Rapid single-step synthesis of porous carbon from an agricultural waste for energy storage application

Waste Management 102 (2020) 330–339

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Rapid single-step synthesis of porous carbon from an agricultural waste for energy storage application Weimin Chen, Xin Wang, Chaozheng Liu, Min Luo, Pei Yang, Xiaoyan Zhou ⇑ College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China Jiangsu Engineering Research Center of Fast-growing Trees and Agri-fiber Materials, Nanjing 210037, China

a r t i c l e

i n f o

Article history: Received 3 April 2019 Revised 17 October 2019 Accepted 31 October 2019

Keywords: Wheat straw Char Microwave Porous carbon Supercapacitor

a b s t r a c t Single-step synthesis of porous carbon (PC) from biomass is a challenge via microwave heating, because biomass rarely absorbs the microwave energy. Herein, wheat-straw-derived char, as a good microwave absorber, was used to achieve rapidly single-step synthesis of PC from an agricultural waste (wheat straw). KOH was used to generate abundant micropores in the PCs. High heating rate caused by microwave heating combined with the pyrolysis gases resulted in the formation of meso-/macropores. A series of post-oxidation reactions between active sites in the PCs and oxygen in the air led to the doping of oxygen-containing chemical groups. Consequently, the obtained PC possessed a high specific surface area of 1905 m2 g
1, a balanced pore distribution with abundant micropores (0.62 cm3 g
1), considerable content of meso-/macropores (0.53 cm3 g
1), and an oxygen-enriched structure (oxygen content up to 21.6%). These characteristics not only contributed to the achievement of a high specific capacitance of 268.5 F g
1 at 0.5 A g
1 for the resultant supercapacitor, but also resulted in an excellent rate capability with a high capacitance retention of 81.2% at 10 A g
1 in a gel electrolyte (polyvinyl alcohol/LiCl). This supercapacitor can extract a high energy density of 21.5 W h kg
1 at 0.5 A g
1 and a high power density of 7.2 kW kg
1 at 10 A g
1. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The global wheat yield reached ~770 million tons in 2017 according to the statistical data from Food and Agriculture Organization. It resulted in the generation of ~1 billion tons for wheat straw after harvest, of which China accounted for 17.8%, ranking first in the world, followed by India (12.8%), Russian (11.1%), and America (6.1%). However, wheat straws were generally burned or buried in farmland directly as a low-value-added agricultural waste, resulting in serious environmental issues. Recently, wheat straw, as a typical biomass waste, had attracted great attention on sustainable production of carbon materials due to its prominent advantages of renewability, low cost, and ease of acquisition (Abioye and Ani, 2015). In the last decades, biomass-based porous carbons (PCs) with a well-developed porosity and a good conductivity has been considered as an appropriate electrode material for energy storage applications (e.g., supercapacitor) (Kuratani et al., 2011; Lv et al., 2012; Zhang et al., 2009).

⇑ Corresponding author at: College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China. E-mail address: [email protected] (X. Zhou). https://doi.org/10.1016/j.wasman.2019.10.058 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.

The PCs produced by the conventional two-step synthesis (carbonization followed by activation with potassium hydroxide) possessed abundant micropores, but much less meso-/macropores, which resulted in a poor rate capability of supercapacitors (Tang et al., 2017). Microporous carbons possessed large specific surface areas (2000–3000 m2 g
1), providing high theoretical capacitances of the prepared supercapacitors (Deng et al., 2015; Han et al., 2014; Wei et al., 2015). Nevertheless, micropores with extremely small size led to poor wettability for electrolyte, thus resulting in a significant reduction in micropores availability. The actual capacitances would be remarkably lower than the theoretical ones. In addition, the poor wettability of microporous carbons also resulted in a poor rate capability, showing a sharply decrease in capacitance at high current loads (Liu et al., 2017; Xing et al., 2016). Contrarily, mesopores can improve the rate capability by providing highways for fast transport of electrolyte ions and improving the accessibility of micropores, thus enhancing the power density of mesoporouscarbon-based supercapacitors (Dai et al., 2017; Li et al., 2017). However, the specific surface areas for charge storage provided by mesoporous carbons were limited. Previous works indicated that macropores can act as bridges for the connection between electrolyte and the interior surface of the carbon material, thus minimizing the ionic diffusion distances (Huang et al., 2016).

W. Chen et al. / Waste Management 102 (2020) 330–339

Hence, obtaining a balanced hierarchically pore distribution (coexistence of micro-/meso-/macropores) in porous carbons is crucial for supercapacitors with both high capacitance and excellent rate capability. High production cost is a dominant challenge for the hierarchical PCs manufacturing which involved a multi-step strategy including the processes of carbonization, activation, and template removal (Selvamani et al., 2016; Xu et al., 2017a; Yang et al., 2017). In addition, the conventional thermal heating used in these processes required long duration, and also resulted in a thermal gradient in the heated substance (Abioye and Ani, 2015; Jin et al., 2014). Thus, developing a single-step and time-saving synthesis of hierarchical PCs from biomass is significant to lower energy consumption and production cost. Recently, microwave heating has been widely used in rapid biomass pyrolysis and PCs production, due to its prominent advantages of uniform heating effect, short duration, and therefore, high energy efficiency (Kong et al., 2019; Mao et al., 2015; Thakur et al., 2007). However, biomass rarely absorbs the microwave energy. In order to obtain the PCs with a well-developed pore structure within short duration, previous studies indicated that biomass-derived char, as a good microwave absorber, can be used as a precursor rather than biomass itself (Mao et al., 2015). Carbonaceous materials (activated carbon, char, et al.) have been applied in microwave-assisted pyrolysis as microwave absorbers (Abubakar et al., 2013; Ng et al., 2017; Su et al., 2017a). These studies demonstrated that a shorter duration could be achieved to reach the target temperature and a wider temperature range could be attained. Previous study also pointed out that the high heating rate caused by microwave irradiation contributed to the formation of mesopores and macropores (Liu et al., 2010). Inspired from these results, biomass-derived char can be expected to serve as a good microwave absorber to promote the single-step conversion from a biomass into a PC material with a desirable pore structure for a

