Layer-stacking activated carbon derived from sunflower stalk as

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Layer-stacking activated carbon derived from sunflower stalk as electrode materials for high-performance supercapacitors Xiaodong Wang, Sining Yun, Wen Fang, Chen Zhang, Xu Liang, Zhibin Lei, and Zong-Huai Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01334 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Sustainable Chemistry & Engineering

INTRODUCTION Prominent environmental issues caused by traditional fossil fueled energy production mean that humans need to search the clean, sustainable, renewable and available resources, and also urgently need to research and develop the highly efficient equipments for energy conversion and storage (fuel cells, supercapacitors and lithium ion batteries, etc).1-3 Supercapacitors (SCs), as one of the very powerful and advanced energy storage systems for electrical vehicles, portable devices, stationary energy storage, etc, have been attracting increasing attentions on account of ultrahigh power density, rapid rate of charge-discharge and excellent cycling life.4,5 SCs are mainly composed of fluid, electrode, electrolyte and diaphragm. The electrode, as an important component of SCs, plays a vital role in the electrical energy storage capacity for SCs.6,7 Reported electrode materials for SCs mainly include the carbon of multidimensional structure, multiple metal oxides and conducting polymers.8,9 Carbon materials, for example, activated carbons,10,11 carbon aerogel,12,13 CNTs,14,15 and graphene,16 have been widely applied as electrode materials based on the properties including outstanding chemical stability, excellent conductivity and high specific capacitance, etc. The characteristics of carbon, crystalline structure, good surface chemical activity and wettability make these carbon materials present aforementioned features. Among them, the activated carbons have competitive advantages in SCs industry due to their diverse morphology and structure, large surface area, outstanding electrochemical properties and low manufacture costs.17,18 Over the past decades, interest in using sustainable biomass (including the agro forestry products and wastes, aquatic animals and plants, etc) which is regarded as the promising precursor materials for preparing activated carbons has increasingly grown, and the activated carbons with good electrochemical performances can be obtained from biomass sources, e.g., elm samara,19 silk,20 willow catkin,21 wood,22 hemp,23 cotton,24 etc. More, sustainable biomass-derived carbons based on artificial materials with different shapes, including 2D materials and nanostructures, have been summarized Table S1 (Supporting Information). Differences in preparation processes and biomass sources mean that there are great distinctions in the morphologies and performances of activated carbons.25-27 Currently, the one-step strategy of carbonization is usually used to prepare carbon materials derived from sustainable waste biomass.28,29 However, this strategy has the disadvantages of high decomposition rate, low yields, uneven pore size and irregular structure for acquired carbons, which greatly influences their performances. Therefore, it is highly desirable for activated carbons used as electrodes in SCs to explore new sustainable raw materials and preparation

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technologies, which is to increase the production of biochar and remove most mineral matter, thereby increasing structure aromaticity. Sunflower stalk is one sustainable biomass source containing substantial cellulose, hemicellulose and lignin, three of which are the promising precursor for producing carbonaceous materials. In this work, sunflower stalk was selected as the starting material to prepare hydrochar (HC), pyrolytic carbon (PC) and activated carbon (AC) through a simple, environment-friendly and low-temperature hydrothermal carbonization (HTC) and an effective activation method, as illustrated in Figure 1. The ready-prepared AC was successfully applied as electrode materials in high-performance SCs. A very high specific capacitance value up to 365 F g-1 was measured through a three-electrode SCs system at 1 A g-1 . Importantly, SCs with AC as electrode materials maintained a high capacitance capability of 81% at 20 A g-1 and an outstanding cycling retention rate as high as 95% following 15,000 cycles. What’s more, the symmetrical SCs delivered a large energy density with about 35.7 Wh kg-1 at the power density of 989 W kg-1. To further understand the superior performance of AC in SCs, the synergistic effects of HTC and high-temperature activation methods were elucidated. This research may present a pathway for effectively utilizing biomass resources to prepare highly active carbon materials for energy storage in SCs. EXPERIMENTAL SECTION Fabrication of sample materials. As illustrated in Figure 1. Sunflower stalk was splitted into small pieces, washed with tap water, and dried at 105 °C in a lab stove. The processed sunflower stalk was crushed into powders, as shown in Figure S1 (Supporting Information), and then grinded and screened by the sieve of 8000 meshes to make the average particle size of smaller than 2 µm. Subsequently, 2 g of the very fine powders were soaked into 40 mL deionized water, and then kept under stirring for 10 h at room temperature in air. The mixture was transferred into polytetrafluoroethylene (PTFE) autoclave (volume, 200 mL), and then the autoclave was put into a high-temperature furnace, and rapidly heating up to 230 °C. After holding time for 24 h at 230 °C, the hydrochar (HC) was acquired by filtering, washing with acid, alcohol and deionized water, and followed by drying. Finally, the resulting HC was mixed with KOH in 20 mL of deionized water, maintaining KOH to HC weight ratios of 2, and then the mixture was fully stirred. After evaporating excess water at 105 °C, the activation treatment was performed at 800 °C for 2 h in a furnace filled with the nitrogen atmosphere. The carbonized product was cleaned with acid and alcohol, and then rinsed with deionized water in order to

