Synthesis of porous carbon material with suitable graphitization

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Synthesis of porous carbon material with suitable graphitization strength for high electrochemical capacitors Linlin Xing, Xun Chen, Zhixiang Tan, Manzhou Chi, Wenting Xie, Jianyu Huang, Yeru Liang, Mingtao Zheng, Hang Hu, Hanwu Dong, Yingliang Liu, and Yong Xiao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05529 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019

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ACS Sustainable Chemistry & Engineering

Synthesis of porous carbon material with suitable graphitization strength for high electrochemical capacitors Linlin Xing†, Xun Chen†, Zhixiang Tan†, Manzhou Chi†, Wenting Xie†, JianYu Huang†, Yeru Liang†, Mingtao Zheng†, Hang Hu†, Hanwu Dong†, Yingliang Liu*†, and Yong Xiao*† †College

of Materials and Energy, South China Agricultural University, Guangzhou

510642, P. R. China

*Corresponding authors E-mail address: [email protected] (Y.X), [email protected] (Y.L.), Tel. and Fax: +86 020 85280319.

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Abstract: It’s a long-lasting challenge to design and synthesize electric double layered electrode materials with suitable graphitization strength, and simultaneously working to achieve a high capacitance for electrochemical capacitors. In our work, starch based porous carbons (SPCs) are obtained via carbonization and mature KOH chemical activation. The prepared materials have a superior specific capacitance of 397, 372, and 337 F g-1 correspond to SPC-5, SPC-15, and SPC-25 (SPC-x, x-represent the starch particle size), as well as a distinguished cycling stability of 97 % capacity retention over 20000 cycles in KOH electrolyte. In addition, in 6 M KOH electrolyte, SPC-5 exhibits a superior energy density of 22.59 W h kg−1 when the power density is 148 W kg-1. This is mainly due to its suitable graphitization strength and particle size. Using of biomass with different amylopectin content to control the degree of graphitization of carbon materials. The strategy demonstrates the way to fabricate a reasonable and graphitized carbons derived from biomass with improved capacitance, which is critical for a broad range of devices for the field of energy storage. Keywords: Supercapacitor, electrode materials, amylopectin, graphitization

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INTRODUCTION Supercapacitors acts as an important part in the field of energy depend on the reason of their rapid charge-discharge capability, high power density, and long lifespan.1-4 People has been seeking for a way to further enhance the capacity of energy storage without sacrificing the excellent rate performance. In the supercapcitor development field, this has been a long-term goal. The performance of a supercapacitor is mainly dominated by its electrode material. As yet, a collection of advanced materials,5 inclusive of substances such as carbon nanofibers, activated carbons (ACs), carbon nanotubes, graphene, conductive polymers, metal oxides, and their complexes have been systematically researched for electrode materials.6-11 Among them, ACs are rapidly increasing interest as supercapacitor electrode due to moderate costs, easily obtained high specific surface area,12 stable physicochemical properties,13 good pore size distribution, and conductivity.14 In addition, these electrode materials are equipped with comparable economic viability yet better capacitance performance, as well as high specific surface areas and superior porosity. Under these circumstances, electrode materials have been deliberated to be a simple strategy when it comes to increasing the electrical double-layer capacitance.15 However, the advantages mentioned are not at all enough without a thorough implementation of the SSA, which needs an applicable framework to enable.16 Further, given the fact that ACs are often microporous carbons, poor capability rate has been the result of these narrow micropores, which builds up to the porosity in these carbons. In order to design high-performance electrode materials,