331

short producing duration. Besides, biomass-derived char can be also converted into PCs. Our previous work (Chen et al., 2019a) reported an approach that using water in humidified nitrogen as initial microwave absorber to facilitate the conversion of a biomass (lignin) into char, then into highly porous carbon in the presence of KOH via microwave heating. However, to obtain highly porous structure, this approach still required a heating duration up to 30 min, to ensure the sufficient contact between steam and lignin/char. Herein, as shown in Fig. 1, a more cost-effectively single-step synthesis of the hierarchical PCs from wheat straw was developed within a very short duration of 5 min to obtain a low-cost supercapacitor with both high capacitance and excellent rate capability. The wheat strawderived char was innovatively used as a microwave absorber to promote this single-step synthesis. Chemical activation was specially used rather than physical activation, since it generally produced a porous carbon with a much more developed porous structure. Additionally, chemical activating agent can be directly mixed with raw wheat straw, thus a shorter isothermal holding duration at the desirable temperature can be achieved than that using physical activation. KOH was specially selected as the chemical activating agent to generate abundant micropores in PCs, while the use of microwave heating and the microwave-assisted pyrolysis of wheat straw were expected to form mesopores and macropores, so as to construct a hierarchical pore structure. To our best knowledge, there was no similar study which reported this synthetic approach. 2. Materials and methods 2.1. Raw materials Wheat straw was obtained from a factory in Nanjing city. It was washed using distilled water for several times to remove surface

Fig. 1. Single-step synthesis of PCs with a balanced pore distribution and an oxygen-enriched structure.

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impurity, and dried in an oven at 100 °C overnight. 10 g of the obtained wheat straw was placed into a tubular furnace (OTF1200X, China), then heated to 500 °C at a heating rate of 10 °C min
1 and held for 60 min to prepare wheat straw-derived char. Nitrogen (99.999%, Praxair) with a flow rate of 50 mL min
1 was purged into the furnace constantly at the whole process. The initial pressure was 1 atm. Table S1 showed the characteristics of wheat straw and wheat straw-derived char in terms of proximate and elemental compositions. Both wheat straw and its derived char were further ground into powder shape with a size less than 150 lm by a micro-grinder and a mesh screen. Then, the obtained char and wheat straw were mixed in a beaker and stirred for 1 h, followed by drying in an oven at 100 °C overnight. The mass proportions of char in the mixture were set at 0%, 10%, 20%, and 50%, respectively. Correspondingly, the resultant PCs were denoted as control sample, PC-10, PC-20, and PC-50. Particularly, for comparison, 100% char-derived PC, denoting as CPC, was also prepared. All chemicals used in this study are of analytical grade. 2.2. Microwave-assisted co-pyrolysis A 2.45 GHz kitchen microwave oven (EM720KG1-PW, Media) was used to synthesize PCs by co-pyrolysis of wheat straw/char/ KOH. As depicted in Scheme1, the mixture of wheat straw and its derived char with a total mass of 2 g were mixed directly with the solid KOH under a mass ratio of 1:3 (mixture: KOH), followed by a quick grinding process in an agate mortar. Then, the mixture was placed into a self-designed quartz reactor with dimensions of 145 mm (length)  35 mm (diameter)  1 mm (wall thickness) for the single-step co-pyrolysis. Two polytetrafluoroethylene (PTFE) pipes were connected to the reactor to let gases in and out. Prior to microwave irradiation, nitrogen was purged into the reactor chamber at a flow rate of 20 mL min
1 for 10 min to remove air. Then, the microwave irradiation with a power of 800 W was constantly applied on the mixture for a duration of 5 min. The obtained solid products were recovered by distilled water and washed using 1 mol L
1 HCl for neutralization, then purged with distilled water for several times until a pH value of 7 was achieved. The PCs were finally obtained after drying in an oven at 100 °C overnight. 2.3. Carbon characterizations A gas adsorption analyzer (AUTOSORB-iQ2-MP, Quantachrome) was used to study the porous structure of the obtained PCs. Nitrogen was used as a filling gas to record the adsorption-desorption isotherms at 77 K. Specific surface area (SBET) was determined based on Brunauer-Emmett-Teller (BET) method (Brunauer et al., 1938). Pore size distribution curves were obtained by the density functional theory (DFT) model which can determine the total pore, micropore, and mesopore volumes (Vt, Vmic, and Vmes) (Ismadji and Bhatia, 2001; Kowalczyk et al., 2003; McCallum et al., 1999). Average pore size (Da) was determined by the equation of ‘‘4Vt/SBET” (Ismadji and Bhatia, 2001). A field emission scanning electron microscopy (FE-SEM, JSM-7600, JEOL Ltd.) and a high-resolution transmission electron microscopy (HRTEM, JEM-2100, JEOL Ltd.) were used to study the micromorphology of PCs. In HRTEM characterization, PCs were first dispersed in ethanol with an ultrasonic dispersion for 5 min. Then, the obtained suspension was scooped onto copper grids which covered with a holey carbon film. An X-ray diffractometer (XRD, Ultima-IV) applied with a Cu Ka radiation (40 kV, 30 mA) was used to investigate the amorphous structure of PCs. The diffraction profiles were recorded at a step rate of 0.02° s
1 in the 2h range of 5–65°.