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make the pH of filtrate reached 7, and subsequently fully dried to acquire the activated carbon (AC). For comparison, we also pyrolyzed directly HC to prepare the pyrolytic carbon (PC) at 800 °C, without using any activation agents. Characterization The morphologies, structures, compositions of three samples were characterized using X-ray diffraction (XRD, D/Max 2200, Japan, with Cu Kα radiation), field-emission scanning electron microscopy (FESEM, JSM-7001F, Japan), transmission electron microscopy (TEM, JEM-3010, Japan), Raman spectroscopy (Renishaw Raman microscope 2000, with an excitation at 532 nm) and X-ray photoelectron spectroscopy (XPS, ThermoFisher K-Alpha, America), respectively. The pore size distribution was analyzed using Nitrogen adsorptiondesorption measurements (TriStar II 3020, at 77 K) after vacuum drying at 200 °C for 8 h. The specific surface area was measured using the Brunauer-Emmett-Teller (BET) method, and the isotherms were further obtained using the Barrett-Joyner-Halenda (BJH) method to derive the pore size distributions. Fabrication of working electrode The preparation of working electrode material was through mixing the sample material (90 wt%) with conductive agent (acetylene black, 5 wt%) and adhesive (PTFE, 5 wt%) in ultrapure water, and then the excess water was evaporated from the solvent. The acquired mixture was painted between two pieces of nickel foam (~1.2 cm2) with spatula and then pressed under a pressure of about 8 MPa for 10 minutes, which is similar to the description in literature.30 After drying at 80 °C for 24 h, the assembled working electrode was prepared, which containes about 2.0~3.0 mg of sample material, and its effective working area is about 0.3 cm2. Electrochemical measurement The three-electrode (Hg/HgO was applied as reference electrode) and symmetric system were applied to measure the electrochemical performances of materials using the test projects of galvanostatic charge-discharge (GCD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), on a test equipment (CHI660E, ChenHua, Shanghai) in 6 M (mol L-1) KOH electrolyte. The specific capacitance (C, F g-1) of SCs in three-electrode system was calculated on the basis of equation 1:13

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C

I t mV

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(1)

In symmetric system, the electrochemical measurements can be performed by equation 2, 3 and 4:17,18 C E

4 I t mV

(2)

1

2  3.6

P  3600

C V 2

(3)