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which can be applied for energy storage devices, activated carbon materials need to be further customized and optimized. Starch, being a natural, renewable, and biodegradable polymer, is produced by a variety of crops, and shockingly demonstrates a content of over 49 % of oxygen. Due to the high composition of oxygen in starch, a big number of hydrophilic groups will emerge on the exterior surface of starch-derived porous. Moreover, this type of porous carbon is cheap and environmental friendly, allowing high application prospects when developing carbon electrode materials from eco-friendly starch. Wu et al. reported that through the method of simultaneous template, starch-derived mesoporous carbons (SMCs) were obtained, giving the result that the specific capacitance of SMC was 144 F g-1 at a current density of 0.05 A g-1 in the 6 M KOH electrolyte.17 Cao et al. used the carbon source biomass starch and MgO as the template for providing pore, showing a specific capacitance of 229 F g-1 at 1 A g-1 under a 6 M KOH electrolyte.18 Chang’s group reported the development of N-doped graphitized carbon nanosheets (NGCNS) originated from starch, the obtained NGCNS indicates that an excellent specific capacitance was 337 F g-1 at 0.5 A g-1 under a 6 M KOH electrolyte.19 Even though all three participants used starch as a carbon source, the electrochemical performances of the derived porous carbon material had better capacitance due to the presence of Fe2O3 as a graphitizing catalyst for producing high graphitization. But their practical application is limited by the template-processes time and energy consuming. Thus, it is highly desirable for people in this field to discover an easier yet economical method

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when producing porous carbon that possesses exceptional electrical conductivity and suitable graphitization. In this paper, we report an approach to attain this goal. This approach mainly involves that porous carbon with different degrees of graphitization was regulated by the content of amylopectin in starch. Through the blue value method, the content of amylose was measured.20 This method provided a further computation of the amylopectin content in taro, sweet potato and potato, which were 84%, 80%, 74%, respectively. Under our current knowledge, the effect of different the content of amylopectin on its graphitization degrees has not yet been investigated. As shown in Figure 1, three main raw materials that the starch provided by taro (particle size ~5 µm),21 sweet potato (~15 µm),22 and potato (~25 µm)23 as the carbon source and graphitize carbon framework on this work, then the most mature KOH chemical activation method in the current activation technology to be chosen to prepare the activated carbon. The highest amylopectin content of taro starch-derived carbon has a highest degree of graphitization, resulting in best electrochemical performance. As mentioned, the purpose of this work is revealed the influence of starch composition structure on the electrochemistry properties of carbon materials. This simple and effective synthesis process is highly likely to generate an easily attainable solution for large-scale production. In near future, this especially points to advanced productions involving a high performance of carbon-based electrodes when supercapacitors are using abundant resources of starch biomass. EXPERIMENTAL SECTION 5



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Synthesis of materials. Preparation of starch pore carbons derived from three biomass which bought from a farm market in Guangdong, China. Hydrochloric acid (36% HCl), potassium hydroxide (KOH), and ethanol (99.8% CH3CH2OH) were used by Shanghai Chemical Reagents. All were analytical reagents grade chemicals that could be served as without more refinement. As obtained three kinds of biomass (taro, sweet potato, potato) were peeled and cut into pieces, then dried them in 105 oC for 48 h and ground into powder. Subsequently, the obtained three kinds of fine powder are transferred to rectangular ceramic boat, heated at 400 oC through a tubular furnace and at a heat rate of 2 oC min-1 under nitrogen for 2 h to obtain precursors. These precursors can be named ta-400, st-400, po-400, respectively. Then the precursors powder is mixed with KOH by the weight ratio 1:3 while activated for 2 h under 700 oC under nitrogen at a heat rate of 5 oC min-1. In order to remove impurities, the above black powder was first washed by 2.0 M HCl, then rinsed again with deionized water for it to be neutral. Finally, the powder was dried at 105 oC overnight, and the final samples were called starch based porous carbons and denoted as SPC-x, where x represent the starch particle size. Material characterization. X-ray diffraction (XRD) patterns was collected using a power diffractometer (Bruker D8, Cu Kα, λ=0.154 nm, 2θ from 5o to 80o). Raman data were gained by Raman spectrometer (Jobin-Yvon, HR800) with spectrophotometer (λ= 457.9 nm). The nitrogen adsorption/desorption data was determined by Micromeritics 3 flex physisorption analyzer, and samples were used the degassing processing at 350 °C for 8 h before use. X-ray photoelectron spectroscopy (XPS, VG ESCALAB Mark 6


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II) was performed for the analysis of composition. The microstructure and surface morphology of the coated samples were observed by field emission electron microscope (FE-SEM, ZEISS Ultra 55) and optical microscope (WSM500D, Micro optical instrument Co. Ltd.). The electrical conductivity of the samples was recorded by a standard four-probe (RTS-9). Static and dynamic contact angle values of water were measured by a contact angle goniometer (JC2000C1). Infrared spectrum was conducted on FTIR spectroscopy instruments (Bruker ISS-88) for identifying functional groups over samples surface. The amylose content was measured by the blue value method. The K+ content was estimated by inductive coupled plasma emission spectrometer (ICP, ICP-6300). Electrochemical

measurement.

