Distinctive Construction of Chitin Derived Hierarchically Porous

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Applications of Polymer, Composite, and Coating Materials

Distinctive Construction of Chitin Derived Hierarchically Porous Carbon Microspheres/Polyaniline for High Rate Supercapacitors Lingfeng Gao, Liukang Xiong, Dingfeng Xu, Jie Cai, Liang Huang, Jun Zhou, and Lina Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05891 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Applied Materials & Interfaces

Distinctive Construction of Chitin Derived Hierarchically Porous Carbon Microspheres/Polyaniline for High Rate Supercapacitors Lingfeng Gao#1, Liukang Xiong#2, Dingfeng Xu1, Jie Cai1, Liang Huang*2, Jun Zhou2, Lina Zhang*1 1

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China. *E-mail: [email protected] 2 Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China. *E-mail: [email protected] # The author contributed equally to this work

Abstract Recently, nanostructured porous carbons are attracting significant interest in various important applications. However, a green and innovative method to fabricate hierarchically porous structured carbon is still a challenge. In present work, hierarchically porous carbon microspheres (HCM) were prepared by pyrolyzing the chitin microspheres fabricated from chitin/chitosan blend solution, in which chitosan was used as forming agent of nanopores/nanochannels to construct the microspheres. The HCM displayed hierarchical porous structure and improved specific surface area (SSA) of 1450 m2/g. For the application of HCM in hybrid electrode materials as supercapacitors, polyaniline (PANI) nanoclusters were further deposited on the surface of HCM. Symmetric supercapacitor (SSC) based on HCM-PANI exhibited high-rate capability with a retaining capacitance over 64% as the scan rate increased from 2 to 500 mV/s. This work introduced a distinctive and green method to fabricate hierarchically porous carbon materials, having considerable application prospect for energy storage. Keywords: hierarchical porosity, high specific surface area, chitin/chitosan, carbon

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microspheres, high rate supercapacitors

1. Introduction Different nanostructured porous carbon materials have been extensively studied for various applications including catalyst supports, adsorption substrates, electrodes for batteries and so on.1,2 In the field of energy storage, porous carbons and their composite materials are attracting significant interest because of their excellent electrical conductivity, remarkable electrochemical stability, and high specific surface area (SSA),3-6 Specifically, carbon-based materials with hierarchically porous structure and high SSA are highly desired. Because high SSA can provides more active sites to facilitate charge capacity, and hierarchical porosity (interconnected macro-meso-micro pores) can promote rapid ion adsorption (charged by pores below 2 nm) and ion transport (charged by mesopores and macropores) with improved capacitance and rate capability.7-9 To achieve this, many researches were reported to prepare hierarchically porous carbon materials with high SSA. For instance, the sacrificial template methods by using silica, and activation methods by using KOH, CO2 and vapor.10-14 However, it still remains a challenge to realizing a hierarchical porosity while maintaining a uniform micro-nanostructure because creating nanopores (especially below 10 nm) with normal activation method will leading to the inevitable collapse of their nanostructures.15 Hence, developing an efficient and green strategy to fabricate porous carbon with controllable high SSA, hierarchical and steady nanostructure, and improved micropore and mesopore volumes is highly desirable. Converting renewable biomass resources into functional materials through green


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technology has profound significance for sustainable development. In recent years, carbon-based materials derived from renewable biomass have been vigorously investigated for the application in supercapacitors, such as wood,16 chitin,17,18 cellulose,19 and chitosan10. It is worth noting that chitin derived from the seafood wastes possess attractive structure and functions,20 and the conversion from raw chitin into novel materials via green technology is a sustainable pathway. Chitin is insoluble in many common solvents, including dilute acid.26 In our laboratory, chitin was successfully dissolved at low temperature through NaOH/urea aqueous system and a series of chitin-derived materials have been constructed from the chitin solution.21-24 More recently, an LiOH/KOH/urea aqueous solution was developed to dissolve the chitin and chitosan with different DA (Degree of Acetylation).25 Thus, based on the above discussions, our strategy was to fabricate a new chitin/chitosan composite microspheres (CCM) by a simplified emulsion method from the alkaline/urea aqueous system for the first time. Subsequently, on the basis of the dissolution of chitosan in acid solution,26,27 the porous chitin microspheres with improved mesopores and micropores can be obtained by soaking in acid solution, owning to the removing of chitosan chains to generate nanopores/nanochannels, and the hierarchically porous carbon microspheres (HCM) with improved SSA and interconnected porosity can be constructed after carbonization. Furthermore, to verify the application prospect of HCM in hybrid electrode materials for supercapacitors, polyaniline was coated on the HCM by in-situ polymerization. Considering polyaniline (PANI) is a crucial electrode material for supercapacitors owning to its