A Raman spectroscopy (DXR532, Themor) equipped with a UIS2 objective (WHN10/22, Olympus) was used to study the defects degree of PCs. Wavenumber range of 1100–1800 cm
1 was recorded by a 532 nm laser radiation at a power of 8 mW. Resolution and exposure time were set at 4 cm
1 and 10 s for each test. Raman spectra presented in this study were obtained by 30 acquisitions. An X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD, Shimadzu) equipped with a monochromated Al Ka X-ray source was used to study the surface atomic compositions and chemical functional groups of the obtained PCs, operating at a voltage of 15 kV and a current of 10 mA. The surface atomic compositions (C, N, and O) were determined based on the low-resolution spectra which were recorded in the binding energy range of 0–1200 eV using a pass energy of 160 eV. The O1s spectra (528–540 eV) were obtained by recording the high-resolution spectra using a pass energy of 40 eV. Then, each spectrum was deconvoluted into 5 oxygen-related peaks (C@O band at 530.6 eV, CAO/CAOH band at 532.2 eV, CAOAC band at 533.3 eV, COOH bond at 534.8 eV, and water at 536.1 eV) by a XPSPEAK Software (version 4.1) (Papirer et al., 1991; Qu and Shi, 1998). 2.4. Supercapacitor assembly and electrochemical measurements Electrode preparation: approximately 5–8 mg of the obtained PC was mixed with acetylene black, and polytetrafluoroethylene emulsion (PTFE, 60% w/w) in an agate mortar at a weight ratio of 8:1:1 and stirred for 30 min. Several ethanol drops were added into the mixture until the slurry was obtained. Then, the mixture was loaded onto a nickel foam with a size of 10 mm  10 mm  1 mm by a brush, and dried in a vacuum oven at 80 °C overnight to remove any ethanol and water. The dried nickel foam was pressed with a 10 MPa force by an extrusion machine to obtain the working electrode. Supercapacitor assembly: in order to better seal the supercapacitor, a gel electrolyte consisting of PVA and LiCl was used. The detailed preparing process can be referred to the previous study (Zeng et al., 2015): briefly, to prepare 5 mol L
1 LiCl/PVA gel electrolyte, 4.24 g LiCl and 2 g PVA were added into 20 mL deionized water, and heated at 85 °C in a water base under constantly stirring for 1 h. Two prepared electrodes with same mass, a polypropylene separator, and a plastic insulator were assembled according to Fig. S1. Prior to assembly, both the electrodes and the polypropylene separator were soaked in the gel electrolyte for 6 h. The assembled supercapacitor was placed for at least 12 h before testing to remove any water. Electrochemical measurements: a three-electrode configuration was applied in an electrochemical working station (Reference 600+, Gamry) to study the electrochemical properties of the prepared electrodes in a 6 mol L
1 KOH electrolyte. A platinum sheet electrode and a saturated calomel electrode served as the counter electrode and the reference electrode, respectively. For cyclic voltammetry (CV) measurements, the potential range was set at
1–0 V. In order to study the rate capability of the electrodes, the scan rates varied from 10 mV s
1 to 100 mV s
1. In order to further evaluate the rate capability of the electrodes quantitatively, galvanostatic charge-discharge (GCD) measurements were conducted at different current densities (0.5 A g
1 to 10 A g
1) for 1000 cycles. Specific capacitance was determined according to the Eq. (1) and averaged based on the obtained 1000 values. Cycle stability of the electrodes was evaluated by consecutive GCD tests for 10,000 cycles at a current density of 5 A g
1. In order to study the resistance of electrode materials, electrolytes, and testing system, electrochemical impedance spectroscopy (EIS) measurements were performed at an amplitude of 5 mV and an open circuit potential. The frequency range was set at 10
2–10
5 Hz.

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C ¼ ðI  t Þ  ðDU  mÞ
1

ð1Þ
1

where C denotes the specific capacitance [F g ], I is the discharging current [A], t is the discharging time [s], 4U is the total change in voltage [V], m is the mass of the PC loaded in each electrode [g]. The prepared electrodes were used to further assemble supercapacitor. A two-electrode configuration was applied to evaluate the electrochemical properties of the supercapacitor. For CV and GCD measurements, potential range was adjusted to
0.4–0.4 V. Specific capacitance of each electrode was determined based on the Eq. (2). Energy density and power density were calculated based on the Eq. (3) and Eq. (4), respectively.