E

(4)

t

for all of the above equations, where I, m, Δt, ΔV, E and P are the discharge current (A), effective weight of material (g), discharge time (s), effective voltage excluding voltage drop (IR drop) in the discharge process (V), energy density (Wh kg−1) and power density (W kg−1 ), respectively. RESULTS AND DISCUSSION Morphology and structure characterization The morphologies of three samples were shown in Figure 2. After HTC treatment, the sunflower stalk powder was transformed into HC, which is composed of the spherical or ellipsoidal particle agglomerates with diameters of several microns (Figure 2a-b). The morphologic and structural formation of HC essentially follows the path of hydrolysis, dehydration, decarboxylation and aromatization processes, which is very similar to the hydrothermal transformation of cellulose, hemicellulose or lignin.31,32 The PC sample shows a similar morphology to coral reefs (Figure 2c). The cause of this result is that the treatment of high-temperature pyrolysis makes the PC further dehydrate and aggregate. Surprisingly, it can be found from the FESEM and TEM images of AC in Figure 2e-i that AC has the layerstacking structure (Figure 2e, 2f and 2i) and mesoporous structure (Figure 2g-h). The mesoporous structure was further confirmed by subsequent Nitrogen adsorption-desorption. A possible reason for the structural transformation and formation is that a chemical reaction happens between KOH and carbon atoms: 6KOH + C  2 K + 3H2 + 2K2 CO3.33 The etching of carbon atoms with KOH is usually accompanied with structure collapse and transformation.34 The mesoporous layer-stacking structure with irregular networks for AC sample is conductive to the adsorption and storage of charged ions, which is highly desired as electrode materials in SCs.

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To further analyze the pore structures of samples, the Nitrogen adsorption-desorption test was measured. As shown in Figure 3, Nitrogen adsorption-desorption isotherms of the three materials are similar, indicating representative Type IV isotherms with H2 hysteresis loops, which are typical of mesoporous structures as classified in the IUPAC.35 At higher relative pressure, compared with HC and PC, the AC sample shows a bigger hysteresis loop, suggesting that there is the larger mesoporous pore diameter.36 From mesopore diameter distribution plots decided by the analysis of adsorption branches of isotherms using BJH method, it can be observed that the distributions peaks were centered at 2.2 and 3.6 nm for PC, and 4.2 nm for AC. Moreover, the mesopore structural parameters of HC, PC and AC samples were summarized in Table 1. The specific surface areas of the three samples were determined to be 720, 1044 and 1505 m2 g-1, respectively, using BET method. Especially, for HC sample, the surface area of about 720 m2 g-1 is much larger than that of literatures reported.22,37 The larger BET surface area depends on these process parameters such as biomass species, carbonization temperature, carbonization time, the proportion of biomass and solvent, and so on. This means that process parameters need to be modified reasonably for achieving the larger BET surface area. In the present work, we obtained a quite high surface area of about 720 m2 g-1, confirmed by the Nitrogen adsorption-desorption tests, which can be contributed to biomass species and the process employed, i.e., the processed sunflower stalk was crushed and grinded into very fine powders, and then screened and soaked, as we employed in the experiment, before the hydrothermal carbonization. By contrast, the reported hydrochar samples with a very low surface area have not given such a special treatment before the hydrothermal carbonization.38,39 This is the main reason that HC possess a quite high surface area of about 720 m2 g-1 in our work. In addition, comparing with HC and PC, the synergistic actions of HTC and pyrolysis activation with KOH agent make AC have the largest specific surface area (1505 m2 g-1 ), total pore volume (Vt, 0.94 cm3 g-1) and average pore size (Da, 3.6 nm). In a word, these distinct structure characteristics may endow the AC with considerable potentials for high-performance SCs. These samples were then identified using XRD, as presented in Figure 4a. The XRD patterns for three samples showed two diffraction peaks centered at about 22° and 43°, corresponding to the (002) and (100) diffractions of carbon, respectively, indicating the presence of turbostratic or disordered carbon.40,41 Usually, the biomass should contain lignin, cellulose and hemicellulose, and these kinds of polysaccarides are not completely decomposed at low-temperature hydrothermal carbonization. It means that the XRD profiles should show some crystalline form of lignin/hemicellulose/cellulose. However, the XRD