low cost, good chemical durability,28-30 and can provide prominent capacitance due to its multiple redox states.31 However, the swelling and shrinkage of PANI in the course of charge-discharge process can result in the collapse of structure.32,33 An effective method is to combine the PANI nanostructure with porous carbon materials to improve their conductivity and rapid ion transference, as well as efficiently promote the rate capability and cycling durability of PANI.34-36 Based on this, we chose PANI as the active component to fabricate composite material with HCM for the electrode materials of supercapacitors to analyze the influences of their hierarchical structure on the electrochemical performances. Herein, a novel hierarchically porous chitin carbon microspheres (HCM) with high SSA and interconnected hierarchical pores was fabricated through an efficient and green method. Moreover, the HCM coated with PANI, coded as HCM-PANI, were used for electrodes in supercapacitor to study the affects of their structure on electrochemical properties. The results showed that the HCM-PANI based symmetric supercapacitor displayed a high rate capability and outstanding cycling durability of 90.6% retaining capacitance after 10000 cycles. This work holds considerable potential in fabricating hierarchically porous carbons from biomass for energy storage applications.


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Figure 1. Graphical illustrations of the formation process (a), two-electrode system(b), and porous structure of the nanofibers (c) for the HCM-PANI.

2. Experimental Section 2.1. Materials. Chitin was provided by Golden Shell Biochemical Co. Ltd. (Zhejiang, China) and purified by using our previous method.22 Chitosan powder (90% DA) was provided by Ruji Biotechnology Co. Ltd (Shanghai, China). Aniline monomers were distilled under vacuum before used, ammonium peroxydisulfate (APS), perchloric acid (HClO4) and other chemical regents were used as received. 2.2. Preparation of nanofibrous chitin/chitosan microspheres (CCM). Chitin and chitosan with different weight ratio was firstly dissolved according to our previous work.25 CCM samples with different chitosan content were named as CCM-X, X represents the mass percentage of chitosan, such as CCM-15 containing 15 wt% chitosan. Typically, for the dissolution of chitin/chitosan (15 wt%) solution,



21.2 g chitin and 3.8 g chitosan were mixed in 600 g solution of LiOH/KOH/urea/H2O (3:8:9.5:79.5 by weight). The solution was frozen under -30 °C for 4 h, and then thawed at 25 °C for 6 h. To fabricate CCM, the obtained solution was added into a pre-cooling solution containing 250 g isooctane, 16 g Span 85, and was kept stirring for 50 min at 0 °C. Subsequently, the reactor was soaked in a boiling bath to induce the formation of nanofibrous microspheres. The regenerated CCM were obtained after adequately rinsed with ethanol and deionized water. 2.3. Preparation of chitin carbon microsphere (HCM) and HCM-PANI. The CCM was soaked in 2 wt% acetic acid for 12 h to remove the chitosan, CCM samples treated with acetic acid were coded as CCM-A, such as CCM-15-A. After that, the microspheres were washed with t-BuOH and then freeze-dried under -50 °C. Subsequently, the freeze-dried microspheres were pyrolyzed under 800 °C in an insert atmosphere, the obtained carbon microsphere was named as HCM. The resulted HCM was rinsed with 1 M HCl, deionized water, and ethanol respectively, and then dried finally at 60 °C for 12 h. The HCM (-0, -5, -10, -15) samples were corresponded to CCM (-0-A, -5-A, -10-A, -15-A), respectively, however, if there are no special instructions, HCM was defaulted to HCM-15. As a contrast, CCM samples were also applied to the same carbonization process as HCM, and coded as CCM-5-C, CCM-10-C, CCM-15-C. HCM-PANI and CCM-C-PANI samples were fabricated by in-situ polymerization. Typically, for HCM-0.05PANI, 60 mg HCM powders were dispersed in 40 mL 1 M HClO4 aqueous, then 182 µL aniline was dropped in and stirred at 0-5 °C. Subsequently, 304.25 mg APS was added under rapid stirring, the