C e ¼ 2ðI  t Þ  ðDU  mÞ
1

ð2Þ

E ¼ C e  ðDU Þ2 =8

ð3Þ

P ¼ 3:6  E  t
1

ð4Þ
1

where Ce denotes the specific capacitance of single electrode [F g ], E and P are the energy density [W h kg
1] and power density [kW kg
1] of the supercapacitor. 3. Results and discussions 3.1. Physical properties Fig. 2a presented the N2 adsorption/desorption isotherms of the PCs. It can be observed that all the PCs demonstrated the vertical increase of N2 absorbed volume at a relative pressure (P/Po) smaller than 0.1, indicating the presence of micropores. It has been

reported that KOH, as a good chemical activator, can generate abundant micropores in PCs framework (Liew et al., 2019; Mao et al., 2015). In the cases of the control sample and the PC-10, higher N2 volumes and the hysteresis loops were observed with the P/Po further increased to 1, implying the presence of prominent mesopores. The DFT method was used to further quantitatively study the pore structure of the PCs based on adsorption isotherms, and the results were shown in Fig. 2b (pore size distribution) and Table 1 (pore structure parameters). The control sample possessed insufficient development of porous structure, demonstrating much lower SBET and Vt values of 560 m2 g
1 and 0.62 cm3 g
1 than other samples. Wheat straw rarely absorbs microwave energy, resulting in a low heating rate. Consequently, the reactions between KOH and carbon framework could not be taken place completely, since the required temperature could not be achieved in reactor during 5 min microwave irradiation. This result can be further evidenced by its highest Da value (4.4 nm) and mesopore ratio (72.6%). All of the PC-10, PC-20, and PC-50 showed well-developed porous structure with high SBET (1555–1905 m2 g
1) and Vt (1.14– 1.21 cm3 g
1) values, evidencing that wheat straw-derived char can serve as a good microwave absorber combined with KOH to achieve a more developed porosity. Also, the significant increase in micropore and mesopore volumes (Vmic and Vmes) were observed. This fact may be attributed to the presence of wheat straw-derived char with a capacity of microwave absorption (Liu et al., 2010). The reactor temperature profiles as a function of microwave heating duration were provided in Fig. S2 to further evaluate the synthesis mechanism. It can be concluded that the heating rate (determined by the slope of the curves in Fig. S2) was increased by raising the amount of wheat straw-derived char, thus resulting in a more

Fig. 2. Physical structure of PCs: (a) N2 adsorption-desorption isotherms, (b) pore size distribution by DFT method, (c) XRD patterns, and (d) Raman spectra.

Table 1 Pore structure parameters of the PCs. Samples

SBET (m2 g
1)

Vtotal (cm3 g
1)

Vmic (cm3 g
1)

Vmes (cm3 g
1)

Vmes/Vtotal (%)

Da (nm)

Control PC-10 PC-20 PC-50

560 1555 1790 1905

0.62 1.17 1.21 1.14

0.17 0.48 0.56 0.62

0.45 0.69 0.65 0.52

72.6 59.0 53.7 45.6

4.4 3.0 2.7 2.4

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developed pore structure with higher specific surface area as concluded in Table 1. This result is ascribed to two reasons: for the first one, adding more amount of char would lead to a higher temperature; for the other one, the higher heating rate would provide longer isothermal holding duration for ensuring sufficient chemical activation via reaching the highest temperature within shorter duration. It is deduced that, in the initial stage, char would act as a good microwave absorber to increase the reactor temperature constantly. In the second stage, wheat straw started to be converted into char when the temperature surpassing 200 °C. In the third stage, both the added char and the converted char acted as microwave absorber to further raise the reactor temperature, so as to trigger the chemical activation for obtaining well-developed porous structure. It should be noted that PC-20 and PC-50 showed a decrease in temperature in the last stage, because the fierce KOH activation would lead to significant carbon loss. Interestingly, it can be observed from Table 1 that mesopore ratio was decreased when raising the amount of char. This result is attributed to that the releasing gases from biomass pyrolysis would produce mesopores and macropores in carbon skeleton, while using char as the initial precursor usually generated abundant micropores but much less meso-/macropores via KOH activation. The above discussion was further confirmed by Fig. S3a-b (pore structure of CPC) which exhibited a type Ⅰ isotherm and a micropore-enriched system with pore size centered at 0.53, 0.91, 1.2, and 1.9 nm. It is generally accepted that meso-/macropores have significantly positive effects on rate capability of supercapacitors, since mesopores in PCs can provide highways for rapid ions transport, and macropores can act as reservoirs to shorten the diffusion distance. Therefore, this approach can adjust the mesopore ratio of PCs even the rate capability of the resultant supercapacitors by simply changing the adding amount of char. Notably, the obtained results suggested that only adding 10% char can greatly boost the single-step conversion during 5 min microwave irradiation. In addition, it can be concluded that the synthetic approach by co-pyrolysis of wheat straw/char/KOH could produce a PC material with a hierarchical pore distribution. Fig. 2c showed the XRD patterns of PCs. All the patterns exhibited two typical diffraction peaks, corresponding to (0 0 2) plane at 23.0° and (1 0 1) plane at 43.0° of graphitic carbon, such intrinsic nature revealed by XRD analysis suggested the good feasibility of the proposed single-step synthesis of carbonaceous materials from wheat straw. Particularly, these two peaks showed broad profiles, indicating the presence of amorphous structure in PCs. It has been reported that, compared with the graphitic structure, amorphous structure can offer larger channels for the transport of electrolyte ions, thus improving the rate capability of supercapacitors (Ou et al., 2015). A Raman spectrometer was used to evaluate the defects degree of the PCs, due to its high sensitivity. The obtained results were presented in Fig. 2d. Two typical peaks, corresponding to the D-band (1352 cm
1) and G-band (1592 cm
1), were observed (Shimodaira and Masui, 2002; Su et al., 2011). All the PCs demonstrated a broad D-band, indicating the presence of defects in PC framework. It should be noted that the presence of defects is beneficial to absorb more ions to increase the capacity contributed by the faradaic reactions, and enhance the double-layer capacitance via changing the surface structure (Vatamanu et al., 2015). Previous studies usually used ID/IG (the intensity ratio between D-band and G-band) as an index to evaluate the defects degree of the PCs (Peng et al., 2013). From Fig. 2d, it can be observed that the control sample had the highest ID/IG value (1.02). This result may be related to the incomplete conversion from wheat straw to a PC material. In addition, the oxygen, as a heteroatom, with a remarkably high content (concluded from XPS characterization) may contribute to the defects via modifying the angles and bond