Each

carbon

sample,

mixed

with

polytetrafluoroethylene (PTFE) and carbon black, was in an 8:1:1 weight ratio in ethanol. Working electrodes were then prepared by applying the slurry to a current collector of foam nickel (1×1cm2) and by vacuum drying at 100 oC overnight. The mass loading of the electrode materials was 4.0 mg cm-2. For the individual electrode, the electrochemical tests were performed in a three-electrode system in which a platinum foil was used as the counter electrodes, Hg / HgO and a carbon electrode electrode was used as the reference electrodes and working electrode respectively. Galvanostatic charge / discharge (GCD) carried out in the range of potentials between −1.0 and 0 V, as well as cyclic voltammetry (CV) measurements, were experimented by using the CHI660E electrochemical station (Shanghai Chenhua Co. Ltd, China). Electrochemical impedance spectroscopy (EIS) was tested in the frequency range of 0.01 ~ 100 kHz 7



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with an amplitude of 5 mV on Im6ex electrochemical workstation (Zahnex Co., Germany). The specific capacitance of electrode calculated from GCD curves by Equation (1): C

I  t m  V

(1)

where C (F g-1), m (g) and I (A) present the specific capacitance of the active materials, the total mass of active materials within the working electrode, and the constant current, respectively; ΔV (V) is the voltage window during the discharge with IR drop corrected, as well, Δt (s) presents the discharge time. To evaluate the SC performance of the SPCs for practical applications, symmetric supercapacitor (SSC), assembled in a configuration with a two-electrode cell, was measured and measured in 6 M KOH electrolytes. To prepare the electrodes, the active material slurry, PTFE binder and acetylene black were pressed and pasted on the nickel foam at a mass ratio of 8:1:1 in aqueous electrolyte cells. Generally, the electrodes were processed through vacuum drying at the temperature of 100 oC overnight. The cellulose membrane was adopted as separator (WM18202, Porosity: 1.6 mm) in aqueous electrolyte cell test systems. Two electrodes with an active material loading of 4 mg per working electrode were made into a symmetric supercapacitor system (called CR2032type coin cell). Through the battery program control system NEWARE (BTS 7.5.x), the electrochemical measurements were carried out well. The GCD and CV test were tested between 0.01 and 1.2 V in aqueous electrolyte system. To get the EIS curves, the experiment was performed at an open circuit voltage by 5 mV in the frequency range of 0.01 Hz to 100 kHz. The specific capacitance Cs was calculated by Eq. (2): 8


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Cs 

2  I  t m  V

(2)

The total capacitance of the cell (Ccell) was obtained by the Eq. (3): Ccell 

Cs 4

(3)

where Cs (F g-1), Ccell (F g-1) present the capacitance of single electrode of the active materials, the capacitance of the whole cell, respectively; m (g) is the active material mass on a single carbon electrode, and I (A) is the constant current. As well as the specific energy density (Ecell, W h kg-1) and the specific power density (Pcell, W kg-1) of symmetric supercapacitors were measured by Eqs. (4) and (5):24-25

Ecell 

Ccell  V 2 2  3.6

(4)

Pcell 

Ecell  3600 t

(5)

where ΔV (V) and Δt (s) are the same as above. RESULTS AND DISCUSSION Microstructures and morphology of as-prepared samples. The powder collected from the three biomass under the same condition were mainly composed of starch, so the three biomass called starch-based biomass. Starch arises in the form of semi-crystalline granules comprising of linear amylose and highly branched amylopectin, the particle size was strongly correlated with the amylopectin content; the larger the starch granule, the lower was the amylopectin content.20 The crystalline structure of starch granules is mainly associated with the amylopectin composition; the higher was the amylopectin content, the stronger degree of crystallization.26 Amylopectin consists of glucosyl units joined by α-1,4 bonds of glycoside and branched 9



