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Nitrogen self-doped porous carbon with layered structure derived from porcine bladders for high-performance supercapacitors
Accepted Manuscript Nitrogen self-doped porous carbon with layered structure derived from porcine bladders for high-performance supercapacitors Dawei Wang, Zongying Xu, Yue Lian, Chaolei Ban, Huaihao Zhang PII: DOI: Reference:
S0021-9797(19)30189-4 https://doi.org/10.1016/j.jcis.2019.02.024 YJCIS 24648
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
2 November 2018 31 January 2019 6 February 2019
Please cite this article as: D. Wang, Z. Xu, Y. Lian, C. Ban, H. Zhang, Nitrogen self-doped porous carbon with layered structure derived from porcine bladders for high-performance supercapacitors, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.02.024
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Nitrogen self-doped porous carbon with layered structure derived from porcine bladders for high-performance supercapacitors Dawei Wang a, Zongying Xu a, Yue Lian a, Chaolei Ban b, Huaihao Zhang a,* a
School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou
225002, PR China b
School of Materials Science and Technology, Liaocheng University, Liaocheng,
252059, PR China Abstract: Nitrogen-doped layered porous carbon has been successfully fabricated from the biomass of porcine bladders via carbonization and KOH activation. The effects of KOH dosage on the structure, composition and capacitive property of carbon were investigated by a variety of means (SEM, HRTEM, XRD, Raman, XPS, BET and electrochemical test). Owing to the unique layered structure and rich heteroatom content of porcine bladders, the sample obtained at a KOH/carbon mass ratio of 2 were endowed with appropriate pore structure, large specific surface area (1881.7 m2 g-1) and high nitrogen content (5.38%). Meanwhile, the sample exhibits the best electrochemical performance in 6 M KOH electrolyte, including high specific capacitance (322.5 F g-1 at a current density of 0.5 A g-1 ), desirable rate capability (79% capacitance retention when current density increases from 0.5 to 10 A g-1) and superior cycling stability (96% capacitance retention after 5000 cycles). Furthermore, the symmetric supercapacitor assembled with this carbon electrode can deliver 10.9 Wh kg-1 energy density at 0.15 kW kg-1 power density, and maintain 95% capacitance after 5000 cycles. The prominent
1
Corresponding author. Tel/Fax: +86 514 87975244 E-mail address: [email protected] (H.H. Zhang)
performance of this material suggests its promising application in supercapacitor electrode. Keywords: Carbon material; Biomass; Porous carbon; Supercapacitor
1. Introduction
With the rapid consumption of fossil fuels and increasing environmental pollution, the development of renewable clean energy storage and conversion technology has become an urgent need of the world [1–3]. As a kind of energy storage device, supercapacitor has good application prospects in portable electronic equipment, electric vehicles and smart grids due to its high power density, fast charge-discharge rate and long cycle life [4–6]. According to the different energy storage mechanisms, supercapacitor can be divided into two types. One is electric double-layer capacitor (EDLC), which relies on electrostatic charge accumulation at the interface between electrode and electrolyte to store energy, using carbon materials as electrode, such as activated carbon, carbon nanotubes and graphene [7,8]. The other is pseudocapacitor, the charge storage of which depends on the fast reversible redox reactions occurring on the surface of electrodes, generally with transition metal oxides and conductive polymers as its electrode materials [9,10]. Activated carbon has become the most widely used supercapacitor electrode material because of its large specific surface area, high conductivity, desirable stability and low cost [11,12]. However, the specific capacitance of activated carbon is relatively low, thus limiting its further application. Based on the energy storage mechanism of 2
EDLC, it is noticeable that the specific surface area and pore structure are important parameters to determine the capacitive performance of carbon materials [13,14]. But other factors, such as surface functional groups and conductivity, can also affect the performance significantly [15,16]. By doping activated carbon with heteroatoms, especially N, O, S, P and B, Faradic reactions generated by functional groups can be introduced to material surface, thus increase the pseudocapacitance and overall electric capacity of supercapacitors [17]. Meanwhile, heteroatom doping can effectively improve the wettability and electronic conductivity of carbon materials [18]. Therefore, pore structure optimization and heteroatom doping are two main means to increase the specific capacitance of activated carbon. In recent years, a variety of carbon precursors, such as ionic liquids [19,20], biomass [21,22] and polymers [23,24], have been reported for in-situ synthesis of heteroatom self-doped carbon materials. Among these precursors, biomass has attracted wide attention for its rich source, low price and environmental benignancy [25]. Furthermore, biomass usually have natural porous structure and abundant elemental composition, making it very suitable to prepare heteroatom-enriched carbon with specific microstructure. As reported, cornstalk [26], rice husk [27], seaweed [28], fungus [29], shrimp shell [30], egg white [31], gelatin [32], etc. have been successfully used to prepare activated carbon with excellent capacitance properties. Porcine bladder, a by-product of pig farming industry, contains carbohydrates and amino acids with abundant C, N and O elements [33], making it an ideal precursor for developing heteroatom-doped carbon. In this work, by choosing porcine bladder as a 3
new precursor, we successfully prepared nitrogen self-doped layered porous carbon (NLPC) via combining carbonization and activation processes. Through introducing KOH as the activation agent, high specific surface area carbon with different pore size distribution can be obtained. The results show that the carbon is not only N-enriched, but also possesses unique layered structure. Meanwhile, the N doping amount and pore structure can be adjusted by changing KOH dosage. More importantly, the porcine-bladder-derived carbon exhibits excellent electrochemical properties, including high specific capacitance of 322.5 F g-1 at 0.5 A g-1, and good cycle stability during 5000 cycles. Besides, the symmetric supercapacitor assembled with the carbon can reach a high energy density of 10.9 Wh kg-1 at a power density of 0.15 kW kg-1.
2. Experimental
2.1. Preparation of nitrogen-doped layered porous carbon
The preparation process of NLPC is depicted in Fig.1. The porcine bladders (Fig. S1A) were soaked and washed with deionized water. After the impurities on surface were removed, the bladders were dried to constant weight at 60 ºC, cut into pieces (Fig. S1B) and transferred into a tube furnace to be carbonized at 500 ºC for 2 h with a heating rate of 1 ºC min-1 under N2 atmosphere. After cooling down to room temperature, the pre-carbonized porcine bladders (denoted as PPB, Fig S1C) were mixed with KOH in crucibles according to certain mass ratios, followed by proper addition of deionized water under continuous stir and drying in oven. Then the crucibles were put into a tube furnace to be activated at 700 ºC for 2 h with a heating rate of 5 ºC min-1 under N2 4
atmosphere. Finally, the carbon materials after cooling down to room temperature were washed with 2 M HCl and deionized water, and then dried at 60 ºC. The obtained samples are denoted as NLPC-x, where x (x = 1, 2 or 3) represents the mass ratio of KOH to PPB (Fig. S1D, E, F).
2.2. Material characterization The specific surface area and pore size distribution of as-prepared carbon materials were characterized by N2 adsorption analyzer (BET, Micromeritics, TriStar 3000, USA). The crystal structure was determined by polycrystal X-ray powder diffractometer (XRD, Bruker AXS, D8 Advance, Germany) with Cu Kα radiation (λ = 1.5406 Å). The graphitization degree was measured by laser confocal Raman spectrometer (Raman, Renishaw, In Via, UK) with argon ion laser (λ = 532 nm). The element content was determined by elemental analyzer (EA, Elementar, Vario EL cube, Germany). The elemental composition was studied by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250Xi, USA). The morphology and microstructure were analyzed by field emission scanning electron microscopy (SEM, Hitachi, S-4800 II, Japan) and high resolution field emission transmission electron microscopy (HRTEM, FEI, Tecnai G2F30 S-TWIN, USA).
