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Biomass-derived microporous carbon with large micropore size for high-performance supercapacitors
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Biomass-derived microporous carbon with large micropore size for high-performance supercapacitors Yubing Li a, Deyi Zhang a, b, *, Yameng Zhang b, Jingjing He a, Yulin Wang a, Kunjie Wang a, Yangtao Xu b, Hongxia Li a, Yi Wang a a b
College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou, 730050, China State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou, 730050, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A large micropore (1 nm < d < 2 nm) dominant microporous carbon was reported. � More than 70.1% of micropore volume is contributed by the large micropores. � Specific capacitance reaches 369 and 398 F g 1 in KOH and H2SO4 electrolyte. � Excellent rate and cycle performance are observed. � A high energy density of 61.2 Wh kg 1 is observed in ionic liquid. A R T I C L E I N F O
A B S T R A C T
Keywords: Microporous carbon Large micropore Biomass Supercapacitors
Large micropore with a pore size of 1–2 nm can allow high charge storage capability while ensure fast ions transport, which serves to improve the energy density of supercapacitors without sacrificing high power density. Herein, we report a large micropore dominant microporous carbon derived from a biomass waste, flaxseed residue from the edible oil industry, for high-performance supercapacitors. The reported material exhibits a large specific surface area of up to 3230 m2 g 1, and more than 70.1% of micropore volume is contributed by large micropores. The specific capacitance of the obtained material reaches up to 369 and 398 F g 1 in KOH and H2SO4 electrolyte, respectively. Meanwhile, the assembled supercapacitor device based on the obtained material ex hibits excellent rate and cycle performance. Over 92.7% of the initial capacitance is retained even under a large current density of 20 A g 1 and the capacitance retention is more than 98.1% after 10000 times cycle in KOH electrolyte. The energy density of the assembled supercapacitor device reaches 61.2 Wh kg 1 at a power density of 468.8 W kg 1, and a high energy density of 43.5 Wh kg 1 is retained at a large power density of 13.3 kW kg 1 in ionic liquid EMIMBF4 electrolyte.
1. Introduction The quick development of electric vehicles and portable electronic devices in recent years arouses growing interest for electrochemical energy storage equipment with both high energy and power density [1, 2]. Supercapacitor is believed to be one of the most promising device to
meet the energy and power density requirements due to its bridging function for the power/energy gap between traditional dielectric capacitor (high power density) and secondary battery (high energy density) [3–5]. Presently, the lower energy density of supercapacitor than secondary battery still hinders its practical application although supercapacitor exhibit some distinguished merits, such as high specific
* Corresponding author. College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou, 730050, China. E-mail address: [email protected] (D. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227396 Received 2 July 2019; Received in revised form 26 September 2019; Accepted 1 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yubing Li, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227396
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power (10 kW kg 1), ultrafast charge-discharge (within seconds) and good cycle performance (>105 times), etc [2,5]. So, it is significant to improve energy density of supercapacitor without sacrificing its high power density for practical application. Porous carbon is an ideal electrode material for supercapacitor owing to its low price, variable and controllable morphology, high specific surface area and good electrical conductivity [6,7]. It has been proved that pore structure has a significant effect on the capacitive properties of porous carbon [8–10]. The solvated electrolyte ions can be transported in small micropore which pore size is less than 1 nm, contributing a high charge storage capability under low current density [11,12]. However, accessibility of electrolyte ions within the small micropore seriously deteriorates with increasing current density, and thus inducing poor rate performance [8]. The pores with size larger than 2 nm, such as mesopores or macropores, are beneficial to rate perfor mance of porous carbon due to the large pore size for fast transport of solvated electrolyte ions even under high current density, but they contribute less in charge storage [8,13]. The large micropore with a pore size of 1–2 nm can provide high specific area for energy storage and enough pore size for fast transport of electrolyte ions [9]. Hence, microporous carbon with large micropore (1 nm < d < 2 nm) dominant pore structure is expected to harvest both high energy and power density. Various biomass, such as garlic skin [14], celery [15], fish scale [16], rice [9], cornstalk [17], sorghum [18], loofah sponge [19], rice straw [20], bagasse [21] and jujube [22], have been widely utilized to prepare porous carbon due to their low price and multiple species. However, in most cases small micropore with a pore size of smaller than 1 nm is the dominant pore structure for the biomass-derived porous carbons ob tained by the conventional carbonization-activation process, which al ways exhibit high charge storage capability at low current density but poor capacitance retention at high current density when used as supercapacitor electrode. For instance, the N, O, P co-doped micropo rous carbon nanosheet derived from fish scale exhibits a high specific capacitance of up to 371 F g 1 at a current density of 0.1 A g 1, but only 49% of initial capacitance is retained at a high current density of 50 A g 1 [16]. Although the heteroatoms doping enhances the wetta bility of the porous surface [23,24], the small micropore (<1 nm) dominant pore structure observably obstructs the transport of electro lyte ions under high current density. Oppositely, a large micropores (1 nm < size < 2 nm) dominant N doped microporous carbon nanosheet derived from rice (more than 68.8% of the total microporous volume is contributed by the large micropores) exhibits a capacitance retention of 72.4% under a current density of 50 A g 1 and a surprising energy density of up to 104 Wh kg 1 in ionic liquid EMIMBF4 electrolyte although rice is a basic food for human being which should be avoided to be used for other purposes [9]. Therefore, large micropore dominant microporous carbon derived from biomass wastes is expected to meet the cost requirement besides high energy and power density for high-performance supercapacitor. Herein, a low-cost large micropore dominant microporous carbon (LMiPC) is successfully obtained by utilizing a biomass waste, flaxseed residue from the edible oil industry, as raw material. The obtained LMiPC material exhibits a large specific surface area of up to 3230 m2 g 1, and more than 70.1% of micropore volume is contributed by the large micropore with a pore size of 1–2 nm. The specific capaci tance of the obtained material reaches up to 369 and 398 F g 1 in KOH and H2SO4 electrolyte, respectively. Meanwhile, the assembled sym metric supercapacitor device based on the obtained material exhibits excellent rate and cycle performance. Over 92.7% of the initial capaci tance is retained even under a large current density of 20 A g 1 and the capacitance retention arrives more than 98.1% after 10000 times charge/discharge cycles in KOH electrolyte. The energy density of the assembled supercapacitor device reaches 61.2 Wh kg 1 at a power density of 468.8 W kg 1, and a high energy density of 43.5 Wh kg 1 is still maintained at a large power density of 13.3 kW kg 1 in ionic liquid