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at a position of α-1,6.27 In generally, with the amylopectin content increasing, the size of the starch granules becomes smaller which means that crystallinity will be improved. Therefore, biomass with high amylopectin content were converted into carbon material with excellent electrochemical capacity and suitable graphitization strength through the carbonization and activation processes. The graphic diagram of the typical preparation process for the SPCs is shown in Figure 1. The three powders with different amylopectin contents are carbonized under nitrogen at the temperature of 400 oC with a heating rate of 2 oC min-1, followed by the chemical activation of the as-obtained precursor with KOH, heated with a rate of 5 oC min-1 under Nitrogen. In the experimental section, details of preparation course can be found.

Figure 1. Scheme of the process to synthesize starch-based porous carbon material (SPCs).

The XRD pattern of the three kinds of raw biomass are shown in Figure S1a. For starches,28 a strong doublet peak at 2θ = 17o and 18o is categorized as A-type allomorphs, 10


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such as taro starch sample. A prominent peak at around 20o is considered as B-type allomorphs, such as potato and sweet potato starch samples. The results match prior observations of the XRD of starches.29 It is notable that the crystallization peak of starch is 24o, which is represent the peak of diffraction of the (002) planes of graphite and reveals the degree of crystallization of starch.30 In addition, through the comparison between the XRD of the raw materials, it was found that the crystallization degree of taro was the strongest among all materials tested. That is, the highest amylopectin content of taro which shows the best crystallinity. As displayed in Figure 2a, an array of sharp peaks of at 28o, 41o, 50o, 58o, 66o, and 73o are illustrated on the pattern of XRD for the three precursors, suggesting the existence of face-centered cubic lattice of KCl (JPDS: No.05-0628).31 The content of KCl is affected by different starch structure during the dehydration and carbonization. The amylose promotes the growth of KCl crystals, while amylopectin inhibits it. Therefore, the largest content of amylopectin in taro starch results in the lowest content of the KCl in the ta-400. The ICP dates were listed in Table S1. It is cleared that there is different amount of KCl in the raw material and precursors, which consistent with the XRD results. Additionally, a wide diffraction peak positioned around 25° was also observed, reflecting that the diffraction of (002) graphite lattice plane indicates to answer that starch was converted into carbon in the carbonization process. After activation with KOH, all the diffraction peak of KCl disappears, only the two characteristic peaks around at 24.7o and 42.9o are observed in Figure 2b. The peak focused at 24.7o correlates to the (002) diffraction of graphite, revealing the great crystalline degree in the order structure. Likewise, the crest at 42.9o 11



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coincides to the (100) plane, intending that a high stack of inter layers is participating in the act of enhancing the electronic conductivity of the samples.32 The empirical parameter (R) was used by Wang et al. , to be defined as the ratio of height from the (002) Bragg peak to the background, and the values of R for each sample are also shown in Figure 2b.33 If the assumption R = 1 is correct, all layers of graphene will be distributed inconstantly as single layers, in a result that the value of R could be proportional to the amount of the carbon surface about edge orientation.34 As to the SPCs, the R values decreased with the increase of starch particle size, such as SPC-5, SPC-15, and SPC-25 are 1.08, 1.05, and 1, respectively, pinpointing a bigger and larger fraction of single carbon sheets in the structure of materials that are as-synthesized. Generally, during the KOH activation, the intercalation of the potassium compounds, destruction of aligned structural dominates in the carbon matrix. Therefore, the smaller the starch particle size, the higher the amylopectin content, resulting in higher starch crystallinity, which is beneficial to improve the graphitization degree of the derivatized carbon material.

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Figure 2. (a) XRD patterns of precursors. (b) XRD patterns, (c) N2 adsorption / desorption isotherms, (d) PSD curves, (e) Raman spectra, and (f) XPS spectra of SPC5, SPC-15, and SPC-25, respectively.