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reaction was kept at 5 °C for 8 h. After polymerization, the precipitate was thoroughly rinsed with deionized water and dried in an oven overnight. For comparison, HCM-0.03PANI and HCM-0.07PANI with AN of 109 µL and 255 µL were fabricated at similar condition. 2.4. Characterization Scanning electron microscopy (SEM) observation was performed on a field emission scanning electron microscopy (Zeiss, SIG-MA). Transmission electron microscope (TEM) image was recorded by aJEM-2010 (HT) transmission electron microscope (JEOL TEM, Japan). Nitrogen physisorption measurements were tested by

Autosorb-iQ,

Brunauer-Emmett-Teller

(BET)

specific

surface

area,

Barrett-Joyner-Halendar (BJH) and Nonlocal Density Functional Theory (NLDFT) analyses were done automatically. Powder X-ray diffraction (XRD) was recorded at XRD-6000, Japan, with Cu Kα radiation of λ = 0.15406 nm. Fourier transform infrared (FTIR) spectra was performed on a FTIR5700 spectrometer. X-ray photoelectron

spectroscopy

(XPS)

analyses

were

performed

by

using

ESCALAB250Xi system. 2.5. Electrochemical measurements The electrodes were obtained by mixing 80 wt% HCM or HCM-PANI powder, 15 wt% acetylene black and 5 wt% poly(tetrafluoroethylene). The composite was stirred for 12 h to evaporate ethanol to form a paste-like mixture, which was then roll-pressed into a 40 µm thick film (Figure S11). The obtained films (weight: 3 mg, surface area: 1 cm2) were dried at oven for 12 h and then dipped into the electrolyte



solution for 2 h for the pre-infiltration of electrolyte solution before the assemble of SSC device. The electrochemical measurements of the fabricated SSC device were carried out in the “Swagleok” cell with 1 M H2SO4 as electrolytes. Detailed calculation method for specific capacitance, energy density, and power density were presented in supporting information together with Figure S8.

3. Results and discussion Both chitin and chitosan were dissolved in alkaline/urea aqueous system at -30 °C to obtain a transparent solution with high transmittance (Figure S1), indicating their good dispersion and excellent blend miscibility in this system. As shown in Figure 1a, chitin/chitosan microspheres (CCM) consisted of nanofibers were obtained by an emulsion method from chitin/chitosan solution, and then were soaked in dilute acetic acid to remove chitosan, leading the formation of micro- and mesopores in the chitin nanofibers. The porous microspheres were carbonized at 800 °C under argon atmosphere to form chitin carbon microspheres (HCM). Subsequently, PANI nanoclusters were uniformly coated on the nanofibers of HCM via in-situ polymerization to construct HCM-PANI. The SSC devices were assembled in a two-electrode system (Figure 1b) with HCM and HCM-PANI composites as the electrode materials, respectively, to investigate their electrochemical performances.


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Figure 2. SEM images of CCM-15 (a, d), CCM-15-A (b, e); Nitrogen adsorption and desorption isotherms (c) and Barrett–Joyner–Halendar (BJH) pore size distribution (f) of CCM-A samples, graphical illustrations for the change of nanofibers during the acid treatment process (g). 3.1. Hierarchically porous structure and formation mechanism of the CCM-A and HCM microspheres As shown in Figure 2a and d, the homogeneous chitin/chitosan microspheres (CCM-15) were weaved with nanofibers with average diameter of 50 nm. It was reported that chitosan and chitin exhibited extended wormlike chain conformation in the alkaline/urea system, which could self-aggregate in parallel to form nanofibers.22,27 Thus, CCM was consisted of the chitin/chitosan nanofibers, in which the chitosan chains were dispersed uniformly in chitin matrix. Importantly, the chitosan chains in the nanofibers of CCM-15 could be dissolved to remove out after acid treatment, coded as CCM-15-A, leading to the formation of more loose and