(EMIMBF4) electrolyte. 2. Experiment 2.1. Preparation of the biomass-derived large micropore dominant microporous carbon The flaxseed residue was obtained from local edible oil factory and used without further treatment. Briefly, 5 g of flaxseed residue was directly carbonized at 600–800 � C under Ar atmosphere for 2 h. The obtained pyrolytic carbon was named as PC-600, PC-700 and PC-800 according to their carbonization temperature, respectively. Then the obtained pyrolytic carbons were mixed with KOH powder in a mass ratio of 1:4 and activated at 700 � C under Ar atmosphere for 1 h. The obtained samples were treated with HCl solution (10 wt%) for 3 h and followed by washing with deionized water until the pH of filtrate arrived about 7.0. After drying at 373 K for 6 h, a series of activated samples was obtained. The names LMiPC1, LMiPC2 and LMiPC3 were assigned to the activated samples of PC-600, PC-700 and PC-800, respectively. 2.2. Characterization The surface morphology of the prepared LMiPC samples were measured by a HitachiS-4800 scanning electron microscopy (SEM, Hitachi, Japan) and a JEOL JEM-2010 transmission electron microscopy (TEM, JEOL, Japan). The surface elements and their chemical circum stance of the obtained LMiPC samples were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250 Xi, USA) using a monochrome Al Kα as the excitation source. Raman spectra of the obtained samples were measured on a Raman spectrometer with an argon ion laser which wavelength is 532 nm (Horiba JY HR800, Japan). N2 adsorption/desorption isotherms were taken in a Micro meritics ASAP 2460 volumetric adsorption analyzer at 77 K (ASAP 2020, Micromeritics, USA). The total specific surface area (SBET) was calcu lated by using Brunauer-Emmett-Teller (BET) formula according to the N2 adsorption data. Density functional theory (DFT) method was uti lized to calculate the pore volume and pore size distribution, and the total pore volume was calculated based on the amount of nitrogen adsorbed at P/P0 ¼ 0.97. The contribution rate of large micropore vol ume to total micropore volume is calculated according to the following formula: � � Vsize � 1 nm CLMi ¼ 1 � 100% (1) Vsize � 2 nm where CLMi represents the contribution of large micropore (1 nm < size < 2 nm) to the total micropore volume, Vsize � 1 nm refers to the pore volume of small micropore (size � 1 nm) and Vsize � 2 nm is the total micropore volume. 2.3. Electrochemical measurements For preparing the working electrode, a slurry of the active material, acetylene black and polytetrafluoroethylene was prepared according to a mass ratio of 8:1:1 and spread on a 10 � 10 mm of foam nickel (used in KOH electrolyte) or stainless steel mesh (used in H2SO4 electrolyte). After drying at 80 � C for 12 h and pressing at a pressure of 10 MPa for 1 min, working electrode with a load mass of ca. 4 mg of active material is obtained. Under three electrode configuration, Hg/HgO electrode and platinum sheet with a size of 10 � 10 mm were used as reference and counter electrode, respectively. Cyclic voltammetry (CV) and galvano static charge-discharge (GCD) tests were performed on the CHI660E electrochemical workstation within a voltage window of 1–0 V and 0.1–0.9 V for 6 M KOH and 1 M H2SO4 electrolytes, respectively. For two-electrode symmetric supercapacitor device, two work electrodes were separated by an interlayer membrane (PP/PE composite 2
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membrane) and a CR2032 type coin cell was assembled using 6 M KOH solution or ionic liquid EMIMBF4 as electrolytes. CV and GCD tests were performed on the CHI660E electrochemical workstation and a computer controlled supercapacitor test system (NEWARE, China), respectively. The voltage windows were set at 0–1 V and 0–3 V when using 6 M KOH and EMIMBF4 as electrolytes, respectively. Electrochemical impedance spectroscopy (EIS) of the assembled devices was also recorded by the CHI660E electrochemical workstation with a frequency range of 0.1 Hz–100 kHz. The specific capacitance of the prepared LMiPC sam ples tested under three-electrode configuration is calculated by the following formula (2), and the specific capacitance of the assembled supercapacitor device is calculated by the following formula (3): Cs ¼
I mdv=dt
Ccell ¼
I mt dv=dt
composite morphology of carbon nanosphere and disorderly stacked amorphous carbon (as shown in Fig. 1a~c). The nanospheres with a smooth surface which is divorced from the semicircular grooves on the surface of the amorphous carbon display a uniform size of ca. 200 nm (Fig. 1b and c). Flaxseed is an important raw material for extracting edible oil which is stored in the spherical lipid bodies, an organelle for holding triglycerides in plant cell [26]. Obviously, the nanospheres in the pyrolysis carbon inherit from the residual lipid bodies in flaxseed residue, and the amorphous carbon derives from the cellulose, lignin and protein in plant tissue of flaxseed. After activated by KOH, the surface of the carbon nanospheres and amorphous carbon becomes rough due to the dramatically improved porosity (as shown in Fig. 1d~f). The improved porosity also can be observed by comparing the TEM images of the pyrolysis carbon PC-700 (Fig. 1g ~ i) with that of the activated sample LMiPC2 (Fig. 1j ~ l). The specific surface area of the sample LMiPC2 arrives 3230 m2 g 1 while that of the typical py rolysis carbon PC-700 only reaches 13 m2 g 1 (as shown in Table 1), also illuminating a dramatic improvement of porosity after activated by KOH. The improved porosity mainly ascribes the etching effect of KOH to carbon material at high temperature based on the following reaction [14]:
(2) (3)
where Cs refers to the single electrode specific capacitance (F g 1) of the prepared LMiPC samples, and Ccell represents the specific capacitance (F g 1) of the assembled coin-type supercapacitor device. I is the applied discharge current (A), m (g) is the mass of active material on the working electrode under three-electrode configuration while mt refers to the total mass of active material on two working electrodes in supercapacitor device, and dV/dt (V s 1) represents the slope obtained by fitting a straight line to the discharge curve. The energy density (E, Wh kg 1) and power density (P, W Kg 1) of the coin-type supercapacitor device can be calculated by the following formula [21,25]: E¼
Ccell � ΔV 2 2 � 3:6
(4)
P¼
E � 3600 Δt
(5)
6KOH þ 2C → 2K þ 3H2 þ 2K2 CO3
(6)
Similar type-I nitrogen adsorption-desorption isotherms are observed for all prepared samples (as shown in Fig. 2a), indicating the microporous nature of the obtained material [27]. As shown in Fig. 2b, the pore size of the sample LMiPC2 mainly centres at 0.78 nm (small micropore), 1.25 and 1.59 nm (large micropore), and 2.2 nm (meso pore). The total volume of LMiPC2 is about 1.67 cm3 g 1 while the micropore and mesoporous volume is 1.41 and 0.26 cm3 g 1, respec tively (as shown in Table 1). It is worth to note that large micropore with a pore size of 1–2 nm is the dominant pore structure in the obtained microporous carbon. The contribution of large micropore volume to total micropore volume calculated according to formula (1) arrives 70.1% for the sample LMiPC2, which value is 62.4% and 100% for the samples LMiPC1 and LMiPC3, respectively (as shown in the Table 1 and Fig. S1), indicating that the contribution of large micropore volume to total micropore volume can be controlled by adjusting the pyrolysis temperature. The higher contribution of large micropore volume to total micropore volume is favorable to ensure high charge storage capability and fast ions transport even under a high current density [9]. The Raman
where ΔV(V) is the voltage window of the supercapacitor device and Δt (s) is the discharge time. 3. Results and discussion The typical SEM images of the pyrolysis carbon PC-700 exhibit a
Fig. 1. Typical SEM images of the pyrolysis carbon PC-700 (a~c) and sample LMiPC2(d ~ f), and TEM images of PC-700 (g ~ i) and LMiPC2(j ~ l). 3
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Table 1 Textural parameters of the prepared samples. Sample ID
Dav (nm)
SBET (m2 g 1)
Sme (m2g 1)
Smi (m2 g
PC-700 LMiPC1 LMiPC2 LMiPC3
14.45 1.87 2.07 3.70
13 3326 3230 1976
12 75 184 518
1 3251 3046 1458
1
Vt (cm3 g
)
0.019 1.56 1.67 1.00
1
)
Vme (cm3 g 0.018 0.1 0.26 0.33
1
)
Vmi (cm3 g 1)
CLMi (%)
0.002 1.46 1.41 0.67
̶ 62.3% 70.1% 100%
Dav: average pore size, SBET: total BET specific surface area, Sme: mesoporous specific surface area, Smi: microporous specific surface area, Vt: total pore volume, Vme: mesoporous volume, Vmi: microporous volume, CLMi: contribution of large micropore volume to total micropore volume.
Fig. 2. Nitrogen adsorption/desorption isotherms of the prepared samples (a) and pore size distribution of the sample LMiPC2 (b), Raman spectrum of the prepared samples (c–e), XPS spectra of the sample LMiPC2 (f) and its high-resolution C1s (g) and O1s (h) XPS spectra.