2.3. Electrochemical measurements
The electrochemical tests of materials were conducted by a three-electrode system in 6 M KOH electrolyte at room temperature, with the NLPC electrode, platinum 5
electrode and Ag/AgCl electrode as the working electrode, counter electrode and reference electrode respectively. To fabricate working electrodes, the active material NLPC-x, conductive graphite and binder polytetrafluoroethylene (PTFE) were mixed in a mass ratio of 80:15:5, followed by addition of absolute ethanol to make the mixture into uniform slurry. Then the slurry was smeared on 1×1 cm2 substrate of nickel foam and dried at 60 ℃. Last, the above materials were pressed into electrode slices under 10 MPa pressure. The mass of NLPC active material on each electrode is about 2.5 ~ 3.0 mg. The symmetric supercapacitor NLPC//NLPC was assembled by symmetrical NLPC-2 electrodes with a polyester fiber paper as the separator. Its electrochemical measurements were carried out in a two-electrode system with 6 M KOH electrolyte at room temperature. In order to achieve a charge storage balance, the active material load mass of two electrodes in NLPC//NLPC are approximately equal. The specific capacitance of electrode was calculated by the following equation:
Cm I t / (m V )
(1)
Where Cm (F g-1) is specific capacitance, I (A) is discharge current, △t (s) is discharge time, m (g) is active material mass and △V (V) is potential window. The energy density and power density of the supercapacitor were calculated according to the following equations respectively: E Cm V / 7.2
(2)
P 3.6 E / t
(3)
2
6
Where E (Wh kg-1) is energy density, Cm (F g-1) is specific capacitance, △V (V) is discharge potential range, P (kW kg-1) is power density and △t (s) is discharge time. The cycling voltammetry (CV) and galvanostatic charge-discharge (GCD) tests were carried out on CHI660E electrochemical workstation. The electrochemical impedance spectroscopy (EIS) was conducted on Autolab-PGSTATA30 workstation at open circuit potential with 10-2-105 Hz frequency range and 5 mV alternating current signal amplitude.
3. Results and discussion
3.1. Morphology and Structure
The morphology of obtained samples were observed by SEM and HRTEM. As presented in Fig. 2A and 2B, the structure of membranous porcine bladder is made up of coarse lamellae, which can not only provide a good basis for producing well-developed micropores via subsequent activation process, but also facilitate the layered structure formation. From Fig. 2C and 2D, the pre-carbonized porcine bladder (PPB) is of distinct multi-layer structure, beneficial to KOH transfer and carbon framework etching, so as to produce abundant pores easily. As observed in Fig. 2E, after KOH etching at high temperature, a large amount of pores and channels were generated in NLPC-2 with still retained layered structure. The magnified SEM image (Fig. 2F) further shows that many interconnected pores are evenly distributed on material surface and interior. Fig. S2A and S2B are the SEM images of NLPC-1 and NLPC-3 respectively. Due to inadequate activation under a low KOH dosage, NLPC-1 did not form a well-developed pore 7
structure. As for NLPC-3, excessive amount of KOH leaded to over-activation, making structure collapse. Comparatively, NLPC-2 has the most suitable layered porous structure, which can enhance the capacitive performance by increasing specific surface area and electrolyte ion transport channels [34], as well as improve the stability by inhibiting material collapse in charging and discharging process [35]. From the HRTEM images (Fig. 2H, S2C and S2D), all NLPC samples show a disordered microporous structure. Particularly, NLPC-2 has unique multilayered structure (Fig. 2G), agreeing with the SEM results. The EDS mappings in Fig. 2I-K indicate NLPC-2 is mainly composed of C, O, N elements, distributed on material surface uniformly. The crystal structure of samples was characterized by XRD. In Fig 3A, the diffraction peaks of (002) and (100) crystal planes can be observed at 2θ = 26º and 43º respectively, indicating the amorphous graphitic structure of all samples [36]. Meanwhile, the intensity of (002) plane diffraction peak of all NLPC samples is weaker than PPB, demonstrating the decreased graphitization degree due to the large number of pores created in KOH activation process [37]. Besides, the graphitization degree of samples was further determined by Raman spectroscopy. In Fig 3B, two characteristic peaks at the wavelengths of 1342 cm-1 and 1588 cm-1, corresponding to D and G band, represents the disordered structure of sp3 hybridization and ordered graphite structure of sp2 hybridization respectively. The relative intensity ratio of D to G band (ID/IG) is used to indicate the graphitization degree of carbon material [38]. Herein, the ID/IG of PPB, NLPC-1, NLPC-2 and NLPC-3 is 0.78, 0.87, 0.91 and 0.92, respectively. Obviously, the
8
graphitization degree gradually declines with the increase of KOH dosage which leads to the formation of defects [39]. The composition of samples was further determined by elemental analysis. As shown in Table 1, the samples mainly consist of C, N and O elements, agreeing with the EDS results. With the increase of KOH dosage, the N content decreased gradually. But benefiting from the rich N in porcine bladders, the N content in NLPC-2 can still reach to a high value of 5.38%. XPS analysis was conducted to determine the surface chemical composition. From the survey spectrum in Fig. 3C, the sample NLPC-2 mainly contains C, O and N elements with the atomic contents of 72.13%, 15.54% and 7.33% respectively. In C 1s spectra (Fig. 3D), there are four kinds of C states: C–C (284.4 eV), C–N (285.1 eV), C–O (286.1 eV) and C=O (288.6 eV) [40]. In N 1s spectrum (Fig. 3E), the four individual peaks located at 397.8 eV, 399.5 eV, 400.6 eV and 402.7 eV represent pyridinic-N (N-6), pyrrolic-N (N-5), quaternary-N (N-Q) and oxidized-N (N-X) respectively [41], the relative contents of which are 27.01%, 49.22%, 15.64% and 8.14%. As reported, the N-6, N-5 and N-X distributed on the material surface and edge are the main providers of pseudocapacitance [42]. Meanwhile, the presence of N-Q can promote electron transfer and enhance conductivity [43]. Therefore, the high-content N with surface activity play an important role in improving the capacitance performance of carbon material. The O 1s spectrum (Fig. 3F) fit well with three peaks at 531.0 eV, 532.6 eV and 535.4 eV, assigned to C=O, C–OH/C–O–C and O–C=O correspondingly [41]. Their relative contents are 33.6%, 55.18% and 11.23%. In conclusion, these N and 9
O-based surface functional groups can change atomic arrangement order, increase reactive sites and enhance conductivity, thus improve the material capacitive performance [44]. The specific surface area (SBET) and pore size distribution of samples were measured by nitrogen adsorption–desorption at 77 K. In Fig. 4A, all NLPC samples display a type-I N2 adsorption/desorption isotherm. At low relative pressure (P/P0 < 0.2), the adsorption capacity increases rapidly. But when the relative pressure increases further, the amount of adsorption will become saturated and gradually form a platform, revealing the micropore structure and narrow pore size distribution [45]. From the pore size distribution plots (Fig. 4B) calculated by nonlocal density functional theory (NLDFT) model, the pore sizes of all samples are mainly distributed around 1 nm, revealing their micropore structure again. The SBET and other pore structure parameters of NLPC samples are listed in Table 2. Clearly, the KOH activation process can significantly increase the SBET and pore volume of carbon. For NLPC-2, the SBET and total pore volume can reach to 1881.7 m2 g-1 and 0.83 cm3 g-1 respectively. But excessive amounts of KOH will cause the collapse of pore structure (like NLPC-3 in Fig. S2B), and greatly reduce the yield of samples (as shown in Table S1). Therefore, reasonable activator amount is critical for the preparation of high-performance carbon [46]. As for NLPC-2, its appropriate pore size distribution and large specific surface area can effectively enhance the material wettability and accelerate the ion transfer, leading to improved capacitive properties [47].