lengths of the graphitic layers (Chen et al., 2017; Li et al., 2012). Typical ID/IG values for PCs were observed in the cases of the PC10 (0.96), PC-20 (0.98), and PC-50 (0.94), indicating that wheat straw-derived char can effectively boost the single-step conversion from wheat straw to a PC material. This result agrees well with the porosity analysis. FE-SEM was used in this study to observe microstructure of the PCs. Micromorphology images of the PCs were presented in Fig. 3a–d. The control sample (Fig. 3a) showed the presence of interconnected macropores. On the one hand, the microwaveassisted pyrolysis of wheat straw involved the release of various gaseous products, resulting in the formation of macropores (Mahinpey et al., 2009); on the other hand, in association with the porosity analysis results, the high heating rate caused by microwave irradiation also had responsibility on the presence of macropores (Liu et al., 2010). Along with the increase in char amount, massive multilayered and inter-connected macropores can be obviously observed in Fig. 3b–d. Wheat straw-derived char could absorb much more microwave energy compared with wheat straw, leading to a more intense reaction of both microwaveassisted pyrolysis and chemical activation. Previous studies reported that microwave-induced conversion from biomass derived-char to the PCs in the presence of KOH mainly generated micropores (Mao et al., 2015). Therefore, in association with the porosity analysis results, it can be concluded that co-pyrolysis of wheat straw/char/KOH could obtain a balanced hierarchical pore distribution consisting of abundant micropores and a considerable content of meso-/macropores. The HRTEM images were presented in Fig. 3e–f (PC-50) and Fig. S4a–b (PC-10 and PC-20) to observe the pore channel ordering of PCs. All of the PCs showed the presence of highly structural disorders. This observation agrees well with the results of XRD and Raman. 3.2. Chemical properties XPS was used to study the surface atomic components and chemical groups of the PCs, and the obtained results were presented in Fig. S5 and Table 2. It can be observed from Table 2 that the control sample possessed the highest oxygen content (33.9%) among other PCs, evidencing the incomplete conversion from wheat straw to a carbonaceous material. This result agrees well with the porosity analysis. Along with the increase in char amount, a decreasing trend in oxygen content was observed. As discussed before, wheat straw-derived char could absorb microwave energy effectively, leading to the increase in heating rate thus achieving the required temperature for chemical activation within a microwave duration of 5 min. However, the PCs obtained by copyrolysis possessed much higher oxygen content (21.6–28.2%) compared to the previously reported values that using conventional contact heating (6.29% (Lin et al., 2018), 3.50–4.87% (Cai et al., 2012), 5.2% (Chen et al., 2016), and 7.4–8.1% (Cheng et al., 2016)). A compared PC sample was specially prepared by singlestep conventional contact heating and characterized by XPS to further prove this result. The resultant low-resolution spectrum (shown in Fig. S6) confirmed a much lower oxygen content of 9.6% in the compared PC than these of PC-10, PC-20, and PC-50. This result could be ascribed to the use of microwave heating which supplied an internal heating energy at a molecular level, resulting in the formation of active centers on the PCs surface (Kappe, 2004; Menendez et al., 2010). After exposed to air atmosphere, a series of post-oxidation reactions between oxygen in the air and these active centers would be occurred, leading to the doping of oxygen-containing groups in the PCs framework. The above results evidenced the superiority of this approach to conventional contact heating on producing oxygen-enriched PCs. It should be noted that the obtained results from Fig. S5 and Table 2 showed

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Fig. 3. Micromorphology images of PCs: FE-SEM images of (a) Control, (b) PC-10, (c) PC-20, and (d) PC-50. HRTEM images of (e)-(f) PC-50.