spectrum of the prepared samples can be deconvoluted into four peaks according to literatures [23,28], corresponding to the G- (around 1580 cm 1), D1- (around 1350 cm 1), D3- (around 1500 cm 1) and D4-band (around 1200 cm 1) of carbon material, respectively (Fig. 2c~e) [23]. In general, the G band refers to the graphite structure of sp2 hybrid carbon atoms, the D1 band corresponds to the disordered structure caused by carbon lattice defects, D3 band indicates amorphous carbon structure and D4 band means polyene or oligomer [24]. The intensity ratio of the D1 band peak to the G band peak (ID1/IG) indicates the graphitization degree of the prepared samples [29]. The ratio of ID1/IG reduces from 1.07 to 1.04, while the ID3/IG also decreases from 0.35 to 0.3 when the pyrolysis temperature raises from 600 to 800 � C (as shown in Table 2), indicating a improving graphitization degree with the increasing pyrolysis temperature, which will enhance the conduc tivity and decrease the charge transfer impedance of the prepared samples. Only carbon and oxygen elements are observed on the surface of the prepared samples. As shown in the XPS survey spectra of the sample LMiPC2 (Fig. 2f), the content of carbon and oxygen element is 94.86 at.% and 5.23 at.%, respectively. With the increasing pyrolysis temperature, the content of oxygen element slightly decreases to 5.02 at. % (as shown in the Table 2). The surface functional groups of the pre pared samples are determined by deconvoluting the high-resolution C1s and O1s XPS spectra. As shown in Fig. 2g, four peaks around 284.5, 284.8, 285.2 and 286.6 eV of C1s spectra relate to graphitic structure
Table 2 Summary of the Raman and XPS spectra analysis on the prepared samples. Sample
LMiPC1
LMiPC2
LMiPC3
ID1/IG ID3/IG C Content (at.%) C1s 284.5 ev 284.8 ev 285.2 ev 286.8 ev O Content (at.%) O1s 531.6 ev 532.8 ev 533.8 ev
1.07 0.35 94.52 28.34% 27.51% 41.21% 2.94% 5.48 15.09% 49.59% 35.32%
1.06 0.33 94.87 25.66% 29.14% 40.50% 4.71% 5.13 9.94% 49.90% 40.16%
1.04 0.30 94.98 30.51% 32.97% 36.53% 0 5.02 1.68% 48.97% 49.35%
(C–C sp2), carbon-carbon single bonds of defects (C–C sp3), carbon-oxygen single bonds (C–O) and carbon-oxygen double bonds – O), respectively [30]. For O1s spectra, three peaks around 531.6, (C– – O, -O- and O–C– – O bonds, respectively 532.8 and 533.8 eV refer to C– [30]. The abundant oxygen-containing functional groups on the surface of the prepared samples would be beneficial to the diffusion and trans port of electrolyte ions in the micropores [31]. With the increasing py rolysis temperature, the relative content of the quinone type group – O) decreases sharply from 15.09% to 1.68% (as shown in the (C– 4
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Table 2), which oxygen-containing functional group is reported to be responsible for the pseudo-capacitance of carbon material by the elec trochemical conversion of benzoquinone-hydroquinone [31–33]. Therefore, the contribution of pseudo-capacitance to charge storage capability of the prepared samples would significantly reduce with the – O functional group. decreasing content of surface C– The electrochemical performance of the prepared samples and commercial activated carbon (AC) YP-50 with a specific surface area of 1687 m2 g 1 is measured firstly under three-electrode configuration using 6 M KOH as the electrolyte. The CV curves of the prepared samples exhibit a distorted rectangular-like shape with apparent hump peaks, indicating the existence of pseudo-capacitance induced by the oxygencontaining functional groups [34,35]. The larger CV curve area of the sample LMiPC2 than that of the LMiPC1, LMiPC3 and YP-50 reflects its higher charge storage capability. Interestingly, although the specific – O functional group (which is surface and content of surface C– responsible for the pseudo-capacitance) of the sample LMiPC2 is lower than that of the sample LMiPC1, a larger charge storage capability is still observed for the sample LMiPC2 due to the higher contribution of large micropore volume to total micropore volume than that of LMiPC1 (as shown in the Table 1 and Fig. S1). With the increasing scan rate, no apparent distortion is observed for the CV curve of the sample LMiPC2 enen under a fast scan rate of 100 mV s 1 (as shown in Fig. 3b), illu minating its excellent rate performance. According to the charge storage mechanism, the capacitance of supercapacitors can be divided into surface control capacitance and diffusion control capacitance [36]. The previous includes the Faraday capacitance induced by the redox reaction on the surface of the material and the double layer capacitance caused by the adsorption-desorption process of the electrolyte ions, while the diffusion control capacitance is mainly derived from the embedding of electrolyte ions [36,37]. In general, the following formula is utilized to estimate the charge storage mechanism of supercapacitor [36,37]. I ¼ avb
parameters and the value of b is generally in the range of 0.5–1. For b ¼ 0.5, the charge storage mechanism is considered to follow the diffusion control process, and for b ¼ 1, the surface control process is suggested [37]. Fig. 3c displays the image of b value under cathode scan and its corresponding fitting variance R2 of the sample LMiPC2, the illustration displays the linear fitting curves of b value at different voltages. As shown in Fig. 3c, b value fluctuates in the range of 0.87–1 with the variational voltage, and its corresponding fitting variance R2 is close to 1, indicating that surface control process is the dominant charge storage mechanism for our reported material. Duun formula is utilized to further clarify the contribution of the surface control capacitance and diffusion control capacitance to the total capacitance [36,37]: IðvÞ ¼ k1 v þ k2 v1=2
(8)
where I(v) represents the response current value at a fixed voltage, k1v and k2v1/2 correspond to the current contribution from the surface capacitive effects and the diffusion-controlled intercalation process, respectively. k1 and k2 are constant and can be obtained by the slope and intercept of the linear equation [36]. The percentage of surface control capacitance contribution to total capacitance at a scan rate of 10 mV s 1 and the columnar percentage map of surface control and diffusion control capacitance contribution at different scan rate (2–100 mV s 1) of the sample LMiPC2 are displayed in Fig. 3d (light blue area) and Fig. 3e, respectively. The contribution of surface control capacitance reaches 67.4% at a low scan rate of 2 mV s 1 and 92.6% at a fast scan rate of 100 mV s 1, indicating a rapid reduction of the contribution of diffusion control capacitance to charge storage capability with the increasing scan rate. Meanwhile, under the same sweep rate, the contribution of surface control capacitance sequentially varies according to the order of LMiPC1 < LMiPC2
(7)
where the measured current I at a fixed voltage obeys a power law relationship with the sweep rate v, both a and b are adjustable
Fig. 3. CV curves of the prepared samples at a scan rate of 2 mV s 1 (a) and the sample LMiPC2 at different scan rate (b) in KOH electrolyte; the b value under cathode scan and its corresponding fitting variance R2 (c) of the sample LMiPC2; the contribution of surface control capacitance at a scan rate of 10 mV s 1 (d); the columnar contribution map of surface control capacitance at different scan rate (e); GCD curves of the sample LMiPC2 at a current density of 0.5 A g 1 (f). 5
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of the triangular-like charge-discharge curve of the sample LMiPC2 also illuminates the additional contribution of pseudo-capacitance to the charge storage capability (Fig. 3f). The specific capacitance of the pre pared samples calculated according to the formula (2) arrives 345, 369, 273 and 210 F g 1 for LMiPC1, LMiPC2, LMiPC3 and YP-50, respec tively, at a current density of 0.5 A g 1. The specific capacitance of the sample LMiPC2 is 1.76 times higher than that of the commercial AC. Meanwhile, an excellent rate capability is observed. As shown in the Fig. S3, the specific capacitance of the sample LMiPC2 arrives 344 and 267 F g 1 under the current density of 1.0 and 20 A g 1, respectively, and more than 77.6% of the initial capacitance is retained as the current density increases from 1.0 to 20 A g 1. In general, more pseudo-capacitance induced by oxygen-containing functional groups could be observed in H2SO4 electrolyte [36,39]. The CV curves of the sample LMiPC2 and commercial activated carbon YP-50 in H2SO4 electrolyte exhibit a quasi-rectangular shape with an apparent redox peak (Fig. 4a), which shape is well retained even under a fast scan rate of 100 mV s 1 (Fig. 4b), indicating the significant contri bution of pseudo-capacitance to the charge storage capability of the obtained material. The apparent pseudo-capacitance exhibits in H2SO4 electrolyte can be further proved by the quasi-triangle GCD curves (Fig. 4c). The specific capacitance of the sample LMiPC2 arrives up to 398 F g 1 in H2SO4 electrolyte at a current density of 0.5 A g 1, while that value of the YP-50 only reaches 221 F g 1. The b value of the sample LMiPC2 varies in the range of 0.7–1 with the variational voltage (Fig. 4d), and the contribution of the surface control capacitance to the total capacitance is ca. 73.6% and 93.3% at a scan rate of 10 mV s 1 and 100 mV s 1, respectively (Fig. 4e and f), which results indicate that the surface control process still is the dominant charge storage mechanism in H2SO4 electrolyte. Table 3 lists the capacitance performance of the biomass-derived porous carbons reported in recent literatures [14, 17–21,40]. As shown in Table 3, our reported material derived from flaxseed residue exhibits a quite competitive capacitance, which value is higher than most of the biomass-derived porous carbon reported in recent literatures, such as sorghum- [18], rice straw- [20], bagasse- [21] and black locust seed-derived porous carbon [40].