10
3.2. Electrochemical performances
The electrochemical properties of NLPC samples were tested by CV, GCD and EIS measurements. In Fig. 5A, all CV curves, at a scan rate of 5 mV s-1 with the potential window of -1~0 V, show a quasi-rectangular shape, indicating a typical behavior of double-layer capacitance [48]. In addition, the apparent humps in CV curves imply the existence of pseudocapacitance, which is generated from the redox reactions of heteroatom functional groups, such as C–N, C=N, C–O, C=O and N–O [49]. Remarkably, the CV curve of NLPC-2 shows the highest peak current and the biggest integral area, illustrating the highest capacitance. Among the obtained samples, although the specific surface area of NLPC-2 is not the largest, its layered structure and suitable pore size distribution are more conducive to the ion transport, and its higher heteroatom contents can produce more pseudocapacitance, thus achieve a better capacitance performance. In Fig. S3 (A, C and E), the CV curves of all samples show no obvious distortion even if the scan rate increases to 100 mV s-1, suggesting their ideal capacitive behavior with fast charge-discharge ability [50]. Fig. 5B displays the GCD curves of samples at the current density of 0.5 A g-1. The slightly curved shape of isosceles triangle further indicates the coexistence of double-layer capacitance and pseudocapacitance [51]. From Fig. S3 (B, D and F), the GCD curves of all samples always maintain good symmetry at different current densities, proving the excellent material reversibility [52].
11
The specific capacitance (Cm) of samples at different current densities is presented in Fig. 5C. Obviously, the Cm of NLPC-2 is the highest, reaching up to 322.5, 305.6, 288.2, 269.5 and 255.8 F g-1 at 0.5, 1, 2, 5 and 10 A g-1 respectively. Meanwhile, with the rise of current density, the capacitance retention of NLPC-2 can reach 79.3%, higher than 76.5% and 72.7% of NLPC-1 and NLPC-3, indicating its good rate capability. Remarkably, as shown in Table S2, the activated carbon derived from porcine bladder exhibits better capacitance performance than other biomass-derived carbon materials. EIS analysis was carried out to further investigate the electrical conductivity of samples. The impedance was fitted by ZSimpWin software based on the equivalent circuit model (Fig. S4D). The corresponding impedance fitting curves and fitting parameters are presented in Fig. S4 (A, C, D) and Table S3, where Rs is equivalent series resistance, Rct is charge transfer resistance, W is Warburg impedance, Cd is double-layer capacitance, and CF is pseudocapacitance. From Fig. 5D, the Nyquist curves of all samples consist of a semicircle in high-frequency region and a straight line in low-frequency region. According to this fitting result, the Rs of NLPC-1, NLPC-2 and NLPC-3 is calculated to be 0.61, 0.54 and 0.59 Ω respectively. As expected, NLPC-2 has the lowest equivalent series resistance, suggesting the good contact between electrolyte and active material [53,54]. Meanwhile, The Rct of NLPC-2 is 0.21 Ω, lower than NLPC-1 (0.39 Ω) and NLPC-3 (0.35 Ω), indicating its better charge-transfer capability and electrical conductivity [55]. Besides, the steep line at low frequency reveals the relatively small W of NLPC-2, manifesting the easy intercalation and deintercalation of ions in electrode [56]. These superiorities of NLPC-2 are largely 12
ascribed to the following two reasons: (1) the abundant micropores and large specific surface area of sample can increase the contact area between electrode and electrolyte, thus provide more electroactive sites; (2) the layered structure can enhance the wettability of material and facilitate the charge transfer and ion diffusion [29]. The cycling stability was measured at a current density of 2 A g-1 for 5000 cycles, as shown in Fig. 6A. With the increase of cycle number, the specific capacitance of NLPC samples decreases gradually and tends to be stable, which is mainly due to the pore-structure collapse of material. After 5000 cycles, 95.5%, 96.4% and 93.8% of the initial Cm can be retained by NLPC-1, NLPC-2 and NLPC-3 respectively. The good cycle stability of NLPC-2 might benefit from the layered structure and suitable pore size distribution, which could buffer the structural collapse caused by material expansion and shrinkage [35]. Fig. 6B displays the GCD curves for the initial and final 5 cycles of NLPC-2. Clearly, the GCD curve shows almost no distortion and keeps good symmetry after cycles, demonstrating the outstanding cycling stability. Fig. 6C is the CV comparison curves of NLPC-2 before and after cycles at the scan rate of 20 mV s-1. No obvious curve contraction and distortion can be observed, again revealing its long cycle life. From the Nyquist plots of NLPC-2 before and after 5000 cycles in Fig. 6D, the diameter of semicircle in high-frequency region increases slightly, while the slope of straight line in low-frequency region decreases a little. Correspondingly, Rs increases from 0.54 Ω to 0.59 Ω, with Rct increasing from 0.21 Ω to 0.29 Ω. Such small resistance changes convincingly prove the desirable cycle stability of NLPC-2.
13
To further explore the practical application value, a symmetric supercapacitor NLPC//NLPC was assembled with NLPC-2 as the electrode material. From Fig. 7A, the CV curves of NLPC//NLPC, with a voltage window of 0~1 V, show no apparent distortion as the scan rate increases, indicating a rapid charge-discharge ability [62]. In Fig. 7B, the good symmetry of GCD curves at different current densities suggests the outstanding capacitive behavior of the device. Based on the discharge curves, the Cm of NLPC//NLPC is calculated to be 78.6, 71.6, 62.7, 52.4 and 37.5 F g-1 at the current density of 0.3, 0.5, 1, 2 and 5 A g-1 respectively, with a capacitance retention of 47.8%, demonstrating the superior rate capability. The Ragone plot obtained from the GCD test at different current densities is illustrated in Fig. 7C. At a power density of 0.15 kW kg-1, NLPC//NLPC can deliver a high energy density of 10.9 Wh kg-1. As shown in Table 3, NLPC//NLPC possesses better capacitive properties than other reported supercapacitors based on biomass carbon materials. Fig. 7D shows the cycle performance of NLPC//NLPC at the current density of 2 A g-1. After 5000 cycles, it can retain 95.3% of the initial Cm and keep nearly 100% Coulombic efficiency, indicating its long cycle life. In a word, the N-doped layered porous carbon, derived from porcine bladders, is a promising electrode material for high-performance supercapacitors.
4. Conclusions In summary, nitrogen self-doped layered porous carbon have been successfully synthesized from porcine bladders through carbonization and KOH activation. Meanwhile, the effect of KOH dosage on the structure and electrochemical properties of 14
carbon was investigated in detail. As KOH dosage increases, the specific surface area of carbon is enlarged, accompanied by reduced heteroatom contents. The electrochemical tests show the sample prepared at KOH/carbon mass ratio of 2 achieves high specific capacitance of 322.5 F g-1 at 0.5 A g-1, as well as good cycling stability with 96% capacitance retention after 5000 cycles. What’s more, the assembled symmetric supercapacitor can deliver
high energy density of 10.9 Wh kg-1 at power density of
0.15 kW kg-1. Such superior capacitive performance of the carbon is attributed to its reasonable pore size distribution, unique multilayered structure and high amount of doped heteroatoms. Therefore, this work is of certain significance to convert layered biomass to promising carbon for energy storage.