Figure 2c illustrates the curve of N2 adsorption and desorption of the SPCs derived from the three kinds of biomass. Type-I isotherms were obvious observed with a lower pressure at (P/Po) of 0∼0.2, indicating the existence of abundant micropores. Moreover, as the particle size increases from 5 to 25, a widening of the knee of the isotherms takes place, which indicates an expansion of the micropore size as particle size increases.35 The porosity distribution (PSD) plotted in Figure 2d further confirm the above facts. The pore size of all samples is focused at ~ 0.7, 1.1, and 1.7 nm, and the micropore size occupies major distribution. Detailed textural parameters of SPCs are summarized in Table S2. Another interesting observation is that the specific surface area of SPCs with simple activation by KOH were 2888, 3072, and 3321 m2 g-1 corresponding to SPC-5, SPC-15, SPC-25, respectively, which is much larger than the other biomass-derived carbon.36-38 This result is attributed to the difference in the amount of KCl in the precursor produced by the different amylopectin content of the raw materials. Besides the activation role of KOH, KCl also can serve as an effective self-template and activator to synergistically enhance the activation process.38 As the result, it is seen that the SPCs have a higher percentage of micropores and improve the specific surface area, thereby increasing surface area is favorable for increasing the specific capacitance in supercapacitor applications. 13



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To further clarify the physical structure of the SPCs, Raman spectra had to be obtained. Two peaks, at 1344 cm-1 (D-band) and 1572 cm-1 (G-band), could be found at the curves (Figure 2e). G band in the Raman spectrum of carbon materials is assigned to the stretching bond of sp2-hybridized carbon. Meanwhile, the D band is attributed to the disorder induced by structural defects and impurities.39 Moreover, the intensity ratio of the G to the D band (IG / ID) is commonly used to estimate the degree of graphitization of carbon based materials.40 From the observation of a narrower G-band and a bigger IG / ID value, it suggested that the carbon materials used has a structure with higher graphitization. About G-bond and D-bond, the ratio of the intensity (IG / ID) is 1.11, 1.05, and 0.85 for SPC-5, SPC-15 and SPC-25, respectively. By comparison, SPC-5 has a greater degree of graphitization than the other samples obtained, which is consist with the XRD analysis results. In addition, these IG / ID values of as-obtained SPCs materials are higher than those of other graphitized carbon materials,41-43 which demonstrates the role of starch crystallinity in producing graphitized structures deeply. The surface elemental compositions of the precursor prepared as shown in Figure S1b, could indicate that the co-existence elements of C, K, N, and O, which is in keeping with the XRD results of the precursor (Figure 2a). After KOH activation, the XPS spectrum of SPCs (Figure 2f) shows that the two peaks were focused at 284 and 532 eV, and can be assigned to C1s and O1s.8 The detail data of as-prepared precursors and SPCs were listed in Table S2. The contents of C1s and O1s were calculated to be SPC-5 (88.5 and 11.5 %), SPC-15 (94.6 and 5.4 %), SPC-25 (94.5 and 5.5 %). The highresolution C1s spectrum of SPC-5 in Figure S1c, where the peaks were located at 14


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283.8, 284.8, and 288.1 eV, due to group C=C, C-O, and O-C=O, respectively.44 Further evidence indicated that group C-O and C=O were shown at 532.8 and 531.6 eV under the relevant high-resolution O1s spectra of SPC-5 in Figure S1d. It is worth mentioning that the oxygen content of SPC-5 was double than SPC-15 and SPC-25. This suggests that the wettability of the electrode can be facilitated by the abundant presence of O functional groups in the prepared materials, leading to an effective electronic transfer. Infrared spectroscopy (IR) is an important indicator to exhibit the functional group information of the materials. As shown in Figure S1e, the spectra of SPCs exhibited similarity absorption bands, the bands at 3400, 2900, 1600 cm−1 suggested the presence of hydroxy, carboxyl, epoxy groups, which consist with the result of XPS of SPCs.45 The power structure of the three biomass and SPCs are shown by using optical and electron microscopy in Figure 3. From the optical image in Figure 3a-c, the particle size is variable and ranges from 5 to 160 µm. The average size of taro ranges from 5 to 10 µm; the size of sweet potato ranges from 15 to 80 mm; the size of potato ranges from 25 to 160 µm. These results match prior observations of the morphology of these starches, but the size of raw biomass was larger than the literature due to the starch particles reunited.46 After dehydration and carbonization processes, the precursors that consist of shrunken particles without a conspicuous porous architecture was observed by using scanning electron microscopy (Figure 3d-f). Moreover, we can observe precursors changes from dispersion to agglomeration, correspond to SPC-5, SPC-15, and SPC-25, respectively. As shown in Figure 3g-i, no well-distributed porous framework was observed on the surface after KOH activated, 15