Table 3 Summary of capacitive performance of biomass-derived carbon-based super capacitor electrodes under three-electrode configuration. Sample type
Raw materials
SBET/ m2 g 1
Electrolyte
Specific capacitance/F g 1
Hierarchical pore carbon with small micropore size Porous carbon sheets Large pore size microporous carbon
Garlic skin
2818
6 M KOH
461 (1 A g
)
[14]
Cornstalk
1588
1 M H2SO4
407 (1 A g 1)
[17]
Flaxseed residue
3326
6 M KOH
This work
Porous carbon
Sorghum
3047
N doped pore carbon Porous activated carbon Carbon spheres@ porous carbon composites Self-doped graphitic biomass carbon
Bagasse
2905
Loofah sponge Rice straw
2718
369 (0.5 A g 1) 344 (1 A g 398 (0.5 A g 1) 329 (1 A g 311 (1 A g 302 (1 A g 351 (1 A g 310 (1 A g
1122
6 M KOH
Black locust seed
2010
6 M KOH
1 M H2SO4 2 M KOH 1 M H2SO4 6 M KOH 1 M H2SO4 6 M KOH
1
1
)
1
Ref.
) ) 1 ) 1 ) 1 )
[18]
1
337 (1 A g )
[20]
333 (1 A g 1)
[40]
1
[21] [19]
To explore the practical application of the obtained material in supercapacitor, symmetric coin-type supercapacitor devices were assembled and tested in 6 M KOH and ionic liquid EMIMBF4, respec tively. The assembled supercapacitor devices based on the samples LMiPC1, LMiPC2, LMiPC3, and YP-50 are named as SC-1, SC-2, SC-3, and SC-4. All of the assembled devices exhibit a similarly symmetrical rectangular CV curve in the 6 M KOH electrolyte whether at a slow scan rate of 5 mV s 1 or at a fast scan rate of 200 mV s 1 (Fig. 5a and Fig. 5b), showing their ideal capacitance properties. Meanwhile, the symmetrical
Fig. 4. CV curves of the sample LMiPC2 at a scan rate of 2 (a) and 100 mV s 1 (b) in H2SO4 electrolyte; GCD curves at a current density of 0.5 A g 1 (c); the b value under cathode scan and its corresponding fitting variance R2 (d); the contribution of surface control capacitance at a scan rate of 10 mV s 1 (e); and the columnar percentage map of surface control capacitance at different scan rate (f). 6
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triangle-like GCD curves with nearly 100% coulomb efficiency (Fig. 5c and Fig. S4), indicating their perfectly reversible electrical double layer capacitance performance. According to the formula (3), the specific capacitances of the devices of SC-1, SC-2, SC-3 and SC-4 at a current density of 0.5 A g 1 arrives 74 F g 1, 76 F g 1, 62 F g 1 and 39 F g 1, respectively. At a high current density of 20 A g 1, more than 85.7%, 92.7%, 91.8% and 82.5% of the initial capacitance are retained for SC-1, SC-2, SC-3 and SC-4, respectively (Fig. 5d). Apparently, the rata per formance of SC-1, SC-2, SC-3 is better than that of SC-4 due to the large micropore dominant pore structure of the prepared samples. The small micropores (size <1 nm) which is the dominant pore structure for the commercial AC obstructs the fast transport of electrolyte ions especially under high current density, which causes an unsatisfactory rata per formance. Due to the higher proportion of large micropore for LMiPC2 than that for LMiPC1, the device SC-2 exhibits a more excellent rata performance than SC-1. Although the proportion of large micropore for LMiPC3 is nearly 100%, the lower surface oxygen content induces a decrease of surface wettability, which is adverse to the fast transport of electrolyte ions, and thus causes a slightly lower rata performance for SC-3. Furthermore, the rate performance of the device SC-2 is superior to that of the supercapacitor devices based on the small micropore domi nant microporous carbons reported in the recent literatures [14,22,27, 41]. For instance, the capacitance retention for the microporous carbons derived from garlic skin [14], jujube [22], pine nut shells [27] and onion [41] is ca. 82%, 75%, 80% and 74% under a current density of 20 A g 1, respectively, which values are far lower than that of our material. Electrochemical impedance spectroscopy (EIS) was employed for further studying the electrochemical performance of the assembled devices (Fig. 5e). The steep linear curve in the low frequency region indicates nearly ideal capacitive performance and small intercept of the semi-circle in the high frequency region illuminates low ohmic imped ance of the assembled devices (0.63Ω, 0.54 and 0.45Ω for SC-1, SC-2 and SC-3, respectively) [42]. Furthermore, the relationship between the real capacitance and imaginary capacitance in EIS reflects the effect of pore structure on ion transport rate in the materials. Fig. 5f is the image of the frequency response of the real capacitance and imaginary capacitance of
the LMiPC2 and YP-50-based supercapacitor devices. The real capaci tance C0 and imaginary capacitance C00 were calculated from the fre quency response analysis using the formulas [8,14]. 0
C ¼
Z00 2π f jZj2
(9)
0
C00 ¼
Z
2π f jZj2
(10)
where f (Hz) represents the frequency, and Z0 and Z00 refer to the real and imaginary parts of the impedance Z, respectively. As shown in the Fig. 5f, the imaginary capacitance of the devices SC-2 and SC-4 reaches their maximum at frequencies of 0.45 and 0.87 Hz, respectively. The corresponding response time (τ) for the devices SC-2 and SC-4 are 1.23s and 2.22s, respectively [8,25]. The lower τ value of the SC-2 than that of the SC-4 indicates the more fast electrolyte ions transport rate of the obtained LMiPC material with large micropore dominant pore structure than commercial AC with a small micropore dominant pore structure. The assembled devices also exhibit excellent cycle performance due to the large micropore dominant pore structure. The capacitance retention of the devices SC-1, SC-2, SC-3 and SC-4 respectively reaches 93.5%, 98.4%, 97.7%, and 96.7% after 5000 times charge-discharge cycles ata current density of 2 A g 1, and more than 98.1% of the initial capaci tance is retained after 10000 times charge-discharge cycles for the de vice SC-2 (Fig. S5). Using ionic liquid as electrolyte can dramatically improve the energy density of symmetric supercapacitors due to the excellent electro chemical stability and broad voltage window [42]. The rectangular-like CV curves even at a fast scan rate of 50 mV s 1 (Fig. 6a) and the triangular-like GCD curves with small voltage drop even at a large current density of 10 A g 1 (Fig. 6b) indicate the good capacitance properties of the device SC-2 in ionic liquid EMIMBF4. The device capacitance of SC-2 is about 48.9 F g 1 at a current density of 0.5 A g 1. The high capacitance retention of 71.2% at a large current density of 10 A g 1 (Fig. 6c) and 65.6% after 5000 times charge-discharge cycles
Fig. 5. CV curves of the assembled supercapacitor devices at a scan rates of 5 mV s 1 (a) and 200 mV s 1 (b); GCD curves of the assembled devices at a current density 0.5 A g 1 (c); rate capability of the assembled devices (d); Nyquist plots of the assembled devices (e); frequency response of the real capacitance and imaginary capacitance of the LMiPC2 and YP-50 based supercapacitor devices (f). 7
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Fig. 6. CV curves of the supercapacitor device SC-2 in EMIMBF4 at various scan rate (a); GCD curves of the devices SC-2 at various current density (b); cycle stability of the devices SC-2 in EMIMBF4 (c); Ragone plot of the devices SC-2 in EMIMBF4 (d).
(Fig. S6) indicate the good rate capability and cycle performance. The device energy density of SC-2 reaches up to 61.2 Wh kg 1 at a low power density of 468.8 W kg 1 and 43.5 Wh kg 1 of energy density retained even at a high power density of 13.3 kW kg 1, which value is higher than most of the porous carbon materials reported in recent literatures (Fig. 6d) [9,29,43–47].
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4. Conclusion A large micropore dominant microporous carbon is successfully prepared by using a biomass waste, flaxseed residue, as raw material. The obtained LMiPC material exhibits a high specific surface area of up to 3230 m2 g 1, and more than 70.1% of micropore volume is contrib uted by the large micropores. The specific capacitance of the obtained material reaches up to 369 and 398 F g 1 in KOH and H2SO4 electrolyte, respectively. Meanwhile, the symmetric supercapacitor devices based on the obtained material exhibit excellent rate and cycle performance. Over 92.7% of the initial capacitance is retained even under a large current density of 20 A g 1 and the capacitance retention is more than 98.1% after 10000 times charge/discharge cycles in KOH electrolyte. The energy density of the assembled supercapacitor devices reaches 61.2 Wh kg 1 at a power density of 468.8 W kg 1, and a high energy density of 43.5 Wh kg 1 is retained at a large power density of 13.3 kW kg 1 in ionic liquid EMIMBF4 electrolyte. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant No. 51462020 and 21867015). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227396.