Acknowledgements This work was financially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Natural Science Foundation of Shandong Province, China (grant number ZR2017MEM019). The related measure and analysis instruments for this work was supported by Testing Center of Yangzhou University.
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25
Fig. 1. Schematic of NLPC preparation.
26
Fig. 2. (A-F) SEM images of samples: porcine bladder (A, B), PPB (C, D), NLPC-2 (E, F); (G, H) HRTEM images of NLPC-2; (I-L) EDS mappings of NLPC-2.
27
B （ 100）
Intensity (a.u.)
（ 002）
G
D
Raman Intensity (a.u.)
A
NLPC-3 NLPC-2 NLPC-1
NLPC-3 NLPC-2 NLPC-1
PPB
PPB 10
20
30
40 50 2degree
C
60
70
900
1500
1800
2100
Raman Shift (cm-1)
D C 1s Intensity (a.u.)
C 1s
Intensity (a.u.)
1200
O 1s N 1s
C-C 284.4eV C-O 286.1eV
C-N 285.1eV
C=O 288.6eV
NLPC-2 1000
E
800
600 400 200 Binding Energy (eV)
0
290
F O 1s
406
N-6 397.8eV
404
402
400
398
396
Intensity (a.u.)
Intensity (a.u.)
N-5 399.5eV
N-X 402.7eV
286
284
282
Binding Energy (eV)
N 1s
N-Q 400.6eV
288
394
C-O 532.6eV
O-C=O 535.4eV
540
Binding Energy (eV)
538
536 534 532 530 Binding Energy (eV)
C=O 531.0eV
528
526
Fig. 3. (A) XRD patterns and (B) Raman spectra of samples; (C-F) XPS spectra of NLPC-2: survey spectrum (C), C 1s (D), N 1s (E) and O 1s (F).
28
700
Quantity adsorbed (cm3g-1)
600 500 400 300 200
NLPC-1 NLPC-2 NLPC-3
100 0
B
0.10
dV/dW (cm3 g-1nm-1)
A
0.08
NLPC-1 NLPC-2 NLPC-3
0.06 0.04 0.02 0.00
0.0
0.2
0.4
0.6
0.8
1.0
1
2
0
3
4
Pore size (nm)
Relative pressure (P/P )
Fig. 4. (A) N2 adsorption/desorption isotherms and (B) pore size distribution curves of NLPC samples.
2
B
1 0 -1 NLPC-1 NLPC-2 NLPC-3
-2 -1.0
-0.8
-0.6
-0.4
-0.2
Potential vs Ag/AgCl (V)
-1
Current density (A g )
A
NLPC-1 NLPC-2 NLPC-3
0.0 -0.2 -0.4 -0.6 -0.8 -1.0
0.0
0
200
400
Potential vs Ag/AgCl (V)
800 1000 1200
D 30
C 400
NLPC-1 NLPC-2 NLPC-3
25
200
NLPC-1 NLPC-2 NLPC-3
100
0 0
2
4
6
8
20
4
15
3
-Z'' (Ohm)
300
-Z'' (Ohm)
Specific capacitance (Fg-1)
600 Time (s)
10
2 NLPC-1 NLPC-2 NLPC-3
1
5 0 0
0 0
10
-1
Current density (A g )
29
5
10
1
2 Z' (Ohm)
15 20 Z' (Ohm)
3
4
25
30
Fig. 5. Electrochemical properties of NLPC samples: (A) CV curves at 5 mV·s-1, (B) GCD curves at 0.5 A·g-1, (C) specific capacitance at different current densities, (D) Nyquist plots (inset is the magnified view).
B 0.2 Potential vs Ag/AgCl (V)
Specific capacitance (F g-1)
A 400 300
200
NLPC-1 NLPC-2 NLPC-3
100
0
0
1000
2000
3000
4000
First 5 cycles Last 5 cycles
0.0 -0.2 -0.4 -0.6 -0.8 -1.0
5000
0
300
Cycle number
D
6 4
1200
1500
4 Before cycles After cycles 3
2 0 -2 -4
2
1 Before cycles After cycles
-6 -8
900
Time (s)
-Z'' (Ohm)
Current density (A g-1)
C
600
-1.0
-0.8
-0.6
-0.4
-0.2
0 0
0.0
1
2
3
Z' (Ohm)
Potential vs Ag/AgCl (V)
Fig. 6. (A) Cycling performance of NLPC samples at 2 A·g-1; (B) GCD curves, (C) CV curves and (D) Nyquist plots of NLPC-2 before and after 5000 cycles.
30
4
B
4
Potential vs Ag/AgCl (V)
2 0 5 mVs-1 10 mVs-1 20 mVs-1 50 mVs-1 100 mVs-1
-2 -4 -6 0.0
0.2
0.4
0.6
0.8
0.3 Ag-1 0.5 Ag-1 1 Ag-1 2 Ag-1 5 Ag-1
1.0 0.8 0.6 0.4 0.2 0.0 0
1.0
100
Potential vs Ag/AgCl (V)
D
this work 10
Ref 59 Ref 61
Ref 57 Ref 28 Ref 32
5
Ref 58 Ref 60
0.1
Specific capacitance (F g-1)
Energy density (Whkg-1)
C
200 300 Time (s)
400 120
100
100
80
80 60 60 40
40
20 0 0
1
Power density (kW kg-1)
20 1000
2000 3000 Cycle number
4000
Coulombic efficiency (%)
Current density (A g-1)
A 6
0 5000
Fig. 7. Capacitive properties of NLPC//NLPC symmetric supercapacitor: (A) CV curves at various scan rates, (B) GCD curves at various current densities, (C) Ragone plots, (D) cycling performance and Coulombic efficiency at the current density of 2 A·g-1.
31
Table 1 Elemental analysis parameters of samples Samples
C (wt%)
N (wt%)
O (wt%)
H (wt%)
Precursor
50.8
14.2
27.3
5.4
PPB
65.7
11.8
17.6
3.2
NLPC-1
72.4
6.5
15.7
2.8
NLPC-2
76.7
5.4
13.5
2.5
NLPC-3
79.7
3.8
11.6
2.3
32
Table 2 Pore structure parameters of NLPC samples a
SBET
b
Smicro
d
c
Vt
c b
e
(nm)
Dm
(m2 g-1)
(m2 g-1)
NLPC-1
1568.4
1317.2
276.6
0.68
0.55
0.13
1.73
NLPC-2
1881.7
1492.8
367.2
0.83
0.64
0.19
1.77
NLPC-3
2079.3
1401.2
663.5
0.98
0.66
0.32
1.89
Sample
Vmicro 3 -1 3 -1 (cm g ) (cm g )
Vmeso (cm3 g-1)
Smeso (m2 g-1)
a
SBET: BET surface area.
b
Smicro: micropore surface area and Vmicro: micropore volume.
c
Smeso: mesopore surface area and Vmeso: mesopore volume.
d
Vt: total pore volume.
e
Dm: average pore size.
33
Table 3 Comparison of energy density and power density of various supercapacitors based on biomass carbon materials Biomass carbon materials
Electrolyte
Energy density (Wh kg-1)
Power density (kW kg-1)
Ref.