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profile of HC shows no crystalline form of cellulose/lignin/hemicellulose in the present work, similar results have been observed in previous reports.31,39,42,43 It may be due to a degradation of lignin/hemicellulose/cellulose for a long-term soaking before hydrothermal carbonization. In addition, as compared with HC and PC, the peak intensity of AC is weaker, indicating that there are lower degree of graphitization and higher density of pores in AC sample. As reported in previous literatures,21,23 the main reason for this difference is that KOH activation and high-temperature pyrolysis under nitrogen atmosphere increase the defects and impurities of AC material, which was further confirmed by Raman spectra and XPS. Furthermore, from the Raman spectra survey (Figure 4b), one peak collected at 1345 cm-1 (D-band) is connected with the amorphous carbon that is caused by defects and impurities, and the other at around 1590 cm-1 (G-band) is associated with the hybridized graphite carbon.24 Note that the intensity ratio (IG /ID) of AC (0.97) is lower than that of HC (1.26) and PC (1.01), as summarized in Table 1, suggesting that there are the very lower degree of graphitization and the larger proportion of defects in AC. The carbon structure formation essentially follows the path of hydrolysis, dehydration, decarboxylation and aromatization processes in the hydrothermal transformation of cellulose, hemicellulose or lignin, and the resulting biomass carbon is essentially a kind of amorphous carbon.31,43 In this work, AC was processed by using the KOH activation under high-temperature condition. There are a large number of defects and impurities in AC, which further damages its structure. A chemical reaction between KOH and carbon atoms is usually accompanied with structure collapse and transformation.34 Therefore, the AC does not show graphitization, which is consistent with the XRD analysis, and has also been verified in previous literature.30 However, a large number of defects promote the more diffusion and adsorption of electrolyte ions inside the materials, which is benefits to improve the electrochemical performances of carbon products.44,45 The compositions of samples were analyzed by XPS (Figure 4c), elements of C, O and N were found in the studied materials (Table 1). However, in the previously reported literature, small amount of transition metal species and other species (such as P, S and K) were detected in activated carbons derived from bio-sources due to the residues from the use of different chemical activated agents and the incomplete washing with solutions.19,22 Compared with HC samples, the element of O is decreased dramatically due to the high-temperature carbonization, so as to increase the content of C in PC and AC samples. In addition, compared to PC, the increase of N and O elements indicates that more defects are present in AC sample, which can be due to the effective activation with KOH in a nitrogen atmosphere. For AC material, the doping of O and N heteroatoms can increased the rate and density of pores and defects, and

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the specific surface area, which could provide more channels for the transmission of charge ions in AC electrode material. Additionally, as presented in Figure 4d, the deconvoluted peaks of C1s spectra were at 284.5, 285.1, 286.6 and 288.4 eV, assigned to C=C (sp2), C-C (sp), CO/C-N and C=O bond, respectively.46,47 More details on the O1s and N1s spectra of HC, PC and AC were shown in Figure S2 (Supporting Information). The relative intensity of C=O and C-O/C-N peaks increases in the order HC < PC < AC, which corresponds to the degree of hybridized carbon and is extremely similar to the results of XRD patterns and Raman spectra. Compared with HC and PC, AC has more functional groups (C=O and C-O/C-N) which can improve the wettability of AC material and the absorption and storage of charge ions at the interface between AC electrode and electrolyte. Moreover, after treatment by KOH, the surface chemistry stability of AC sample can be also increased, and which is beneficial to reduce the loss of irreversible capacitance in SCs with KOH as electrolyte.48 In a word, these positive effects of surface chemistry on AC are beneficial to increase the electrochemical properties of SCs, as demonstrated in the literatures.49 Electrochemical performances in three-electrode system The electrochemical behaviors of acquired samples were firstly examined using a threeelectrode system. For the three kinds of electrode materials, CV curves collected at a scan rate of 20 mV s-1 were presented in Figure 5a. Apparently, compared with HC, the CV profiles of PC and AC show the approximately perfect rectangular shape with the steep current change, indicating that the electrochemical performances of PC and AC have been largely improved. Furthermore, the AC sample has a larger rectangular profile, suggesting that it has a higher capacitance than PC. More details on the comparation of capacitance values obtained from CV cures for HC, PC and AC were shown in Figure S3a (Supporting Information). Figure 5c exhibited the gravimetric capacitance of three materials under the condition of various current densities. Capacitance values gathered at 1 A g-1 of 169, 200 and 365 F g-1 were calculated for HC, PC and AC samples, respectively, which are consistent with the achieved results from GCD profiles (Figure 5b). Moreover, upon increasing the current density to 20 A g -1, the capacitance retention rates of 71, 79 and 81% were still retained for HC, PC and AC, respectively (Figure 5c). For AC sample, after treatment with KOH, the stability of surface chemistry can be also increased, and which is beneficial to reduce the loss of irreversible capacitance. It is well worth emphasizing that the gravimetric capacitance of 365 F g-1 is much higher than that of most of the previously researched biomass-derived carbons (Table S1, Supporting Information). Meanwhile, it’s also worth noting that the relationship between