Table 2 Compositions of surface atomic elements and oxygen-containing chemical groups. Properties

Carbon samples (%) Control

PC-10

PC-20

PC-50

Atomic components (%) O N C

33.9 0.4 65.7

28.2 0.5 71.3

22.1 0.4 77.5

21.6 0.6 77.8

O1s (%) C@O CAO/CAOH CAOAC COOH Water

31.4 27.9 13.1 16.6 11.0

27.7 31.1 13.4 12.2 15.6

9.8 32.8 22.7 23.1 11.6

11.7 32.4 19.5 19.7 16.7

the high content of the ACOOH group, implying the presence of a hydrophilic surface (Xu et al., 2017b). 3.3. Electrochemical properties An electrochemical working station was used to evaluate the electrochemical properties of the PC-based electrodes. CV measurements were performed at different scan rates (10, 20, 50, and 100 mV s
1) to investigate the rate capability of the prepared electrodes. Fig. 4a–b (Control sample and PC-50) and Fig. S7a-b (PC-10 and PC-20) depicted the CV profiles of the prepared electrodes. All electrodes demonstrated a distorted rectangle at a low scan rate of 10 mV s
1. This result indicates that the capacitance of the prepared electrodes consisted of not only EDLC but also pseudocapacitance. The high oxygen content on the PCs may have responsibility for the presence of pseudo-capacitance. The PC-10, PC-20, and PC-50 showed a slightly distorted rectangle even at a high scan rate of 100 mV s
1, indicating the good rate capability of these electrodes. On the one hand, all of the PC-10, PC-20, and PC-50 possessed the high content of surface oxygen element (21.6–28.2%), which not only would improve surface wettability of the PCs but

also serve as active sites for Faradic reactions; on the other hand, all of these PCs possessed a hierarchical pore distribution with a remarkable high contents of meso-/macropores, these merits are greatly beneficial for rapid ions transport via providing highways and shortening the diffusion distance, thus improving the rate capability of the resultant electrodes. (Su et al., 2017b; Zhang et al., 2016). GCD measurements were carried out at different current densities (0.5, 1, 2, 5, and 10 A g
1) to further study the rate capability of the electrodes quantitatively (as presented in Fig. 4c). It can be concluded that the PCs synthesized by co-pyrolysis demonstrated higher capacitance retentions (>80.7%) compared with the control sample (77.1%) at a high current density of 10 A g
1, further confirming the good rate capability. Fig. 4d depicted the GCD curves of the electrodes at a current density of 0.5 A g
1 to compare their specific capacitances. It can be observed that PC-50 possessed the highest specific capacitance up to 325.0 F g
1 among all PCs, due to its highest specific surface area (1905 m2 g
1) and micropore volume (0.62 cm3 g
1). Abundant micropores in PC-50 ensured high specific surface area, offering sufficient effective sites for ions or charges accumulation. However, the ultra-narrow pore size of

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Fig. 4. Electrochemical properties of the prepared electrodes in a 6 mol L
1 KOH electrolyte: (a) CV profile of the Control sample, (b) CV profile of the PC-50, (c) Specific capacitances at different current densities, (d) GCD profiles at a current density of 0.5 A g
1, (e) Nyquist plots, and (f) cycle stability tested at a current density of 5 A g
1.

these micropores would also result in poor wettability between electrolyte and PCs surface, generating large ions-non-accessible surface area, thus yielding a remarkably lower actual capacitance (Liu et al., 2017; Chen et al., 2019b), while the presence of considerable meso-/macropores in PC-50 is greatly favorable to provide more ions-accessible surface area via buffering ions congestion, thus improving the availability of micropores for ions storage (Campbell et al., 2015). Interestingly, Fig. S8 showed that CPC possessed a higher specific capacitance of 352 F g
1 than PC-50, due likely to the higher micropore volumes (0.73 vs. 0.62 cm3 g
1) and larger SBET value (2090 m2 g
1) observed from Fig. S3a) of CPC. However, a lower rate capability of 75.9% was observed in CPC compared to that (81.0%) of PC-50. As discussed before, this result can be attributed to the higher mesopores volumes (0.52 cm3 g
1 vs. 0.39 cm3 g
1) in PC-50. Based on the above discussion, it can be deduced that there is a trade-off between capacitance and rate capability, and mainly depending on the mass ratio of wheat straw to wheat straw-derived char in the precursor. Notably, only adding 10% char (PC-10) could remarkably increase the specific capacitance by 133% compared with that of the control sample. Also, the rate capability was enhanced, showing the increase in capacitance retention from 77.1% to 84.3%. The specific capacitances and porosities reported from previous works were listed in Table 3 which showed the superior specific capacitance

of the PC-50 despite of the moderate SBET and Vt values, even higher than that (285 F g
1) of the post-modified PC (Chen et al., 2019c). This result is attributed to the relative much higher oxygen content in PC-50, supplying additional pseudo-capacitance, and the presence of hierarchical pore distribution with considerable meso-/ macropores contents, providing more accessible surface area for electrolyte ions. It should be noted from Table S2 that, owing to its shorter production duration and more energy-efficient heating method, the PC-50 also possessed a much lower energy consumption by ~89.1% in terms of heating, thus giving a remarkably lower estimated production cost by ~87.6% (~6.54 CNY kg
1) than that (~52.67 CNY kg
1) of the single-step approach using conventional contact heating, and a comparable value to that (~4.90 CNY kg
1) of the recently reported work (Yek et al., 2019) using microwave heating. Therefore, this economical approach shows great commercialization potential on the PCs production from waste agricultural biomass. To study the capacitive behavior of the prepared electrodes, Nyquist plots (as presented in Fig. 4e) were drawn at a frequency range of 10
2–105 Hz. All the prepared electrodes demonstrated a slight deviation from vertical line at the low-frequency region, indicating the presence of dominant capacitive behavior accompanied with pseudo-capacitive contribution (Tang, et al., 2017). The PCs prepared by co-pyrolysis showed a much smaller diameter of

Table 3 Comparison between this work and the reported works on the specific capacitance and porosity of PCs.

a

Precursor

Specific capacitancea F g
1

Current density A g
1

SBET m2 g
1

Vt cm3 g
1

Ref.