Pueraria
6 M KOH
8.5
0.12
[57]
Tobacco rods
6 M KOH
5.5
1.50
[58]
Pomelo peel
6 M KOH
9.4
0.10
[59]
Seaweed
1 M H2SO4
8.0
0.10
[28]
Shrimp shells
6 M KOH
5.2
1.16
[60]
Auricularia
6 M KOH
9.4
0.05
[61]
Gelatin
1 M H2SO4
6.2
0.50
[32]
Porcine bladders
6 M KOH
10.9
0.15
This work
34
Graphical abstract
35
S0021-9797(19)30189-4 https://doi.org/10.1016/j.jcis.2019.02.024 YJCIS 24648
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
2 November 2018 31 January 2019 6 February 2019
Please cite this article as: D. Wang, Z. Xu, Y. Lian, C. Ban, H. Zhang, Nitrogen self-doped porous carbon with layered structure derived from porcine bladders for high-performance supercapacitors, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.02.024
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nitrogen self-doped porous carbon with layered structure derived from porcine bladders for high-performance supercapacitors Dawei Wang a, Zongying Xu a, Yue Lian a, Chaolei Ban b, Huaihao Zhang a,* a
School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou
225002, PR China b
School of Materials Science and Technology, Liaocheng University, Liaocheng,
252059, PR China Abstract: Nitrogen-doped layered porous carbon has been successfully fabricated from the biomass of porcine bladders via carbonization and KOH activation. The effects of KOH dosage on the structure, composition and capacitive property of carbon were investigated by a variety of means (SEM, HRTEM, XRD, Raman, XPS, BET and electrochemical test). Owing to the unique layered structure and rich heteroatom content of porcine bladders, the sample obtained at a KOH/carbon mass ratio of 2 were endowed with appropriate pore structure, large specific surface area (1881.7 m2 g-1) and high nitrogen content (5.38%). Meanwhile, the sample exhibits the best electrochemical performance in 6 M KOH electrolyte, including high specific capacitance (322.5 F g-1 at a current density of 0.5 A g-1 ), desirable rate capability (79% capacitance retention when current density increases from 0.5 to 10 A g-1) and superior cycling stability (96% capacitance retention after 5000 cycles). Furthermore, the symmetric supercapacitor assembled with this carbon electrode can deliver 10.9 Wh kg-1 energy density at 0.15 kW kg-1 power density, and maintain 95% capacitance after 5000 cycles. The prominent
1
Corresponding author. Tel/Fax: +86 514 87975244 E-mail address: [email protected] (H.H. Zhang)
performance of this material suggests its promising application in supercapacitor electrode. Keywords: Carbon material; Biomass; Porous carbon; Supercapacitor
1. Introduction
With the rapid consumption of fossil fuels and increasing environmental pollution, the development of renewable clean energy storage and conversion technology has become an urgent need of the world [1–3]. As a kind of energy storage device, supercapacitor has good application prospects in portable electronic equipment, electric vehicles and smart grids due to its high power density, fast charge-discharge rate and long cycle life [4–6]. According to the different energy storage mechanisms, supercapacitor can be divided into two types. One is electric double-layer capacitor (EDLC), which relies on electrostatic charge accumulation at the interface between electrode and electrolyte to store energy, using carbon materials as electrode, such as activated carbon, carbon nanotubes and graphene [7,8]. The other is pseudocapacitor, the charge storage of which depends on the fast reversible redox reactions occurring on the surface of electrodes, generally with transition metal oxides and conductive polymers as its electrode materials [9,10]. Activated carbon has become the most widely used supercapacitor electrode material because of its large specific surface area, high conductivity, desirable stability and low cost [11,12]. However, the specific capacitance of activated carbon is relatively low, thus limiting its further application. Based on the energy storage mechanism of 2
EDLC, it is noticeable that the specific surface area and pore structure are important parameters to determine the capacitive performance of carbon materials [13,14]. But other factors, such as surface functional groups and conductivity, can also affect the performance significantly [15,16]. By doping activated carbon with heteroatoms, especially N, O, S, P and B, Faradic reactions generated by functional groups can be introduced to material surface, thus increase the pseudocapacitance and overall electric capacity of supercapacitors [17]. Meanwhile, heteroatom doping can effectively improve the wettability and electronic conductivity of carbon materials [18]. Therefore, pore structure optimization and heteroatom doping are two main means to increase the specific capacitance of activated carbon. In recent years, a variety of carbon precursors, such as ionic liquids [19,20], biomass [21,22] and polymers [23,24], have been reported for in-situ synthesis of heteroatom self-doped carbon materials. Among these precursors, biomass has attracted wide attention for its rich source, low price and environmental benignancy [25]. Furthermore, biomass usually have natural porous structure and abundant elemental composition, making it very suitable to prepare heteroatom-enriched carbon with specific microstructure. As reported, cornstalk [26], rice husk [27], seaweed [28], fungus [29], shrimp shell [30], egg white [31], gelatin [32], etc. have been successfully used to prepare activated carbon with excellent capacitance properties. Porcine bladder, a by-product of pig farming industry, contains carbohydrates and amino acids with abundant C, N and O elements [33], making it an ideal precursor for developing heteroatom-doped carbon. In this work, by choosing porcine bladder as a 3
new precursor, we successfully prepared nitrogen self-doped layered porous carbon (NLPC) via combining carbonization and activation processes. Through introducing KOH as the activation agent, high specific surface area carbon with different pore size distribution can be obtained. The results show that the carbon is not only N-enriched, but also possesses unique layered structure. Meanwhile, the N doping amount and pore structure can be adjusted by changing KOH dosage. More importantly, the porcine-bladder-derived carbon exhibits excellent electrochemical properties, including high specific capacitance of 322.5 F g-1 at 0.5 A g-1, and good cycle stability during 5000 cycles. Besides, the symmetric supercapacitor assembled with the carbon can reach a high energy density of 10.9 Wh kg-1 at a power density of 0.15 kW kg-1.
2. Experimental
2.1. Preparation of nitrogen-doped layered porous carbon
The preparation process of NLPC is depicted in Fig.1. The porcine bladders (Fig. S1A) were soaked and washed with deionized water. After the impurities on surface were removed, the bladders were dried to constant weight at 60 ºC, cut into pieces (Fig. S1B) and transferred into a tube furnace to be carbonized at 500 ºC for 2 h with a heating rate of 1 ºC min-1 under N2 atmosphere. After cooling down to room temperature, the pre-carbonized porcine bladders (denoted as PPB, Fig S1C) were mixed with KOH in crucibles according to certain mass ratios, followed by proper addition of deionized water under continuous stir and drying in oven. Then the crucibles were put into a tube furnace to be activated at 700 ºC for 2 h with a heating rate of 5 ºC min-1 under N2 4
atmosphere. Finally, the carbon materials after cooling down to room temperature were washed with 2 M HCl and deionized water, and then dried at 60 ºC. The obtained samples are denoted as NLPC-x, where x (x = 1, 2 or 3) represents the mass ratio of KOH to PPB (Fig. S1D, E, F).
2.2. Material characterization The specific surface area and pore size distribution of as-prepared carbon materials were characterized by N2 adsorption analyzer (BET, Micromeritics, TriStar 3000, USA). The crystal structure was determined by polycrystal X-ray powder diffractometer (XRD, Bruker AXS, D8 Advance, Germany) with Cu Kα radiation (λ = 1.5406 Å). The graphitization degree was measured by laser confocal Raman spectrometer (Raman, Renishaw, In Via, UK) with argon ion laser (λ = 532 nm). The element content was determined by elemental analyzer (EA, Elementar, Vario EL cube, Germany). The elemental composition was studied by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250Xi, USA). The morphology and microstructure were analyzed by field emission scanning electron microscopy (SEM, Hitachi, S-4800 II, Japan) and high resolution field emission transmission electron microscopy (HRTEM, FEI, Tecnai G2F30 S-TWIN, USA).