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the capacitive performance and surface area is nonlinear, to a degree, which is because that not all pores and surface areas of materials are effective in charge transmission and storage.50 In the Figure 5d, we compared the Nyquist plots of HC, PC and AC. As is known to all, in low frequency range, the straight line should be 45ºand the phase angle should be 90ºfor a capacitive structure.51,52 The relationship between impedance phase angle and frequency was presented in Figure S4 (Supporting Information). All of the electrode materials exhibit excellent capacitive behaviors, and a smaller semicircle is formed in AC Nyquist plot at low frequency region, which suggests that AC has the smaller charge-transfer resistance (Rct) and series resistance (Rs) than another two samples (see fitted results in Table 1 and Figure 5d). In general, the superior electrochemical performances of AC in SCs can be attributed to that it has the mesoporous layer-stacking structure with irregular networks and large specific surface area, which could provide more channels for the transmission of charge ions. As compared with HC and PC, the superior performance of AC demonstrates its great potential as electrode material for SCs application. To comprehensively investigate the AC by activation with KOH agent, we further tested its electrochemical performance, as shown in Figure 6. GCD curves exhibit the perfectly symmetrical charge-discharge profiles, demonstrating that it has good electrochemical reversibility and coulomb efficiency. Meanwhile, the regular CV shapes are similar to rectangle (Figure 6b), even at the various scan rates, indicating that it has an outstanding rate capability. More, the cyclic stability was examined at 20 A g-1 (Figure 6c), and the capacitance retention was approximately maintained at 95% after 15,000 cycles and the voltage curve exhibited a small IR drop of 0.15 V, suggesting its very excellent long-term cycling stability. The capacitance retention rates, after 10,000 and 5,000 charge-discharge cycles, were presented in Figure S5 (Supporting Information). Furthermore, Figure 6d presents the almost identical CV curves before and after 15,000 cycles at 100 mV s -1, and a specific capacitance retention as high as 94% was obtained, as shown in Figure S3b (Supporting Information), which further confirms that the AC material has the outstanding electrochemical cycling stability in SCs applications. Electrochemical properties in symmetrical system The different capacitive behaviors of AC sample electrode were also investigated using a self-assemble symmetrical system. As presented in Figure 7a, the CV curves still lookes like rectangular at various scan rates, which is similar to that measured in three-electrode SCs. A capacitance value calculated from GCD curve (Figure 7b) was close to 263 F g-1 at 0.5 A g-1.