PC-50 Lignin Tabacco rods Lignin Shiitake mushroom Cotton Rice brans Reed straw Willow leaves Willow catkins Bamboo

325/306 253 263.6 285 306 283 323 297 216 306 301

0.5/1.0 0.5 0.5 0.5 1.0 1.0 0.1 0.1 0.1 0.1 0.1

1905 3130 2115 3198 2988 2436 2475 2497 1031 2054 1472

1.14 1.67 1.22 2.21 1.76 1.27 1.21 1.61 0.66 1.06 0.61

This work Wang et al., 2016b Zhao et al., 2016 Chen et al., 2019c Cheng et al., 2015 Cheng et al., 2016 Hou et al., 2014 Xie et al., 2016 Liu et al., 2016 Wang et al., 2016c Tian et al., 2015

The specific capacitances were all obtained using 6 mol L
1 KOH solution as electrolyte.

W. Chen et al. / Waste Management 102 (2020) 330–339

337

Fig. 5. Electrochemical properties of the prepared supercapacitor in a gel electrolyte: (a) CV profiles at different scan rates, (b) GCD profiles at different current densities, (c) Nyquist plots, (d) Ragone plots, and (e) cycle stability tested at a current density of 10 A g
1.

semicircle at the medium-frequency region comparing with the control sample, indicating that a hierarchical pore distribution could promote the reduction on the charge-transfer resistance (Tang, et al., 2017). All the prepared electrodes demonstrated much lower values of the real axis intercept (0.25–0.27 X) at the highfrequency region compared to these of the previously reported studies (0.71 X (Hao et al., 2015) and 0.7–0.8 X (Zhao et al., 2016)), implying the presence of a low equivalent serial internal resistance. These results indicate that the PCs prepared by copyrolysis possessed a good electrical conductivity which accompanied with a large micropore volume and a hierarchical pore distribution to endow a high specific capacitance and an excellent rate capability. As shown in Fig. 4f, all prepared electrodes exhibited a good cycle stability, showing the capacitance retention of 92.0% (control sample), 94.0% (PC-10), 90.2% (PC-20), and 93.9% (PC-50) after 10,000 charge-discharge cycles. The PC-50 which has the highest specific capacitance among all samples was further assembled into supercapacitor to study the electrochemical properties in practical application. Gel electrolyte (PVA/LiCl) was used to ensure the good system tightness and further evaluate the rate capability of the prepared supercapacitor, since ions are far more difficult to diffuse in a gel electrolyte compared to an aqueous electrolyte, like 6 mol L
1 KOH. Potential range in CV tests was specially determined based on Fig. S9 (at a scan rate of 50 mV s
1) which displayed reversible and stable CV profiles up to a potential difference of 0.8 V. Fig. 5a presented the CV profiles with a typical rectangular shape, indicating the characteristic of EDLC. The presence of dominant micropores in the PC framework provided adsorption sites for electrolyte ions to contribute the EDLC. Since gel electrolyte is more difficult to diffuse into the inner layer of each electrode than KOH electrolyte (Zeng et al., 2015), a higher equivalent serial internal resistance (0.49 X) was observed in Fig. 5c. However, it can be concluded from Fig. 5b that the prepared supercapacitor demonstrated a high capacitance retention of 81.2% when the current density was increased from 0.5 A g
1 to 10 A g
1, further evidencing the good rate capability. The hierarchical pore distribution could shorten the diffusion pathway for ions to improve the availability of micropores. The presence of a high surface oxygen content also has