2.3. Electrochemical measurements
The electrochemical tests of materials were conducted by a three-electrode system in 6 M KOH electrolyte at room temperature, with the NLPC electrode, platinum 5
electrode and Ag/AgCl electrode as the working electrode, counter electrode and reference electrode respectively. To fabricate working electrodes, the active material NLPC-x, conductive graphite and binder polytetrafluoroethylene (PTFE) were mixed in a mass ratio of 80:15:5, followed by addition of absolute ethanol to make the mixture into uniform slurry. Then the slurry was smeared on 1×1 cm2 substrate of nickel foam and dried at 60 ℃. Last, the above materials were pressed into electrode slices under 10 MPa pressure. The mass of NLPC active material on each electrode is about 2.5 ~ 3.0 mg. The symmetric supercapacitor NLPC//NLPC was assembled by symmetrical NLPC-2 electrodes with a polyester fiber paper as the separator. Its electrochemical measurements were carried out in a two-electrode system with 6 M KOH electrolyte at room temperature. In order to achieve a charge storage balance, the active material load mass of two electrodes in NLPC//NLPC are approximately equal. The specific capacitance of electrode was calculated by the following equation:
Cm I t / (m V )
(1)
Where Cm (F g-1) is specific capacitance, I (A) is discharge current, △t (s) is discharge time, m (g) is active material mass and △V (V) is potential window. The energy density and power density of the supercapacitor were calculated according to the following equations respectively: E Cm V / 7.2
(2)
P 3.6 E / t
(3)
2
6
Where E (Wh kg-1) is energy density, Cm (F g-1) is specific capacitance, △V (V) is discharge potential range, P (kW kg-1) is power density and △t (s) is discharge time. The cycling voltammetry (CV) and galvanostatic charge-discharge (GCD) tests were carried out on CHI660E electrochemical workstation. The electrochemical impedance spectroscopy (EIS) was conducted on Autolab-PGSTATA30 workstation at open circuit potential with 10-2-105 Hz frequency range and 5 mV alternating current signal amplitude.
3. Results and discussion
3.1. Morphology and Structure
The morphology of obtained samples were observed by SEM and HRTEM. As presented in Fig. 2A and 2B, the structure of membranous porcine bladder is made up of coarse lamellae, which can not only provide a good basis for producing well-developed micropores via subsequent activation process, but also facilitate the layered structure formation. From Fig. 2C and 2D, the pre-carbonized porcine bladder (PPB) is of distinct multi-layer structure, beneficial to KOH transfer and carbon framework etching, so as to produce abundant pores easily. As observed in Fig. 2E, after KOH etching at high temperature, a large amount of pores and channels were generated in NLPC-2 with still retained layered structure. The magnified SEM image (Fig. 2F) further shows that many interconnected pores are evenly distributed on material surface and interior. Fig. S2A and S2B are the SEM images of NLPC-1 and NLPC-3 respectively. Due to inadequate activation under a low KOH dosage, NLPC-1 did not form a well-developed pore 7
structure. As for NLPC-3, excessive amount of KOH leaded to over-activation, making structure collapse. Comparatively, NLPC-2 has the most suitable layered porous structure, which can enhance the capacitive performance by increasing specific surface area and electrolyte ion transport channels [34], as well as improve the stability by inhibiting material collapse in charging and discharging process [35]. From the HRTEM images (Fig. 2H, S2C and S2D), all NLPC samples show a disordered microporous structure. Particularly, NLPC-2 has unique multilayered structure (Fig. 2G), agreeing with the SEM results. The EDS mappings in Fig. 2I-K indicate NLPC-2 is mainly composed of C, O, N elements, distributed on material surface uniformly. The crystal structure of samples was characterized by XRD. In Fig 3A, the diffraction peaks of (002) and (100) crystal planes can be observed at 2θ = 26º and 43º respectively, indicating the amorphous graphitic structure of all samples [36]. Meanwhile, the intensity of (002) plane diffraction peak of all NLPC samples is weaker than PPB, demonstrating the decreased graphitization degree due to the large number of pores created in KOH activation process [37]. Besides, the graphitization degree of samples was further determined by Raman spectroscopy. In Fig 3B, two characteristic peaks at the wavelengths of 1342 cm-1 and 1588 cm-1, corresponding to D and G band, represents the disordered structure of sp3 hybridization and ordered graphite structure of sp2 hybridization respectively. The relative intensity ratio of D to G band (ID/IG) is used to indicate the graphitization degree of carbon material [38]. Herein, the ID/IG of PPB, NLPC-1, NLPC-2 and NLPC-3 is 0.78, 0.87, 0.91 and 0.92, respectively. Obviously, the
8
graphitization degree gradually declines with the increase of KOH dosage which leads to the formation of defects [39]. The composition of samples was further determined by elemental analysis. As shown in Table 1, the samples mainly consist of C, N and O elements, agreeing with the EDS results. With the increase of KOH dosage, the N content decreased gradually. But benefiting from the rich N in porcine bladders, the N content in NLPC-2 can still reach to a high value of 5.38%. XPS analysis was conducted to determine the surface chemical composition. From the survey spectrum in Fig. 3C, the sample NLPC-2 mainly contains C, O and N elements with the atomic contents of 72.13%, 15.54% and 7.33% respectively. In C 1s spectra (Fig. 3D), there are four kinds of C states: C–C (284.4 eV), C–N (285.1 eV), C–O (286.1 eV) and C=O (288.6 eV) [40]. In N 1s spectrum (Fig. 3E), the four individual peaks located at 397.8 eV, 399.5 eV, 400.6 eV and 402.7 eV represent pyridinic-N (N-6), pyrrolic-N (N-5), quaternary-N (N-Q) and oxidized-N (N-X) respectively [41], the relative contents of which are 27.01%, 49.22%, 15.64% and 8.14%. As reported, the N-6, N-5 and N-X distributed on the material surface and edge are the main providers of pseudocapacitance [42]. Meanwhile, the presence of N-Q can promote electron transfer and enhance conductivity [43]. Therefore, the high-content N with surface activity play an important role in improving the capacitance performance of carbon material. The O 1s spectrum (Fig. 3F) fit well with three peaks at 531.0 eV, 532.6 eV and 535.4 eV, assigned to C=O, C–OH/C–O–C and O–C=O correspondingly [41]. Their relative contents are 33.6%, 55.18% and 11.23%. In conclusion, these N and 9
O-based surface functional groups can change atomic arrangement order, increase reactive sites and enhance conductivity, thus improve the material capacitive performance [44]. The specific surface area (SBET) and pore size distribution of samples were measured by nitrogen adsorption–desorption at 77 K. In Fig. 4A, all NLPC samples display a type-I N2 adsorption/desorption isotherm. At low relative pressure (P/P0 < 0.2), the adsorption capacity increases rapidly. But when the relative pressure increases further, the amount of adsorption will become saturated and gradually form a platform, revealing the micropore structure and narrow pore size distribution [45]. From the pore size distribution plots (Fig. 4B) calculated by nonlocal density functional theory (NLDFT) model, the pore sizes of all samples are mainly distributed around 1 nm, revealing their micropore structure again. The SBET and other pore structure parameters of NLPC samples are listed in Table 2. Clearly, the KOH activation process can significantly increase the SBET and pore volume of carbon. For NLPC-2, the SBET and total pore volume can reach to 1881.7 m2 g-1 and 0.83 cm3 g-1 respectively. But excessive amounts of KOH will cause the collapse of pore structure (like NLPC-3 in Fig. S2B), and greatly reduce the yield of samples (as shown in Table S1). Therefore, reasonable activator amount is critical for the preparation of high-performance carbon [46]. As for NLPC-2, its appropriate pore size distribution and large specific surface area can effectively enhance the material wettability and accelerate the ion transfer, leading to improved capacitive properties [47].