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but it can be observed many big holes in the SPCs. In addition, the extensive creation of pores was often caused by KOH, a common activating agent. Undoubtedly, this will lead to the collapse of its nanostructures, which in turn leads to ambiguous pore structures that ultimately lead to overall low yields. In the preparation of SPCs, the precursor of the KOH ratio was chosen to be 1:3, the starch-derived carbon has the high specific surface area (>2800 m2 g-1) under the circumstances. Interestingly, even with a ratio of 1:3 (precursors / KOH), the SPC-5 maintained a high yield of 54 wt%, rating at much higher than that of the other biomass.4, 47-48 This can be attributed to the high amylopectin and carbon content in raw materials, which can provide more stable carbon framework and high yield of SPCs.

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Figure 3. (a-c) Optical microscope image of three kinds of raw biomass taro, sweet potato, potato, respectively. (d-f) SEM images of precursors at same magnifications. (g-i) SEM images of SPCs at different magnifications. While, the inset in (g) shows the local magnification of the SPC-5. The inset in (h) shows the local magnification of the SPC-15. The inset in (i) shows the local magnification of the SPC-25, respectively.

Due to the hydrophilicity of the electrode, it is easy to promote the penetration of the electrolyte. Therefore, it is critical to improve the affinity of SPC-based electrodes to improve their electrochemical performance. Figure S2 show the contact angle of SPCs, 17



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with the starch particle size increasing from 5, 15, to 25, the contact angle also gradually increasing from 54o, 61o, to 78o, indicating a reduced hydrophilicity of SPCs. This significant improvement in hydrophilicity was associated with a reduction in particle size. On the one hand, small particle size can be in full contact with the electrolyte, so that the hydrophilicity of SPCs can be improved. On the other hand, the presence of an abundant amount of O functional groups in the prepared materials promotes the wettability of the electrode, hence resulting in an improvement in hydrophilicity. In addition, the SPCs’ electronic conductivity was given a further measurement through the method of 4-probe. The values of conductivity of SPCs reveal that SPC-15 and SPC-25 is decreased to 2.20 and 2.10 S cm-1 from 2.27 S cm-1 of SPC-5. After collecting the results, it is suggested that the carbon samples’ conductivity is not only affected by oxygen contents, but also by the starch crystallinity due to the different amylopectin contents. The improved surface wettability and conductivity make the SPCs an excellent candidate for surpercapacitor electrode materials. Electrochemical activity. To investigate the intrinsic electrochemical properties of SPCs, collections of data were estimated in 6 M KOH under a three-electrode system. At the same time, the curves of GCD collected from the related resultant SPCs materials at a current density of 0.5 A g-1 are compared in Figure S3a. About the shapes, the charge curves indicate almost linear in the range studied of potentials, while the discharge curves are nearly symmetric with their related charge curves, all that above show the good capacitive properties of SPCs electrodes. Markedly, the SPC-5 displays the best performance among these three samples, and compared with commercial 18