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9
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Biomass-derived microporous carbon with large micropore size for high-performance supercapacitors Yubing Li a, Deyi Zhang a, b, *, Yameng Zhang b, Jingjing He a, Yulin Wang a, Kunjie Wang a, Yangtao Xu b, Hongxia Li a, Yi Wang a a b
College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou, 730050, China State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou, 730050, China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A large micropore (1 nm < d < 2 nm) dominant microporous carbon was reported. � More than 70.1% of micropore volume is contributed by the large micropores. � Specific capacitance reaches 369 and 398 F g 1 in KOH and H2SO4 electrolyte. � Excellent rate and cycle performance are observed. � A high energy density of 61.2 Wh kg 1 is observed in ionic liquid. A R T I C L E I N F O
A B S T R A C T
Keywords: Microporous carbon Large micropore Biomass Supercapacitors
Large micropore with a pore size of 1–2 nm can allow high charge storage capability while ensure fast ions transport, which serves to improve the energy density of supercapacitors without sacrificing high power density. Herein, we report a large micropore dominant microporous carbon derived from a biomass waste, flaxseed residue from the edible oil industry, for high-performance supercapacitors. The reported material exhibits a large specific surface area of up to 3230 m2 g 1, and more than 70.1% of micropore volume is contributed by large micropores. The specific capacitance of the obtained material reaches up to 369 and 398 F g 1 in KOH and H2SO4 electrolyte, respectively. Meanwhile, the assembled supercapacitor device based on the obtained material ex hibits excellent rate and cycle performance. Over 92.7% of the initial capacitance is retained even under a large current density of 20 A g 1 and the capacitance retention is more than 98.1% after 10000 times cycle in KOH electrolyte. The energy density of the assembled supercapacitor device reaches 61.2 Wh kg 1 at a power density of 468.8 W kg 1, and a high energy density of 43.5 Wh kg 1 is retained at a large power density of 13.3 kW kg 1 in ionic liquid EMIMBF4 electrolyte.
1. Introduction The quick development of electric vehicles and portable electronic devices in recent years arouses growing interest for electrochemical energy storage equipment with both high energy and power density [1, 2]. Supercapacitor is believed to be one of the most promising device to
meet the energy and power density requirements due to its bridging function for the power/energy gap between traditional dielectric capacitor (high power density) and secondary battery (high energy density) [3–5]. Presently, the lower energy density of supercapacitor than secondary battery still hinders its practical application although supercapacitor exhibit some distinguished merits, such as high specific
* Corresponding author. College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou, 730050, China. E-mail address: [email protected] (D. Zhang). https://doi.org/10.1016/j.jpowsour.2019.227396 Received 2 July 2019; Received in revised form 26 September 2019; Accepted 1 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yubing Li, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227396
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power (10 kW kg 1), ultrafast charge-discharge (within seconds) and good cycle performance (>105 times), etc [2,5]. So, it is significant to improve energy density of supercapacitor without sacrificing its high power density for practical application. Porous carbon is an ideal electrode material for supercapacitor owing to its low price, variable and controllable morphology, high specific surface area and good electrical conductivity [6,7]. It has been proved that pore structure has a significant effect on the capacitive properties of porous carbon [8–10]. The solvated electrolyte ions can be transported in small micropore which pore size is less than 1 nm, contributing a high charge storage capability under low current density [11,12]. However, accessibility of electrolyte ions within the small micropore seriously deteriorates with increasing current density, and thus inducing poor rate performance [8]. The pores with size larger than 2 nm, such as mesopores or macropores, are beneficial to rate perfor mance of porous carbon due to the large pore size for fast transport of solvated electrolyte ions even under high current density, but they contribute less in charge storage [8,13]. The large micropore with a pore size of 1–2 nm can provide high specific area for energy storage and enough pore size for fast transport of electrolyte ions [9]. Hence, microporous carbon with large micropore (1 nm < d < 2 nm) dominant pore structure is expected to harvest both high energy and power density. Various biomass, such as garlic skin [14], celery [15], fish scale [16], rice [9], cornstalk [17], sorghum [18], loofah sponge [19], rice straw [20], bagasse [21] and jujube [22], have been widely utilized to prepare porous carbon due to their low price and multiple species. However, in most cases small micropore with a pore size of smaller than 1 nm is the dominant pore structure for the biomass-derived porous carbons ob tained by the conventional carbonization-activation process, which al ways exhibit high charge storage capability at low current density but poor capacitance retention at high current density when used as supercapacitor electrode. For instance, the N, O, P co-doped micropo rous carbon nanosheet derived from fish scale exhibits a high specific capacitance of up to 371 F g 1 at a current density of 0.1 A g 1, but only 49% of initial capacitance is retained at a high current density of 50 A g 1 [16]. Although the heteroatoms doping enhances the wetta bility of the porous surface [23,24], the small micropore (<1 nm) dominant pore structure observably obstructs the transport of electro lyte ions under high current density. Oppositely, a large micropores (1 nm < size < 2 nm) dominant N doped microporous carbon nanosheet derived from rice (more than 68.8% of the total microporous volume is contributed by the large micropores) exhibits a capacitance retention of 72.4% under a current density of 50 A g 1 and a surprising energy density of up to 104 Wh kg 1 in ionic liquid EMIMBF4 electrolyte although rice is a basic food for human being which should be avoided to be used for other purposes [9]. Therefore, large micropore dominant microporous carbon derived from biomass wastes is expected to meet the cost requirement besides high energy and power density for high-performance supercapacitor. Herein, a low-cost large micropore dominant microporous carbon (LMiPC) is successfully obtained by utilizing a biomass waste, flaxseed residue from the edible oil industry, as raw material. The obtained LMiPC material exhibits a large specific surface area of up to 3230 m2 g 1, and more than 70.1% of micropore volume is contributed by the large micropore with a pore size of 1–2 nm. The specific capaci tance of the obtained material reaches up to 369 and 398 F g 1 in KOH and H2SO4 electrolyte, respectively. Meanwhile, the assembled sym metric supercapacitor device based on the obtained material exhibits excellent rate and cycle performance. Over 92.7% of the initial capaci tance is retained even under a large current density of 20 A g 1 and the capacitance retention arrives more than 98.1% after 10000 times charge/discharge cycles in KOH electrolyte. The energy density of the assembled supercapacitor device reaches 61.2 Wh kg 1 at a power density of 468.8 W kg 1, and a high energy density of 43.5 Wh kg 1 is still maintained at a large power density of 13.3 kW kg 1 in ionic liquid