responsibility on the excellent rate capability by improving the electrical conductivity and surface wettability of each electrode (Wang et al., 2016a; Wu and Xia, 2015). From Fig. 5d (Ragone plots), a high energy density of 21.5 W h kg
1 (at 0.5 A g
1) and a high power density of 7.2 kW kg
1 (at 10 A g
1) were observed. Fig. 5e displayed an excellent cycle stability of the prepared supercapacitor, showing a high capacitance retention of 90.7% after 10,000 charge-discharge cycles. 4. Conclusion In summary, benefiting from the good microwave absorbing ability of the wheat straw-derived char and the chemical activation induced by KOH, waste wheat straw was successfully converted into high-performance PCs with a balanced hierarchically pore distribution (mesopore ratios up to 45.6–59.0%) and an oxygenenriched structure (oxygen contents as high as 21.6–28.2%) by microwave heating. Compared to the conventional approach using contact heating, the proposed synthesis is more cost-effective, owing to its much shorter production duration of only 5 min and lower energy consumption by ~89.1% in terms of heating. The resultant electrode employed PC-50 as its active material showed attractive capacitive performances, achieving a high specific capacitance of 325 F g
1 at 0.5 A g
1 and a high capacitance retention of 81.0% at 10 A g
1. In addition, the supercapacitor assembled by this electrode also exhibited an excellent rate capability with a high capacitance retention of 81.2% at 10 A g
1 even in a gel electrolyte (PVA/LiCl). This supercapacitor can deliver a high energy density of 21.5 W h kg
1 at 0.5 A g
1 and a high power density of 7.2 kW kg
1 at 10 A g
1. These promising results are significant to costeffectively utilize such an agricultural biomass waste to rapidly obtain a value-added PC material for advanced energy storage application. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgement The authors are grateful for the start-up funds for scientific research at the Nanjing Forestry University (163020126), National Science and Technology Achievements Project in Forestry (Grant No. [2016]42), Natural Science Foundation of the Jiangsu Province (Grant No. BK20161524), National Natural Science Foundation of China (Grant No. 31400515), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This study was also sponsored by the Qing Lan Project. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.10.058. References Abioye, A.M., Ani, F.N., 2015. Recent development in the production of activated carbon electrodes from agricultural waste biomass for supercapacitors: a review. Renew. Sust. Energy Rev. 52, 1282–1293. Abubakar, Z., Salema, A.A., Ani, F.N., 2013. A new technique to pyrolyse biomass in a microwave system: effect of stirrer speed. Bioresour. Technol. 128, 578–585. Brunauer, S., Emmett, P.H., Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60, 309–319. Cai, J.Y., Min, J., McDonnell, J., Church, J.S., Easton, C.D., Humphries, W., Lucas, S., Woodhead, A.L., 2012. An improved method for functionalisation of carbon nanotube spun yarns with aryldiazonium compounds. Carbon 50, 4655–4662. Campbell, B., Ionescu, R., Favors, Z., Ozkan, C.S., Ozkan, M., 2015. Bio-derived, binderless, hierarchically porous carbon anodes for Li-ion batteries. Sci. Rep., 5 Chen, J.Z., Xu, J.L., Zhou, S., Zhao, N., Wong, C.P., 2016. Nitrogen-doped hierarchically porous carbon foam: A free-standing electrode and mechanical support for high-performance supercapacitors. Nano Energy 25, 193–202. Chen, W.M., Wang, X., Feizbakhshan, M., Liu, C.Z., Hong, S., Yang, P., Zhou, X.Y., 2019a. Preparation of lignin-based porous carbon with hierarchical oxygenenriched structure for high-performance supercapacitors. J. Colloid Interf. Sci. 540, 524–534. Chen, W.M., Luo, M., Liu, C.Z., Hong, S., Wang, X., Yang, P., Zhou, X.Y., 2019b. Fast microwave self-activation from chitosan hydrogel bead to hierarchical and O, N co-doped porous carbon at an air-free atmosphere for high-rate electrodes material. Carbohyd. Polym. 219, 229–239. Chen, W.M., Wang, X., Luo, M., Yang, P., Zhou, X.Y., 2019c. Fast one-pot microwave preparation and plasma modification of porous carbon from waste lignin for energy storage application. Waste Manage. 89, 129–140. Chen, W., Zhou, X., Shi, S., Thiphuong, N., Chen, M., 2017. Synergistical enhancement of the electrochemical properties of lignin-based activated carbon using NH3H2O dielectric barrier discharge plasma. RSC Adv. 7, 7392–7400. Cheng, P., Gao, S.Y., Yang, X.F., Bai, Y.L., Xu, H., Liu, Z.H., Lei, Z.B., 2015. Hierarchically porous carbon by activation of shiitake mushroom for capacitive energy storage. Carbon 93, 315–324. Cheng, P., Li, T., Yu, H., Zhi, L., Liu, Z.H., Lei, Z.B., 2016. Biomass-derived carbon fiber aerogel as a binder-free electrode for high-rate supercapacitors. J. Phys. Chem. C 120. Dai, C.C., Wan, J.F., Geng, W.D., Song, S.J., Ma, F.W., Shao, J.Q., 2017. KOH direct treatment of kombucha and in situ activation to prepare hierarchical porous carbon for high-performance supercapacitor electrodes. J. Solid State Electr. 21, 2929–2938. Deng, X., Zhao, B., Zhu, L., Shao, Z., 2015. Molten salt synthesis of nitrogen-doped carbon with hierarchical pore structures for use as high-performance electrodes in supercapacitors. Carbon 93, 48–58. Han, Y., Liu, S., Li, D., Li, X., 2014. Three-dimensionally hierarchical porous carbon creating high-performance electrochemical capacitors. Electrochim. Acta 138, 193–199. Hao, P., Zhao, Z.H., Leng, Y.H., Tian, J., Sang, Y.H., Boughton, R.I., Wong, C.P., Liu, H., Yang, B., 2015. Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high performance supercapacitors. Nano Energy 15, 9–23. Hou, J.H., Cao, C.B., Idrees, F., Xu, B., Hao, X., Lin, W., 2014. From rice bran to high energy density supercapacitors: a new route to control porous structure of 3D carbon. Sci. Rep. 4, 7260. Huang, G.X., Kang, W.W., Xing, B.L., Chen, L.J., Zhang, C.X., 2016. Oxygen-rich and hierarchical porous carbons prepared from coal based humic acid for supercapacitor electrodes. Fuel Process. Technol. 142, 1–5. Ismadji, S., Bhatia, S.K., 2001. A modified pore-filling isotherm for liquid-phase adsorption in activated carbon. Langmuir 17, 1488–1498. Jin, H., Wang, X.M., Shen, Y.B., Gu, Z.R., 2014. A high-performance carbon derived from corn stover via microwave and slow pyrolysis for supercapacitors. J. Anal. Appl. Pyrol. 110, 18–23. Kappe, C.O., 2004. Controlled microwave heating in modern organic synthesis. Angew. Chem. Int. Edit. 43, 6250–6284.

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