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activated carbon YP-50, the electrochemical properties of SPCs were better. As shown in Figure S3c, the capacitance values of SPC-5, SPC-15, SPC-25, and YP-50 calculated from GCD curves were 397, 372, 337, and 163 F g-1, respectively. Such result is higher than the activity of the major biomass which is derived from porous carbons reported above, such as tobacco rods-derived carbon (286.6 F g-1 at 0.5 A g-1),49 biomass waste coconut shell (268 F g-1 at 1 A g-1),50 pomelo peel followed (342 F g-1 at 0.2 A g-1),51 willow catkin (298 F g-1 at 0.5 A g-1).52 Compared with that of SPC-15 and SPC-25, the capacitive property which had been enhanced of SPC-5 should be attributed to the improvement of conductivity and the improved surface wettability between electrolyte and electrode materials. Considering the satisfactory capacitance of the optimal SPC5, the CV and GCD curves of SPC-5 was further systematically investigated. The CV curves of SPC-5 is depicted in Figure 4a, the CV curves still remain a rectangular shape with the scan rate from 5 to 200 mV s-1, indicating that they have excellent rate performance. GCD curves of symmetric supercapacitor at different current density in SPC-5 indicated quasi-triangular shape (Figure 4b), meaning the electrodes possess an excellent charge/discharge invertibility. Moreover, the specific capacitance of SPCs at different current densities is marked in Figure 4c. SPC-5 exhibits a high specific capacitance of 276 F g-1 even at a current density of 50 A g-1 (69.5% capacitance retention), revealing a good rate performance. The excellent rate capacity is mainly due to short ion spread distance and low ion transport resistance in the process of charge/discharge.53 In order to study the electrolyte ion diffusion and transmission kinetics deeply, electrochemical impedance spectroscopy (EIS) was further 19



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experimented in the study. Figure 4d shown the Nyquist plots of SPCs electrodes that all have a semicircle, which is consisted of a 45° diagonal line and a vertical line. Notability, the more the line verticals, the closer the supercapacitor acts as a perfect and ideal capacitor in the low-frequency region. In contrast, in the high-frequency region, the 45° diagonal line dedicates the stand-out pore accessibility for the electrolyte, the width of the semicircular impedance loop reveals the charge-transfer resistance in the electrode materials, and the real axis intercept is the equivalent series resistance (ESR). Compared with the SPC-15 and SPC-25 electrodes, SPC-5 shows the straightest line and the smallest Rct, which is a feature of better capacitive behavior. In addition, the fitting equivalent circuit model is shown in the inset of Figure 4d. Here, the total capacitor circuit, composed of the equivalent series resistances (Rs) and the charge transfer resistance (Rct), connected with the capacitor layer obtained from the process of charge/discharge reaction, as well as C and Q. It is clearly that the lowest Zw and Rct values of SPC-5 (0.44, 0.22 Ω) than SPC-15 (0.46, 0.24 Ω) and SPC-25 (0.48, 0.26 Ω) electrodes, indicates lowest contact resistance and smallest intrinsic ohmic resistance electrode of SPC-5 that has a superior electric conductivity in aqueous electrolyte. So, the EIS results further prove that the best electrical conductivity and graphitization in the electrode materials described above can improve the capacity of rapid electron and ion transmission, and the result which is related with the above electrochemical characterization. As listed in Table S4, all samples exhibit small Rs values, indicating the low intrinsic resistance, good electrical conductivity. In addition,

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suitable graphitization strength and large specific surface area makes the SPCs an outstanding candidate for surpercapacitor electrode materials.

Figure 4. SPCs for electrochemical testing in the electrolyte with 6 M KOH in a threeelectrode system. Cyclic voltammograms (a) and GCD curves (b) of SPC-5. Rate performance (c) and Nyquist plots (d) of the SPCs based electrodes.

SPC-5 with the excellent characteristics above will be ideal electrodes when used for supercapacitors. To evaluate the performance of SPC-5 for practical applications, in a two-electrode cell configuration, a symmetric supercapacitor was fabricated and tested in electrolytes with 6 M KOH. CVs measurements of the device were carried in 21



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different voltage windows (Figure S4a) and the curve in the range of 1.2–1.4 V shows gas evolution from electrolysis. Thus, all further measurements were performed with a voltage range of 0–1.2 V. The size dimension of the SPC-5 electrode is 12 mm in diameter for the two-electrode symmetrical supercapacitor (Figure S4b), the loading of electrode is 2.0 mg/cm2, the thickness is 0.02mm, the density is 0.88 g cm-3. Apparently, the secure rectangular shape of the CV curve was constantly maintained without any deformation even at an extreme scan rate of up to 200 mV s−1, indicative of a typical double-layer capacitive behaviour and an excellent rate performance (Figure 5a). Moreover, a symmetrically linear form and low IR drop was presented by the GCD statistics of SPC-5 (Figure 5b) when measured at different current densities, further verifying an super charge/discharge reversibility, as well as low internal series resistance.54 As presented in Figure 5c, the impedance curve of the cell proves that the vertical line has a closely ideal capacitive behaviour at the low-frequencies and that an ionic resistance is as low as 0.42 Ω at the same condition, suggesting a speedy and competent ion transfer in the structure of pore.55 Ragone plots are purposed to connect the power and energy density as a very useful indicator to positively provide aid to heap the performance of supercapacitor devices. As shown in Figure 5d, when the power density is 148 W kg-1, the maximum energy density is 22.59 W h kg-1, and decreases to 15.51 W h kg-1 slightly while power density increases to 14911 W kg-1. At high discharge rates, the device offers the highest energy density, compared with that reported on other biomass which was derived from the supercapacitors of carbon-based symmetric, which are in alkaline aqueous electrolytes.56-62 In Figure 5e, the statistic 22