(EMIMBF4) electrolyte. 2. Experiment 2.1. Preparation of the biomass-derived large micropore dominant microporous carbon The flaxseed residue was obtained from local edible oil factory and used without further treatment. Briefly, 5 g of flaxseed residue was directly carbonized at 600–800 � C under Ar atmosphere for 2 h. The obtained pyrolytic carbon was named as PC-600, PC-700 and PC-800 according to their carbonization temperature, respectively. Then the obtained pyrolytic carbons were mixed with KOH powder in a mass ratio of 1:4 and activated at 700 � C under Ar atmosphere for 1 h. The obtained samples were treated with HCl solution (10 wt%) for 3 h and followed by washing with deionized water until the pH of filtrate arrived about 7.0. After drying at 373 K for 6 h, a series of activated samples was obtained. The names LMiPC1, LMiPC2 and LMiPC3 were assigned to the activated samples of PC-600, PC-700 and PC-800, respectively. 2.2. Characterization The surface morphology of the prepared LMiPC samples were measured by a HitachiS-4800 scanning electron microscopy (SEM, Hitachi, Japan) and a JEOL JEM-2010 transmission electron microscopy (TEM, JEOL, Japan). The surface elements and their chemical circum stance of the obtained LMiPC samples were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250 Xi, USA) using a monochrome Al Kα as the excitation source. Raman spectra of the obtained samples were measured on a Raman spectrometer with an argon ion laser which wavelength is 532 nm (Horiba JY HR800, Japan). N2 adsorption/desorption isotherms were taken in a Micro meritics ASAP 2460 volumetric adsorption analyzer at 77 K (ASAP 2020, Micromeritics, USA). The total specific surface area (SBET) was calcu lated by using Brunauer-Emmett-Teller (BET) formula according to the N2 adsorption data. Density functional theory (DFT) method was uti lized to calculate the pore volume and pore size distribution, and the total pore volume was calculated based on the amount of nitrogen adsorbed at P/P0 ¼ 0.97. The contribution rate of large micropore vol ume to total micropore volume is calculated according to the following formula: � � Vsize � 1 nm CLMi ¼ 1 � 100% (1) Vsize � 2 nm where CLMi represents the contribution of large micropore (1 nm < size < 2 nm) to the total micropore volume, Vsize � 1 nm refers to the pore volume of small micropore (size � 1 nm) and Vsize � 2 nm is the total micropore volume. 2.3. Electrochemical measurements For preparing the working electrode, a slurry of the active material, acetylene black and polytetrafluoroethylene was prepared according to a mass ratio of 8:1:1 and spread on a 10 � 10 mm of foam nickel (used in KOH electrolyte) or stainless steel mesh (used in H2SO4 electrolyte). After drying at 80 � C for 12 h and pressing at a pressure of 10 MPa for 1 min, working electrode with a load mass of ca. 4 mg of active material is obtained. Under three electrode configuration, Hg/HgO electrode and platinum sheet with a size of 10 � 10 mm were used as reference and counter electrode, respectively. Cyclic voltammetry (CV) and galvano static charge-discharge (GCD) tests were performed on the CHI660E electrochemical workstation within a voltage window of 1–0 V and 0.1–0.9 V for 6 M KOH and 1 M H2SO4 electrolytes, respectively. For two-electrode symmetric supercapacitor device, two work electrodes were separated by an interlayer membrane (PP/PE composite 2
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membrane) and a CR2032 type coin cell was assembled using 6 M KOH solution or ionic liquid EMIMBF4 as electrolytes. CV and GCD tests were performed on the CHI660E electrochemical workstation and a computer controlled supercapacitor test system (NEWARE, China), respectively. The voltage windows were set at 0–1 V and 0–3 V when using 6 M KOH and EMIMBF4 as electrolytes, respectively. Electrochemical impedance spectroscopy (EIS) of the assembled devices was also recorded by the CHI660E electrochemical workstation with a frequency range of 0.1 Hz–100 kHz. The specific capacitance of the prepared LMiPC sam ples tested under three-electrode configuration is calculated by the following formula (2), and the specific capacitance of the assembled supercapacitor device is calculated by the following formula (3): Cs ¼
I mdv=dt
Ccell ¼
I mt dv=dt
composite morphology of carbon nanosphere and disorderly stacked amorphous carbon (as shown in Fig. 1a~c). The nanospheres with a smooth surface which is divorced from the semicircular grooves on the surface of the amorphous carbon display a uniform size of ca. 200 nm (Fig. 1b and c). Flaxseed is an important raw material for extracting edible oil which is stored in the spherical lipid bodies, an organelle for holding triglycerides in plant cell [26]. Obviously, the nanospheres in the pyrolysis carbon inherit from the residual lipid bodies in flaxseed residue, and the amorphous carbon derives from the cellulose, lignin and protein in plant tissue of flaxseed. After activated by KOH, the surface of the carbon nanospheres and amorphous carbon becomes rough due to the dramatically improved porosity (as shown in Fig. 1d~f). The improved porosity also can be observed by comparing the TEM images of the pyrolysis carbon PC-700 (Fig. 1g ~ i) with that of the activated sample LMiPC2 (Fig. 1j ~ l). The specific surface area of the sample LMiPC2 arrives 3230 m2 g 1 while that of the typical py rolysis carbon PC-700 only reaches 13 m2 g 1 (as shown in Table 1), also illuminating a dramatic improvement of porosity after activated by KOH. The improved porosity mainly ascribes the etching effect of KOH to carbon material at high temperature based on the following reaction [14]:
(2) (3)
where Cs refers to the single electrode specific capacitance (F g 1) of the prepared LMiPC samples, and Ccell represents the specific capacitance (F g 1) of the assembled coin-type supercapacitor device. I is the applied discharge current (A), m (g) is the mass of active material on the working electrode under three-electrode configuration while mt refers to the total mass of active material on two working electrodes in supercapacitor device, and dV/dt (V s 1) represents the slope obtained by fitting a straight line to the discharge curve. The energy density (E, Wh kg 1) and power density (P, W Kg 1) of the coin-type supercapacitor device can be calculated by the following formula [21,25]: E¼
Ccell � ΔV 2 2 � 3:6
(4)
P¼
E � 3600 Δt
(5)
6KOH þ 2C → 2K þ 3H2 þ 2K2 CO3
(6)
Similar type-I nitrogen adsorption-desorption isotherms are observed for all prepared samples (as shown in Fig. 2a), indicating the microporous nature of the obtained material [27]. As shown in Fig. 2b, the pore size of the sample LMiPC2 mainly centres at 0.78 nm (small micropore), 1.25 and 1.59 nm (large micropore), and 2.2 nm (meso pore). The total volume of LMiPC2 is about 1.67 cm3 g 1 while the micropore and mesoporous volume is 1.41 and 0.26 cm3 g 1, respec tively (as shown in Table 1). It is worth to note that large micropore with a pore size of 1–2 nm is the dominant pore structure in the obtained microporous carbon. The contribution of large micropore volume to total micropore volume calculated according to formula (1) arrives 70.1% for the sample LMiPC2, which value is 62.4% and 100% for the samples LMiPC1 and LMiPC3, respectively (as shown in the Table 1 and Fig. S1), indicating that the contribution of large micropore volume to total micropore volume can be controlled by adjusting the pyrolysis temperature. The higher contribution of large micropore volume to total micropore volume is favorable to ensure high charge storage capability and fast ions transport even under a high current density [9]. The Raman
where ΔV(V) is the voltage window of the supercapacitor device and Δt (s) is the discharge time. 3. Results and discussion The typical SEM images of the pyrolysis carbon PC-700 exhibit a
Fig. 1. Typical SEM images of the pyrolysis carbon PC-700 (a~c) and sample LMiPC2(d ~ f), and TEM images of PC-700 (g ~ i) and LMiPC2(j ~ l). 3
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Table 1 Textural parameters of the prepared samples. Sample ID
Dav (nm)
SBET (m2 g 1)
Sme (m2g 1)
Smi (m2 g
PC-700 LMiPC1 LMiPC2 LMiPC3
14.45 1.87 2.07 3.70
13 3326 3230 1976
12 75 184 518
1 3251 3046 1458
1
Vt (cm3 g
)
0.019 1.56 1.67 1.00
1
)
Vme (cm3 g 0.018 0.1 0.26 0.33
1
)
Vmi (cm3 g 1)
CLMi (%)
0.002 1.46 1.41 0.67
̶ 62.3% 70.1% 100%
Dav: average pore size, SBET: total BET specific surface area, Sme: mesoporous specific surface area, Smi: microporous specific surface area, Vt: total pore volume, Vme: mesoporous volume, Vmi: microporous volume, CLMi: contribution of large micropore volume to total micropore volume.
Fig. 2. Nitrogen adsorption/desorption isotherms of the prepared samples (a) and pore size distribution of the sample LMiPC2 (b), Raman spectrum of the prepared samples (c–e), XPS spectra of the sample LMiPC2 (f) and its high-resolution C1s (g) and O1s (h) XPS spectra.