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Especially, a good capacitance retention rate and an outstanding stability value maintained after 15,000 cycles could be achieved up to 72% and 93% (Figure 7c), respectively. Also, more detailed information about the capacitance values and retention obtained from CV curve of AC was shown in Figure S3c-d (Supporting Information). In general, these CV results are very close to the above results calculated from GCD curves. The Nyquist plot was shown in Figure 7d, which has similarity with the results in Figure 5d, demonstrating the suitable conductivity of AC electrode material in KOH electrolyte. These results further confirm that the AC electrode material has very nice electrochemical features in not only three-electrode system but symmetrical SCs system. Both the energy density and the power density, which are the most important performance measures for SCs, were gained in the symmetrical system. Figure 7e displays the Ragone plot of SCs with AC, and the energy density at about 35.7 Wh kg-1 could be achieved under the condition of a power density up to 989 W kg-1. At the same time, even if the power density was enhanced into 31 kW kg-1, an energy density was still maintained at 15.6 Wh kg-1. These numerical values are much higher than that of previously researched cativated carbons derived from other biomass resource in the symmetric SCs (see also Table S1 for more details in Supporting Information). The "button type symmetric SCs device" was assembled in order to demonstrate the potential applications of our prepared AC electrode, as shown in Figure 7f, and the fabrication details and schematic representation of "button type symmetric SCs device" were shown in Figure S6 (Supporting Information). Two "button type symmetric SCs devices" in serial circuit could make a red light-emitting indicator light up (1.8-2.0 V) for about 200 s after being charged to 2 V. Furthermore, three "button type symmetric SCs devices" in series could drive a series of indicators in parallel (2.8-3.0 V) for about 130 s and could still supply enough energy for a fan to run for about 40 s after being charged to 3 V (Figure 7g-i and see also video 1, 2 and 3 in Supporting Information). These results indicate that the as-prepared AC with mesoporous layer-stacking structure, using a simple HTC process combining an effective activation method, is a promising candidate for practical SCs applications. Further, we also evaluated the application potential of this AC material as counter electrode (CE) catalyst in iodine-mediated dye-sensitised solar cells (DSSCs) using N719 dye. The CV, EIS, Tafel and J-V curves were shown in Figure S7 (Supporting Information). From J-V curves, a higher power conversion efficiency (PCE) of 6.56% and fill factor (FF) of 0.64 were achieved, as summarized in Table S2 (Supporting Information). The optimization of photovoltaic performance of new-system DSSCs (copper-based electrolyte and D35/Y123 dye)

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with this bio-based carbon material is ongoing. For more details on fabrication and characterization of DSSCs, please consult our previous pubulications.53-60 CONCLUSION In summary, the AC derived from sustainable sunflower stalk was successfully synthesized through the HTC process combining an effective activation method. A large specific surface area (1505 m2 g-1) and a special mesoporous layer-stacking random-network structure were achieved. As electrode material for SCs, the AC demonstrated high electrochemical storage capacity, with the excellent capacitance of 365 and 259 F g-1 measured by three-electrode and symmetrical SCs, respectively. Further, an outstanding capacitance retention rate and a good stability value maintained after 15,000 cycles were close to 81% and 95%, respectively, which were acquired at 20 A g-1 in three-electrode system. Importantly, an energy density as high as 35.7 Wh kg-1 was obtained using the symmetrical system, even at the power density of 989 W kg-1 . The electrochemical properties of prepared AC electrode material in SCs are higher than that reported in previous studies. Waste sunflower stalk biomass can be used as a starting material to manufacture the activated carbons, considering the abundance, high electrochemical storage capacity and recyclability of the resultant AC. This work may provide a strategy for comprehensively and effectively utilizing sustainable waste biomass resources to reduce environmental pollution and prepare highly active carbons as electrode materials for the high-performance energy storage and conversion devices. ASSOCIATED CONTENT Supporting Information. FESEM image of sunflower stalk powders, plots of O1s and N1s spectra, Bode, capacitance retentions and values, table of reported ACs derived from different bio-sources materials for SCs, schematic representation of fabricated symmetrical SCs, curves of photocurrent density-photovoltage (J-V), CV, Nyquist and Tafel, parameter table of DSSCs using AC as counter electrode (CE), and the Video 1, Video 2 and Video 3 that related to this article. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] ORCID iD

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Sining Yun: 0000-0003-3197-5559 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Financial support from NSFC (51672208), National Science & Technology Pillar Program during the Twelfth Five-year Plan Period (2012BAD47B02), Sci-Tech R&D Program of Shaanxi Province (2010K01-120 and 2015JM5183), and Shaanxi Provincial Department of Education (2013JK0927) are greatly acknowledged. The project was partly sponsored by SRF for ROCS, SEM. REFERENCES 1.