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with an initial specific capacitance of about 97%, detected from the cycling stability test of SPC-5, which was finally retained after the mode of consecutive 20000 charging / discharging cycles at a constant current density of 5 A g−1 in 6 M KOH. By revealing the great potential of SPC-5 to meet practical application standards, the finding above foreshadows superior durablity and long-term stability for further investigations and developments.

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Figure 5. Showing the electrochemical performance in the cell above making use of SPC-5 electrode in the electrolyte with 6 M KOH. (a) CV curves, (b) Charge-discharge curves, (c) Nyquist plots, (d) Ragone plot, and (e) Cycle stability profiles of SPC-5.

CONCLUSION In summary, starch based porous carbon (SPCs) have been successfully synthesized from three kinds of biomass through a low-cost and easy carbonation and activation approach. The low temperature carbonization is an essential process for reserving the same properties as raw materials and KOH treatment as they tend to generate a lot of microporous within the structure of the material. The result indicates that suitable graphitization strength is controlled by amylopectin content. As the content of amylopectin increases, the particle size become smaller, resulting in the degree of graphitization increases, hence improve the electrochemical performance. As expected, SPC-5 has superior specific capacitance and rate capacitance (397 F g-1 at 0.5 A g-1 and 276 F g-1 at 50 A g-1), high energy and power density (22.59 Wh kg-1 vs 148 W kg-1) and excellent stability (97 % of capacitance retention after 20 000 cycles at 5 A g-1) in aqueous 6 M KOH electrolyte. Moreover, the yield of SPC-5 is up to 54 % after KOH activation. Taking together, we have demonstrated the combining of graphitic carbon skeleton and particle size as well as oxygen surface functional is extremely effective to boost the capacitance of porous carbons. In addition, this work is of general significance for the production of highly porous and graphitized carbon

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from suitable feedstocks for energy storage, supercapacitors, gas sorption, or other applications. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on ACS Publications website at XRD pattern, XPS spectra, FTIR spectra, Contact angle, Porosity parameters, Element contents, and GCD curves and CV. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Tel. and Fax: +86-02085280319 (Y.X.) *E-mail: [email protected] (Y.L.). ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21671069, U1501242, 21571066, 51602107), and National Undergraduate Training Program for Innovation and Entrepreneurship (201810564057). REFERENCES (1) Li, Z.; Gadipelli, S.; Yang, Y.; Guo, Z. Design of 3D graphene-oxide spheres and their derived hierarchical porous structures for high performance supercapacitors. Small. 2017, 13 (44), 1702474, DOI 10.1002/smll.201702474. (2) Sun, Z.; Zheng, M.; Hu, H.; Dong, H.; Liang, Y.; Xiao, Y.; Lei, B.; Liu, Y. From biomass wastes to vertically aligned graphene nanosheet arrays: A catalyst-free synthetic strategy towards high-quality graphene for electrochemical energy storage. Chem. Eng. J. 2018, 336, 550-561, DOI 10.1016/j.cej.2017.12.019. 25



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TOC The amylopectin content controls the degree of graphitization. SPC-5 has smallest particle size and highest amylopectin content, exhibits a surprisingly high specific capacitance of 397 F g−1 at 0.5 A g−1 and 97 % capacity retention after 20 000 cycles.

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Synthesis of porous carbon material with suitable graphitization

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