10
3.2. Electrochemical performances
The electrochemical properties of NLPC samples were tested by CV, GCD and EIS measurements. In Fig. 5A, all CV curves, at a scan rate of 5 mV s-1 with the potential window of -1~0 V, show a quasi-rectangular shape, indicating a typical behavior of double-layer capacitance [48]. In addition, the apparent humps in CV curves imply the existence of pseudocapacitance, which is generated from the redox reactions of heteroatom functional groups, such as C–N, C=N, C–O, C=O and N–O [49]. Remarkably, the CV curve of NLPC-2 shows the highest peak current and the biggest integral area, illustrating the highest capacitance. Among the obtained samples, although the specific surface area of NLPC-2 is not the largest, its layered structure and suitable pore size distribution are more conducive to the ion transport, and its higher heteroatom contents can produce more pseudocapacitance, thus achieve a better capacitance performance. In Fig. S3 (A, C and E), the CV curves of all samples show no obvious distortion even if the scan rate increases to 100 mV s-1, suggesting their ideal capacitive behavior with fast charge-discharge ability [50]. Fig. 5B displays the GCD curves of samples at the current density of 0.5 A g-1. The slightly curved shape of isosceles triangle further indicates the coexistence of double-layer capacitance and pseudocapacitance [51]. From Fig. S3 (B, D and F), the GCD curves of all samples always maintain good symmetry at different current densities, proving the excellent material reversibility [52].
11
The specific capacitance (Cm) of samples at different current densities is presented in Fig. 5C. Obviously, the Cm of NLPC-2 is the highest, reaching up to 322.5, 305.6, 288.2, 269.5 and 255.8 F g-1 at 0.5, 1, 2, 5 and 10 A g-1 respectively. Meanwhile, with the rise of current density, the capacitance retention of NLPC-2 can reach 79.3%, higher than 76.5% and 72.7% of NLPC-1 and NLPC-3, indicating its good rate capability. Remarkably, as shown in Table S2, the activated carbon derived from porcine bladder exhibits better capacitance performance than other biomass-derived carbon materials. EIS analysis was carried out to further investigate the electrical conductivity of samples. The impedance was fitted by ZSimpWin software based on the equivalent circuit model (Fig. S4D). The corresponding impedance fitting curves and fitting parameters are presented in Fig. S4 (A, C, D) and Table S3, where Rs is equivalent series resistance, Rct is charge transfer resistance, W is Warburg impedance, Cd is double-layer capacitance, and CF is pseudocapacitance. From Fig. 5D, the Nyquist curves of all samples consist of a semicircle in high-frequency region and a straight line in low-frequency region. According to this fitting result, the Rs of NLPC-1, NLPC-2 and NLPC-3 is calculated to be 0.61, 0.54 and 0.59 Ω respectively. As expected, NLPC-2 has the lowest equivalent series resistance, suggesting the good contact between electrolyte and active material [53,54]. Meanwhile, The Rct of NLPC-2 is 0.21 Ω, lower than NLPC-1 (0.39 Ω) and NLPC-3 (0.35 Ω), indicating its better charge-transfer capability and electrical conductivity [55]. Besides, the steep line at low frequency reveals the relatively small W of NLPC-2, manifesting the easy intercalation and deintercalation of ions in electrode [56]. These superiorities of NLPC-2 are largely 12
ascribed to the following two reasons: (1) the abundant micropores and large specific surface area of sample can increase the contact area between electrode and electrolyte, thus provide more electroactive sites; (2) the layered structure can enhance the wettability of material and facilitate the charge transfer and ion diffusion [29]. The cycling stability was measured at a current density of 2 A g-1 for 5000 cycles, as shown in Fig. 6A. With the increase of cycle number, the specific capacitance of NLPC samples decreases gradually and tends to be stable, which is mainly due to the pore-structure collapse of material. After 5000 cycles, 95.5%, 96.4% and 93.8% of the initial Cm can be retained by NLPC-1, NLPC-2 and NLPC-3 respectively. The good cycle stability of NLPC-2 might benefit from the layered structure and suitable pore size distribution, which could buffer the structural collapse caused by material expansion and shrinkage [35]. Fig. 6B displays the GCD curves for the initial and final 5 cycles of NLPC-2. Clearly, the GCD curve shows almost no distortion and keeps good symmetry after cycles, demonstrating the outstanding cycling stability. Fig. 6C is the CV comparison curves of NLPC-2 before and after cycles at the scan rate of 20 mV s-1. No obvious curve contraction and distortion can be observed, again revealing its long cycle life. From the Nyquist plots of NLPC-2 before and after 5000 cycles in Fig. 6D, the diameter of semicircle in high-frequency region increases slightly, while the slope of straight line in low-frequency region decreases a little. Correspondingly, Rs increases from 0.54 Ω to 0.59 Ω, with Rct increasing from 0.21 Ω to 0.29 Ω. Such small resistance changes convincingly prove the desirable cycle stability of NLPC-2.
13
To further explore the practical application value, a symmetric supercapacitor NLPC//NLPC was assembled with NLPC-2 as the electrode material. From Fig. 7A, the CV curves of NLPC//NLPC, with a voltage window of 0~1 V, show no apparent distortion as the scan rate increases, indicating a rapid charge-discharge ability [62]. In Fig. 7B, the good symmetry of GCD curves at different current densities suggests the outstanding capacitive behavior of the device. Based on the discharge curves, the Cm of NLPC//NLPC is calculated to be 78.6, 71.6, 62.7, 52.4 and 37.5 F g-1 at the current density of 0.3, 0.5, 1, 2 and 5 A g-1 respectively, with a capacitance retention of 47.8%, demonstrating the superior rate capability. The Ragone plot obtained from the GCD test at different current densities is illustrated in Fig. 7C. At a power density of 0.15 kW kg-1, NLPC//NLPC can deliver a high energy density of 10.9 Wh kg-1. As shown in Table 3, NLPC//NLPC possesses better capacitive properties than other reported supercapacitors based on biomass carbon materials. Fig. 7D shows the cycle performance of NLPC//NLPC at the current density of 2 A g-1. After 5000 cycles, it can retain 95.3% of the initial Cm and keep nearly 100% Coulombic efficiency, indicating its long cycle life. In a word, the N-doped layered porous carbon, derived from porcine bladders, is a promising electrode material for high-performance supercapacitors.
4. Conclusions In summary, nitrogen self-doped layered porous carbon have been successfully synthesized from porcine bladders through carbonization and KOH activation. Meanwhile, the effect of KOH dosage on the structure and electrochemical properties of 14
carbon was investigated in detail. As KOH dosage increases, the specific surface area of carbon is enlarged, accompanied by reduced heteroatom contents. The electrochemical tests show the sample prepared at KOH/carbon mass ratio of 2 achieves high specific capacitance of 322.5 F g-1 at 0.5 A g-1, as well as good cycling stability with 96% capacitance retention after 5000 cycles. What’s more, the assembled symmetric supercapacitor can deliver
high energy density of 10.9 Wh kg-1 at power density of
0.15 kW kg-1. Such superior capacitive performance of the carbon is attributed to its reasonable pore size distribution, unique multilayered structure and high amount of doped heteroatoms. Therefore, this work is of certain significance to convert layered biomass to promising carbon for energy storage.
Acknowledgements This work was financially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Natural Science Foundation of Shandong Province, China (grant number ZR2017MEM019). The related measure and analysis instruments for this work was supported by Testing Center of Yangzhou University.