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Agriculture and forestry processing residues Crop straw Household waste Animal dung Fuel wood

Biology resources

Pyrolysis Picking

Crushing

HTC

Washing

Screening

230 °C 24h

Sunflower

Sunflower stalk

Biomass

Advantages

Powder

Activation

Abundant

FESEM Pyrolytic carbon 800 °C

PC

FESEM Hydrochar HC

KOH

FESEM

Sustainable Environment friendly

Activated carbon

AC

Figure 1. Schematic illustration of preparing carbon materials derived from sunflower stalk (FESEM scale bar: 1m).

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(b)

(a)

(c)

1 μm

5 μm

Page 18 of 26

1 μm

Cellulose

Sunflower

HTC

KOH

Hemicellulose

Stalk

Lignin

(d)

(e)

10 μm

1 μm

(g)

(f)

200 nm

(i)

(h)

Figure 2. FESEM images of (a-b) HC, (c) PC and (d-f) AC samples, and (g-h) TEM images of AC.

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Figure 3. Nitrogen adsorption-desorption isotherms of (a) HC, (b) PC and (c) AC samples (inset: showing the pore diameter distributions, respectively).

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Figure 4. (a) XRD patterns, (b) Raman spectra, (c) XPS survey and (d) C1s spectra of the HC, PC and AC samples.

Table 1. Structural parameters, compositions, and capacitive performances of the samples. Samples HC PC AC

Elemental composition [at.%] C O N 13.19 85.82 0.99 89.69 9.14 1.17 88.98 9.82 1.20

IG /ID

SBET [m2 g-1]

Vta) [cm3 g-1]

Dab) [nm]

Rsc) []

Rctd) []

Ce) [F g-1]

1.26 1.01 0.97

720 1044 1505

0.39 0.58 0.94

2.7 2.6 3.6

0.73 0.73 0.72

0.17 0.42 0.15

169 200 365

a)

Vt: Total pore volume measured at relative pressure of 0.99; Da: Average pore diameter analyzed using the BJH method; c) Rs: Series resistance; d) Rct: Charge-transfer resistance; e) C: Specific capacitance measured at current density of 1 A g-1. b)

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Figure 5. Electrochemical properties of the HC, PC and AC samples investigated using the three-electrode system in 6 M KOH electrolyte: (a) CV profiles at a scan rate of 20 mV s-1; (b) GCD curves at the current density of 1 A g-1; (c) Gravimetric capacitances at various current densities from 1 to 20 A g-1 ; (d) Nyquist plots (inset: showing the Nyquist plots in the high frequency region).

22



Figure 6. Electrochemical behaviors of the AC sample examined using the three-electrode system in 6 M KOH electrolyte: (a) GCD curves at different scan rates; (b) CV profiles at various current densities; (c) Cycling stability at 20 A g-1 (inset: showing the charge-discharge curves with before and after 15,000 cycles); (d) CV curves of before and after 15,000 cycles at 100 mV s-1.

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(f)

(g)

(h)

(i)

Figure 7. Electrochemical properties of the AC electrode investigated using the symmetric system in 6 M KOH electrolyte: (a) CV profiles at different scan rates; (b) GCD curves at various current densities; (c) Gravimetric capacitances with various current densities from 0.5 to 20 A g-1 and cycling stability measured at 20 A g-1 ; (d) Nyquist plot (inset: showing the Nyquist plot expanded in high frequency region); (e) Ragone plot of AC and other previously

24



reported activated carbons. Digital photos of (f) the button-type symmetric SCs devices and (g-i) the various electronic devices powered by the SCs devices (see also video 1, 2, and 3 in the Supporting Information).

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Graphical abstract: Layer-stacking activated carbon derived from sunflower stalk as electrode material in supercapacitors (SCs) shows outstanding electrochemical performances.

TOC figure


Layer-stacking activated carbon derived from sunflower stalk as

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