomass. CONCLUSIONS The three-dimensional structural HPCs with excellent electrochemical performance were successfully prepared through the synergistic reaction of OWL and lignin followed by KOH activation. Meanwhile, the hierarchical porous structure is beneficial to improve the rate performance of carbon-based electrode material for EDLC, and oxygen doping is remarkable for enhancing specific capacitance by generating pseudocapacitance. When 6 M KOH is used as the electrolyte, 1-HPC600-600-5 demonstrates high specific capacitance (428 F g-1 at 0.04 A g-1), good energy density (12 Wh kg-1) and superb capacitance retention (96% after 12000 cycles). The electrochemical performance of 1-HPC600-600-5 is obviously better than that of porous carbon prepared by pure lignin and OWL under identical preparation conditions. Hence, this facile and novel strategy not only prepares promising carbon-based electrode material for EDLC on a large scale, but also achieves the green and high value-added application of biomass waste of lignin and OWL. ■ ASSOCIATED CONTENT


Page 17 of 36 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


Supporting Information The following files are available free of charge.

Lignin structure (Fig. S1), Morphology pictures (Fig. S2), XRD (Fig. S3), TG/DTG curves (Fig. S4), XPS (Fig. S5), Crosslinking process (Fig. S6), N2 adsorption-desorption isotherms and pore size distributions (Fig. S7), GCD curve (Fig. S8), Textural parameters of samples (Table S1), The comparison of oxygen content between this work and other high oxygen-content carbon-based electrodes for EDLC (Table S2) and Comparison of electrochemical performance (Table S3). ■ AUTHOR INFORMATION Corresponding Authors *E-mail address: [email protected]. Tel.: +86-516-83591059. ORCID Jing-Pei Cao: 0000-0002-1544-7441 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was subsidized by National Natural Science Foundation of China (Grants U1710103 and 21676292), the National Key Research and Development Program (Grant 2018YFB0604600), the Natural Science Foundation of Jiangsu Province (BK20161180), and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References 1.

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Fig. 1. Graphical schematic for preparation of HPCs.


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a

b

c

d

e

f

Fig. 2. TEM images of CSM prepared at 600 oC (a and b); SEM images (c and d) and TEM images (e and f) of 1 -HPC600-600-5 under different magnifications.



a

b

100

CSM 1-HPC600-600-5

D

0.00 -0.05

Experimental

Weight loss (%)

80

Theoretical

-0.10 -0.15

60

-0.20

DTG (%/ oC)

G

Intensity

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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40 -0.25

2D

-0.30

20 500

1000 1500 2000 2500 3000 3500 4000 4500 Raman Shift (cm-1)

100

200

300

400 500 600 700 Temperature (oC)

800

900

Fig. 3. Raman spectra of CSM and 1-HPC600-600-5 (a); experimental and theoretical TG/DTG curves of SM (b).


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CSM CLG

COWR Intensity

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


-OH/C=C Aryl ether -CH

OWR

C=O -OH

4000

3500

3000 2000 -1 Wavenumbers (cm )

1500

Fig. 4. FTIR spectra of OWR, COWR, CLG and CSM.


1000


a

1.2

800

dV(d) (cm3 nm-1 g-1)

Volume adsorbed (cm3 g-1)

600

400 1-HPC600-600-5

200

b

1-HPC600-600-5 ALG600-600-5 AWSR 600-600-5

0.9

0.6

0.3

ALG 600-600-5 AWSR600-600-5

0 0.0

0.8 0.2 0.4 0.6 Relative pressure (P/P0)

c

0.0 1

1.0

2

4 8 Pore width (nm)

d

Raw

16

32

Raw Fitted line

Fitted line

C=O

Intensity

C=C/C-C C-O

Intensity

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

Page 32 of 36

C=O

O=C-O H2O/O 2 -OH

C=O -COOH

282

284

286 288 Binding energy (eV)

290

292

528

530

532 534 536 Binding energy (eV)

538

540

Fig. 5. N2 adsorption-desorption isotherms of 1-HPC600-600-5, ALG600-600-5 and AOWR600-600-5 (a); pore size distributions of 1-HPC600-600-5, ALG600-600-5 and AOWR600-600-5 (b); high-resolution XPS spectra of C 1s and O 1s peaks of 1-HPC600-600-5 (c and d).


Page 33 of 36

1000

a

1-HPC600-600-5

400

2

Current (mA)

Voltage (mV)

ALG600-600-5

600

b

4

AOWR600-600-5

800

0

-2 AOWR600-600-5

200

ALG600-600-5

-4 0

0

2000

4000

6000 Time (s)

c

8000

10000

1-HPC600-600-5

0.0 100

d

AOWR600-600-5 ALG600-600-5

400

0.3

80

1-HPC600-600-5 2.0

60

1.5

200

40

100

1000

-Z" ()

-Z" ()

300

20

0

2

4

6

8

0

10

1.0 0.5 0.0 0.4

0

10

0.8 20

0.5 A g-1

400

1 A g-1

800

2 A g-1 3 A g-1

600

4 A g-1 5 A g-1

400

1.2 Z' () 30

1.6

2.0

40

50

Z' ()

Current density (A g-1)

e

0.9

ALG600-600-5

AOWR600-600-5

1-HPC600-600-5

0

0.6 Potential (V)

f

200

Current (mA)

Specific capacitance (F g-1)

500

Voltage (mV)

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


0

-200 10 mV s-1

200

20 mV s-1

-400

100 mV s

0 0

100

200

300 400 Time (s)

500

600

700

0.0

50 mV s-1 200 mV s-1

-1

0.3

0.6

0.9

Potential (V)

Fig. 6. GCD curves of 1-HPC600-600-5, ALG600-600-5 and AOWR600-600-5 at 0.04 A g-1 (a); CV curves of 1-HPC600-600-5, ALG600-600-5 and AOWR600-600-5 at 2 mV s-1 (b); the specific capacitances of 1-HPC600-600-5, ALG600-600-5 and AOWR600-600-5 at different current densities (c); Nyquist plots of 1-HPC600-600-5, ALG600-600-5 and AOWR600-600-5 (d); GCD curves of 1-HPC600-600-5 at different current densities (e); CV curves of 1-HPC600-600-5 at different scan rates (f).



Page 34 of 36

Fig. 7. Schematic for storage behavior and surface faradic redox reactions of 1-HPC600-600-5 in EDLC system.


a

60

60

40

40

20

20

0

2000

4000 6000 8000 Cycle numbers

10000

0 12000

Coulombic efficiency (%)

80

80

0

b12

100

100

Capacitance retention (%)

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


Energy density (Wh kg-1)

Page 35 of 36

9

6

3

0

10

This work Ref. 52

Ref. 53

Ref. 54 Ref. 56

Ref. 55 Ref. 57

100 1000 Power density (W kg-1)

Fig. 8. Cycle stability of 1-HPC600-600-5 at 5 A g-1 (a); Ragone plot for 1-HPC600-600-5 (b).


10000


Page 36 of 36

TABLE OF CONTENTS

SYNOPSIS: Oxygen-rich hierarchical porous carbon with three-dimensional structure, applied to EDLC, was prepared by the synergistic reaction and activation between organic waste liquid and lignin.


Three-dimensional hierarchical porous carbon with high oxygen

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