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25
Fig. 1. Schematic of NLPC preparation.
26
Fig. 2. (A-F) SEM images of samples: porcine bladder (A, B), PPB (C, D), NLPC-2 (E, F); (G, H) HRTEM images of NLPC-2; (I-L) EDS mappings of NLPC-2.
27
B （ 100）
Intensity (a.u.)
（ 002）
G
D
Raman Intensity (a.u.)
A
NLPC-3 NLPC-2 NLPC-1
NLPC-3 NLPC-2 NLPC-1
PPB
PPB 10
20
30
40 50 2degree
C
60
70
900
1500
1800
2100
Raman Shift (cm-1)
D C 1s Intensity (a.u.)
C 1s
Intensity (a.u.)
1200
O 1s N 1s
C-C 284.4eV C-O 286.1eV
C-N 285.1eV
C=O 288.6eV
NLPC-2 1000
E
800
600 400 200 Binding Energy (eV)
0
290
F O 1s
406
N-6 397.8eV
404
402
400
398
396
Intensity (a.u.)
Intensity (a.u.)
N-5 399.5eV
N-X 402.7eV
286
284
282
Binding Energy (eV)
N 1s
N-Q 400.6eV
288
394
C-O 532.6eV
O-C=O 535.4eV
540
Binding Energy (eV)
538
536 534 532 530 Binding Energy (eV)
C=O 531.0eV
528
526
Fig. 3. (A) XRD patterns and (B) Raman spectra of samples; (C-F) XPS spectra of NLPC-2: survey spectrum (C), C 1s (D), N 1s (E) and O 1s (F).
28
700
Quantity adsorbed (cm3g-1)
600 500 400 300 200
NLPC-1 NLPC-2 NLPC-3
100 0
B
0.10
dV/dW (cm3 g-1nm-1)
A
0.08
NLPC-1 NLPC-2 NLPC-3
0.06 0.04 0.02 0.00
0.0
0.2
0.4
0.6
0.8
1.0
1
2
0
3
4
Pore size (nm)
Relative pressure (P/P )
Fig. 4. (A) N2 adsorption/desorption isotherms and (B) pore size distribution curves of NLPC samples.
2
B
1 0 -1 NLPC-1 NLPC-2 NLPC-3
-2 -1.0
-0.8
-0.6
-0.4
-0.2
Potential vs Ag/AgCl (V)
-1
Current density (A g )
A
NLPC-1 NLPC-2 NLPC-3
0.0 -0.2 -0.4 -0.6 -0.8 -1.0
0.0
0
200
400
Potential vs Ag/AgCl (V)
800 1000 1200
D 30
C 400
NLPC-1 NLPC-2 NLPC-3
25
200
NLPC-1 NLPC-2 NLPC-3
100
0 0
2
4
6
8
20
4
15
3
-Z'' (Ohm)
300
-Z'' (Ohm)
Specific capacitance (Fg-1)
600 Time (s)
10
2 NLPC-1 NLPC-2 NLPC-3
1
5 0 0
0 0
10
-1
Current density (A g )
29
5
10
1
2 Z' (Ohm)
15 20 Z' (Ohm)
3
4
25
30
Fig. 5. Electrochemical properties of NLPC samples: (A) CV curves at 5 mV·s-1, (B) GCD curves at 0.5 A·g-1, (C) specific capacitance at different current densities, (D) Nyquist plots (inset is the magnified view).
B 0.2 Potential vs Ag/AgCl (V)
Specific capacitance (F g-1)
A 400 300
200
NLPC-1 NLPC-2 NLPC-3
100
0
0
1000
2000
3000
4000
First 5 cycles Last 5 cycles
0.0 -0.2 -0.4 -0.6 -0.8 -1.0
5000
0
300
Cycle number
D
6 4
1200
1500
4 Before cycles After cycles 3
2 0 -2 -4
2
1 Before cycles After cycles
-6 -8
900
Time (s)
-Z'' (Ohm)
Current density (A g-1)
C
600
-1.0
-0.8
-0.6
-0.4
-0.2
0 0
0.0
1
2
3
Z' (Ohm)
Potential vs Ag/AgCl (V)
Fig. 6. (A) Cycling performance of NLPC samples at 2 A·g-1; (B) GCD curves, (C) CV curves and (D) Nyquist plots of NLPC-2 before and after 5000 cycles.
30
4
B
4
Potential vs Ag/AgCl (V)
2 0 5 mVs-1 10 mVs-1 20 mVs-1 50 mVs-1 100 mVs-1
-2 -4 -6 0.0
0.2
0.4
0.6
0.8
0.3 Ag-1 0.5 Ag-1 1 Ag-1 2 Ag-1 5 Ag-1
1.0 0.8 0.6 0.4 0.2 0.0 0
1.0
100
Potential vs Ag/AgCl (V)
D
this work 10
Ref 59 Ref 61
Ref 57 Ref 28 Ref 32
5
Ref 58 Ref 60
0.1
Specific capacitance (F g-1)
Energy density (Whkg-1)
C
200 300 Time (s)
400 120
100
100
80
80 60 60 40
40
20 0 0
1
Power density (kW kg-1)
20 1000
2000 3000 Cycle number
4000
Coulombic efficiency (%)
Current density (A g-1)
A 6
0 5000
Fig. 7. Capacitive properties of NLPC//NLPC symmetric supercapacitor: (A) CV curves at various scan rates, (B) GCD curves at various current densities, (C) Ragone plots, (D) cycling performance and Coulombic efficiency at the current density of 2 A·g-1.
31
Table 1 Elemental analysis parameters of samples Samples
C (wt%)
N (wt%)
O (wt%)
H (wt%)
Precursor
50.8
14.2
27.3
5.4
PPB
65.7
11.8
17.6
3.2
NLPC-1
72.4
6.5
15.7
2.8
NLPC-2
76.7
5.4
13.5
2.5
NLPC-3
79.7
3.8
11.6
2.3
32
Table 2 Pore structure parameters of NLPC samples a
SBET
b
Smicro
d
c
Vt
c b
e
(nm)
Dm
(m2 g-1)
(m2 g-1)
NLPC-1
1568.4
1317.2
276.6
0.68
0.55
0.13
1.73
NLPC-2
1881.7
1492.8
367.2
0.83
0.64
0.19
1.77
NLPC-3
2079.3
1401.2
663.5
0.98
0.66
0.32
1.89
Sample
Vmicro 3 -1 3 -1 (cm g ) (cm g )
Vmeso (cm3 g-1)
Smeso (m2 g-1)
a
SBET: BET surface area.
b
Smicro: micropore surface area and Vmicro: micropore volume.
c
Smeso: mesopore surface area and Vmeso: mesopore volume.
d
Vt: total pore volume.
e
Dm: average pore size.
33
Table 3 Comparison of energy density and power density of various supercapacitors based on biomass carbon materials Biomass carbon materials
Electrolyte
Energy density (Wh kg-1)
Power density (kW kg-1)
Ref.
Pueraria
6 M KOH
8.5
0.12
[57]
Tobacco rods
6 M KOH
5.5
1.50
[58]
Pomelo peel
6 M KOH
9.4
0.10
[59]
Seaweed
1 M H2SO4
8.0
0.10
[28]
Shrimp shells
6 M KOH
5.2
1.16
[60]
Auricularia
6 M KOH
9.4
0.05
[61]
Gelatin
1 M H2SO4
6.2
0.50
[32]
Porcine bladders
6 M KOH
10.9
0.15
This work
34
Graphical abstract
35