spectrum of the prepared samples can be deconvoluted into four peaks according to literatures [23,28], corresponding to the G- (around 1580 cm 1), D1- (around 1350 cm 1), D3- (around 1500 cm 1) and D4-band (around 1200 cm 1) of carbon material, respectively (Fig. 2c~e) [23]. In general, the G band refers to the graphite structure of sp2 hybrid carbon atoms, the D1 band corresponds to the disordered structure caused by carbon lattice defects, D3 band indicates amorphous carbon structure and D4 band means polyene or oligomer [24]. The intensity ratio of the D1 band peak to the G band peak (ID1/IG) indicates the graphitization degree of the prepared samples [29]. The ratio of ID1/IG reduces from 1.07 to 1.04, while the ID3/IG also decreases from 0.35 to 0.3 when the pyrolysis temperature raises from 600 to 800 � C (as shown in Table 2), indicating a improving graphitization degree with the increasing pyrolysis temperature, which will enhance the conduc tivity and decrease the charge transfer impedance of the prepared samples. Only carbon and oxygen elements are observed on the surface of the prepared samples. As shown in the XPS survey spectra of the sample LMiPC2 (Fig. 2f), the content of carbon and oxygen element is 94.86 at.% and 5.23 at.%, respectively. With the increasing pyrolysis temperature, the content of oxygen element slightly decreases to 5.02 at. % (as shown in the Table 2). The surface functional groups of the pre pared samples are determined by deconvoluting the high-resolution C1s and O1s XPS spectra. As shown in Fig. 2g, four peaks around 284.5, 284.8, 285.2 and 286.6 eV of C1s spectra relate to graphitic structure
Table 2 Summary of the Raman and XPS spectra analysis on the prepared samples. Sample
LMiPC1
LMiPC2
LMiPC3
ID1/IG ID3/IG C Content (at.%) C1s 284.5 ev 284.8 ev 285.2 ev 286.8 ev O Content (at.%) O1s 531.6 ev 532.8 ev 533.8 ev
1.07 0.35 94.52 28.34% 27.51% 41.21% 2.94% 5.48 15.09% 49.59% 35.32%
1.06 0.33 94.87 25.66% 29.14% 40.50% 4.71% 5.13 9.94% 49.90% 40.16%
1.04 0.30 94.98 30.51% 32.97% 36.53% 0 5.02 1.68% 48.97% 49.35%
(C–C sp2), carbon-carbon single bonds of defects (C–C sp3), carbon-oxygen single bonds (C–O) and carbon-oxygen double bonds – O), respectively [30]. For O1s spectra, three peaks around 531.6, (C– – O, -O- and O–C– – O bonds, respectively 532.8 and 533.8 eV refer to C– [30]. The abundant oxygen-containing functional groups on the surface of the prepared samples would be beneficial to the diffusion and trans port of electrolyte ions in the micropores [31]. With the increasing py rolysis temperature, the relative content of the quinone type group – O) decreases sharply from 15.09% to 1.68% (as shown in the (C– 4
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Table 2), which oxygen-containing functional group is reported to be responsible for the pseudo-capacitance of carbon material by the elec trochemical conversion of benzoquinone-hydroquinone [31–33]. Therefore, the contribution of pseudo-capacitance to charge storage capability of the prepared samples would significantly reduce with the – O functional group. decreasing content of surface C– The electrochemical performance of the prepared samples and commercial activated carbon (AC) YP-50 with a specific surface area of 1687 m2 g 1 is measured firstly under three-electrode configuration using 6 M KOH as the electrolyte. The CV curves of the prepared samples exhibit a distorted rectangular-like shape with apparent hump peaks, indicating the existence of pseudo-capacitance induced by the oxygencontaining functional groups [34,35]. The larger CV curve area of the sample LMiPC2 than that of the LMiPC1, LMiPC3 and YP-50 reflects its higher charge storage capability. Interestingly, although the specific – O functional group (which is surface and content of surface C– responsible for the pseudo-capacitance) of the sample LMiPC2 is lower than that of the sample LMiPC1, a larger charge storage capability is still observed for the sample LMiPC2 due to the higher contribution of large micropore volume to total micropore volume than that of LMiPC1 (as shown in the Table 1 and Fig. S1). With the increasing scan rate, no apparent distortion is observed for the CV curve of the sample LMiPC2 enen under a fast scan rate of 100 mV s 1 (as shown in Fig. 3b), illu minating its excellent rate performance. According to the charge storage mechanism, the capacitance of supercapacitors can be divided into surface control capacitance and diffusion control capacitance [36]. The previous includes the Faraday capacitance induced by the redox reaction on the surface of the material and the double layer capacitance caused by the adsorption-desorption process of the electrolyte ions, while the diffusion control capacitance is mainly derived from the embedding of electrolyte ions [36,37]. In general, the following formula is utilized to estimate the charge storage mechanism of supercapacitor [36,37]. I ¼ avb
parameters and the value of b is generally in the range of 0.5–1. For b ¼ 0.5, the charge storage mechanism is considered to follow the diffusion control process, and for b ¼ 1, the surface control process is suggested [37]. Fig. 3c displays the image of b value under cathode scan and its corresponding fitting variance R2 of the sample LMiPC2, the illustration displays the linear fitting curves of b value at different voltages. As shown in Fig. 3c, b value fluctuates in the range of 0.87–1 with the variational voltage, and its corresponding fitting variance R2 is close to 1, indicating that surface control process is the dominant charge storage mechanism for our reported material. Duun formula is utilized to further clarify the contribution of the surface control capacitance and diffusion control capacitance to the total capacitance [36,37]: IðvÞ ¼ k1 v þ k2 v1=2
(8)
where I(v) represents the response current value at a fixed voltage, k1v and k2v1/2 correspond to the current contribution from the surface capacitive effects and the diffusion-controlled intercalation process, respectively. k1 and k2 are constant and can be obtained by the slope and intercept of the linear equation [36]. The percentage of surface control capacitance contribution to total capacitance at a scan rate of 10 mV s 1 and the columnar percentage map of surface control and diffusion control capacitance contribution at different scan rate (2–100 mV s 1) of the sample LMiPC2 are displayed in Fig. 3d (light blue area) and Fig. 3e, respectively. The contribution of surface control capacitance reaches 67.4% at a low scan rate of 2 mV s 1 and 92.6% at a fast scan rate of 100 mV s 1, indicating a rapid reduction of the contribution of diffusion control capacitance to charge storage capability with the increasing scan rate. Meanwhile, under the same sweep rate, the contribution of surface control capacitance sequentially varies according to the order of LMiPC1 < LMiPC2
(7)
where the measured current I at a fixed voltage obeys a power law relationship with the sweep rate v, both a and b are adjustable
Fig. 3. CV curves of the prepared samples at a scan rate of 2 mV s 1 (a) and the sample LMiPC2 at different scan rate (b) in KOH electrolyte; the b value under cathode scan and its corresponding fitting variance R2 (c) of the sample LMiPC2; the contribution of surface control capacitance at a scan rate of 10 mV s 1 (d); the columnar contribution map of surface control capacitance at different scan rate (e); GCD curves of the sample LMiPC2 at a current density of 0.5 A g 1 (f). 5
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of the triangular-like charge-discharge curve of the sample LMiPC2 also illuminates the additional contribution of pseudo-capacitance to the charge storage capability (Fig. 3f). The specific capacitance of the pre pared samples calculated according to the formula (2) arrives 345, 369, 273 and 210 F g 1 for LMiPC1, LMiPC2, LMiPC3 and YP-50, respec tively, at a current density of 0.5 A g 1. The specific capacitance of the sample LMiPC2 is 1.76 times higher than that of the commercial AC. Meanwhile, an excellent rate capability is observed. As shown in the Fig. S3, the specific capacitance of the sample LMiPC2 arrives 344 and 267 F g 1 under the current density of 1.0 and 20 A g 1, respectively, and more than 77.6% of the initial capacitance is retained as the current density increases from 1.0 to 20 A g 1. In general, more pseudo-capacitance induced by oxygen-containing functional groups could be observed in H2SO4 electrolyte [36,39]. The CV curves of the sample LMiPC2 and commercial activated carbon YP-50 in H2SO4 electrolyte exhibit a quasi-rectangular shape with an apparent redox peak (Fig. 4a), which shape is well retained even under a fast scan rate of 100 mV s 1 (Fig. 4b), indicating the significant contri bution of pseudo-capacitance to the charge storage capability of the obtained material. The apparent pseudo-capacitance exhibits in H2SO4 electrolyte can be further proved by the quasi-triangle GCD curves (Fig. 4c). The specific capacitance of the sample LMiPC2 arrives up to 398 F g 1 in H2SO4 electrolyte at a current density of 0.5 A g 1, while that value of the YP-50 only reaches 221 F g 1. The b value of the sample LMiPC2 varies in the range of 0.7–1 with the variational voltage (Fig. 4d), and the contribution of the surface control capacitance to the total capacitance is ca. 73.6% and 93.3% at a scan rate of 10 mV s 1 and 100 mV s 1, respectively (Fig. 4e and f), which results indicate that the surface control process still is the dominant charge storage mechanism in H2SO4 electrolyte. Table 3 lists the capacitance performance of the biomass-derived porous carbons reported in recent literatures [14, 17–21,40]. As shown in Table 3, our reported material derived from flaxseed residue exhibits a quite competitive capacitance, which value is higher than most of the biomass-derived porous carbon reported in recent literatures, such as sorghum- [18], rice straw- [20], bagasse- [21] and black locust seed-derived porous carbon [40].
Table 3 Summary of capacitive performance of biomass-derived carbon-based super capacitor electrodes under three-electrode configuration. Sample type
Raw materials
SBET/ m2 g 1
Electrolyte
Specific capacitance/F g 1
Hierarchical pore carbon with small micropore size Porous carbon sheets Large pore size microporous carbon
Garlic skin
2818
6 M KOH
461 (1 A g
)
[14]
Cornstalk
1588
1 M H2SO4
407 (1 A g 1)
[17]
Flaxseed residue
3326
6 M KOH
This work
Porous carbon
Sorghum
3047
N doped pore carbon Porous activated carbon Carbon spheres@ porous carbon composites Self-doped graphitic biomass carbon
Bagasse
2905
Loofah sponge Rice straw
2718
369 (0.5 A g 1) 344 (1 A g 398 (0.5 A g 1) 329 (1 A g 311 (1 A g 302 (1 A g 351 (1 A g 310 (1 A g
1122
6 M KOH
Black locust seed
2010
6 M KOH
1 M H2SO4 2 M KOH 1 M H2SO4 6 M KOH 1 M H2SO4 6 M KOH
1
1
)
1
Ref.
) ) 1 ) 1 ) 1 )
[18]
1
337 (1 A g )
[20]
333 (1 A g 1)
[40]
1
[21] [19]
To explore the practical application of the obtained material in supercapacitor, symmetric coin-type supercapacitor devices were assembled and tested in 6 M KOH and ionic liquid EMIMBF4, respec tively. The assembled supercapacitor devices based on the samples LMiPC1, LMiPC2, LMiPC3, and YP-50 are named as SC-1, SC-2, SC-3, and SC-4. All of the assembled devices exhibit a similarly symmetrical rectangular CV curve in the 6 M KOH electrolyte whether at a slow scan rate of 5 mV s 1 or at a fast scan rate of 200 mV s 1 (Fig. 5a and Fig. 5b), showing their ideal capacitance properties. Meanwhile, the symmetrical
Fig. 4. CV curves of the sample LMiPC2 at a scan rate of 2 (a) and 100 mV s 1 (b) in H2SO4 electrolyte; GCD curves at a current density of 0.5 A g 1 (c); the b value under cathode scan and its corresponding fitting variance R2 (d); the contribution of surface control capacitance at a scan rate of 10 mV s 1 (e); and the columnar percentage map of surface control capacitance at different scan rate (f). 6
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triangle-like GCD curves with nearly 100% coulomb efficiency (Fig. 5c and Fig. S4), indicating their perfectly reversible electrical double layer capacitance performance. According to the formula (3), the specific capacitances of the devices of SC-1, SC-2, SC-3 and SC-4 at a current density of 0.5 A g 1 arrives 74 F g 1, 76 F g 1, 62 F g 1 and 39 F g 1, respectively. At a high current density of 20 A g 1, more than 85.7%, 92.7%, 91.8% and 82.5% of the initial capacitance are retained for SC-1, SC-2, SC-3 and SC-4, respectively (Fig. 5d). Apparently, the rata per formance of SC-1, SC-2, SC-3 is better than that of SC-4 due to the large micropore dominant pore structure of the prepared samples. The small micropores (size <1 nm) which is the dominant pore structure for the commercial AC obstructs the fast transport of electrolyte ions especially under high current density, which causes an unsatisfactory rata per formance. Due to the higher proportion of large micropore for LMiPC2 than that for LMiPC1, the device SC-2 exhibits a more excellent rata performance than SC-1. Although the proportion of large micropore for LMiPC3 is nearly 100%, the lower surface oxygen content induces a decrease of surface wettability, which is adverse to the fast transport of electrolyte ions, and thus causes a slightly lower rata performance for SC-3. Furthermore, the rate performance of the device SC-2 is superior to that of the supercapacitor devices based on the small micropore domi nant microporous carbons reported in the recent literatures [14,22,27, 41]. For instance, the capacitance retention for the microporous carbons derived from garlic skin [14], jujube [22], pine nut shells [27] and onion [41] is ca. 82%, 75%, 80% and 74% under a current density of 20 A g 1, respectively, which values are far lower than that of our material. Electrochemical impedance spectroscopy (EIS) was employed for further studying the electrochemical performance of the assembled devices (Fig. 5e). The steep linear curve in the low frequency region indicates nearly ideal capacitive performance and small intercept of the semi-circle in the high frequency region illuminates low ohmic imped ance of the assembled devices (0.63Ω, 0.54 and 0.45Ω for SC-1, SC-2 and SC-3, respectively) [42]. Furthermore, the relationship between the real capacitance and imaginary capacitance in EIS reflects the effect of pore structure on ion transport rate in the materials. Fig. 5f is the image of the frequency response of the real capacitance and imaginary capacitance of
the LMiPC2 and YP-50-based supercapacitor devices. The real capaci tance C0 and imaginary capacitance C00 were calculated from the fre quency response analysis using the formulas [8,14]. 0
C ¼
Z00 2π f jZj2
(9)
0
C00 ¼
Z
2π f jZj2
(10)
where f (Hz) represents the frequency, and Z0 and Z00 refer to the real and imaginary parts of the impedance Z, respectively. As shown in the Fig. 5f, the imaginary capacitance of the devices SC-2 and SC-4 reaches their maximum at frequencies of 0.45 and 0.87 Hz, respectively. The corresponding response time (τ) for the devices SC-2 and SC-4 are 1.23s and 2.22s, respectively [8,25]. The lower τ value of the SC-2 than that of the SC-4 indicates the more fast electrolyte ions transport rate of the obtained LMiPC material with large micropore dominant pore structure than commercial AC with a small micropore dominant pore structure. The assembled devices also exhibit excellent cycle performance due to the large micropore dominant pore structure. The capacitance retention of the devices SC-1, SC-2, SC-3 and SC-4 respectively reaches 93.5%, 98.4%, 97.7%, and 96.7% after 5000 times charge-discharge cycles ata current density of 2 A g 1, and more than 98.1% of the initial capaci tance is retained after 10000 times charge-discharge cycles for the de vice SC-2 (Fig. S5). Using ionic liquid as electrolyte can dramatically improve the energy density of symmetric supercapacitors due to the excellent electro chemical stability and broad voltage window [42]. The rectangular-like CV curves even at a fast scan rate of 50 mV s 1 (Fig. 6a) and the triangular-like GCD curves with small voltage drop even at a large current density of 10 A g 1 (Fig. 6b) indicate the good capacitance properties of the device SC-2 in ionic liquid EMIMBF4. The device capacitance of SC-2 is about 48.9 F g 1 at a current density of 0.5 A g 1. The high capacitance retention of 71.2% at a large current density of 10 A g 1 (Fig. 6c) and 65.6% after 5000 times charge-discharge cycles
Fig. 5. CV curves of the assembled supercapacitor devices at a scan rates of 5 mV s 1 (a) and 200 mV s 1 (b); GCD curves of the assembled devices at a current density 0.5 A g 1 (c); rate capability of the assembled devices (d); Nyquist plots of the assembled devices (e); frequency response of the real capacitance and imaginary capacitance of the LMiPC2 and YP-50 based supercapacitor devices (f). 7
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Fig. 6. CV curves of the supercapacitor device SC-2 in EMIMBF4 at various scan rate (a); GCD curves of the devices SC-2 at various current density (b); cycle stability of the devices SC-2 in EMIMBF4 (c); Ragone plot of the devices SC-2 in EMIMBF4 (d).
(Fig. S6) indicate the good rate capability and cycle performance. The device energy density of SC-2 reaches up to 61.2 Wh kg 1 at a low power density of 468.8 W kg 1 and 43.5 Wh kg 1 of energy density retained even at a high power density of 13.3 kW kg 1, which value is higher than most of the porous carbon materials reported in recent literatures (Fig. 6d) [9,29,43–47].
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4. Conclusion A large micropore dominant microporous carbon is successfully prepared by using a biomass waste, flaxseed residue, as raw material. The obtained LMiPC material exhibits a high specific surface area of up to 3230 m2 g 1, and more than 70.1% of micropore volume is contrib uted by the large micropores. The specific capacitance of the obtained material reaches up to 369 and 398 F g 1 in KOH and H2SO4 electrolyte, respectively. Meanwhile, the symmetric supercapacitor devices based on the obtained material exhibit excellent rate and cycle performance. Over 92.7% of the initial capacitance is retained even under a large current density of 20 A g 1 and the capacitance retention is more than 98.1% after 10000 times charge/discharge cycles in KOH electrolyte. The energy density of the assembled supercapacitor devices reaches 61.2 Wh kg 1 at a power density of 468.8 W kg 1, and a high energy density of 43.5 Wh kg 1 is retained at a large power density of 13.3 kW kg 1 in ionic liquid EMIMBF4 electrolyte. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant No. 51462020 and 21867015). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227396.
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