porous nanofibers (Figure 2b, e), this could be explained by the disturbance of parallel gathered chains during the dissolution process, as exhibited in Figure 2g. Furthermore, the SEM images in Figure S2 clearly indicated that with an increase of the dissolved chitosan (from CCM-0-A to CCM-15-A), the nanofiber become more multihole, further confirming that the chitosan acted as forming agent of pores to generate the nanopores/nanochannels in the nanofibers. This phenomenon was similar to those of sea-island fibers, in which one nanofiber-like component can be extracted out from the bicomponent fiber to form the superfine fibers.37 Nitrogen adsorption-desorption isotherms of CCM-A are shown in Figure 2c, and the corresponding SSAs and pore volumes are displayed in Table S1. The SSA values increased from 228 m2/g (CCM-0-A) to 350 m2/g (CCM-15-A) with an increase of dissolved chitosan amount. Moreover, from the analysis of the pore size distributions (Figure 2f), the micropores (below 2 nm) and small-scale mesopores (3-10 nm) in the CCM-A microspheres increased significantly with an enhancement of the chitosan content after the acid treatment, further proving that chitosan contributed to the nanopores formation. These results demonstrated that the chitosan chains and thinner nanofibers were uniformly dispersed in the chitin matrix to occupy a space, thus the nanopores/nanochannels were created after removing chitosan by acid, leading the formation of the porous nanofibers described in Figure 1a. The formation mechanism and porous structure of the CCM-A, HCM and HCM-PANI microspheres were further confirmed by the following results.


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Figure 3. SEM images (a, d) and TEM images (b, e) of HCM-15, Nitrogen adsorption and desorption isotherms (c) and Nonlocal Density Functional Theory (NLDFT) pore size distribution (f) of different HCM samples. As displayed in Figure 3a and d, the morphology of the HCM remained well after carbonization, only slightly shrink, compared to CCM-A. Moreover, the differences between the nanofiber structure of various HCM were no more distinguishing, due to the slight shrinkage of their fibers during pyrolysis process (Figure S3). The TEM images (Figure 3b and e) indicated that the carbon nanofibers of HCM with diameter from 50 to 100 nm were connected with each other to form multi-channeled carbon networks. From the analysis of nitrogen adsorption-desorption isotherms of HCM samples (Figure 3c), the SSA of HCM-15 was 1450 m2/g, higher than that of HCM-10 (1190 m2/g), HCM-5 (801 m2/g), and HCM-0 (555 m2/g) (Table S2). The micropores and total pore volumes of HCM also increased obviously from HCM-0 to HCM-15 (Figure 3f). Especially, amount of micropores and small-scale mesopores enhanced, leading to the increased SSA. These results further supported the existence of nanopores/nanochannels in the carbon fibers, as shown in Figure 1c. It was



noteworthy that the SSAs (540 m2/g, 568 m2/g, 606 m2/g) of the CCM-C microspheres without removing chitosan (Figure S4) were much lower than that of HCM. This proven strongly that chitosan as forming agent of nanopores/nanochannels played an important role on the increasing SSA and porosity for the HCM. In view of these results, the HCM had various pore sizes including 0.5-2 nm and 2-10 nm resulted from removing chitosan (Figure 3f), and 50-200 nm which generated from the gaps between carbon nanofibers (Figure 3d), namely macro-meso-micro pores. What’s more, differ from many reported methods in fabricating hierarchical pores, our method can create many micro-mesopores gently without any collapse of the nanostructure.

Figure 4. SEM images of HCM-0.05PANI at different scale (a-c); TEM images of HCM-0.05PANI at different scale (d, e); EDX spectrum (f), SEM image and the elemental mappings of C, N, O for HCM-0.05PANI (g). PANI was in-situ polymerized on the surface of HCM to demonstrate the potential


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application of HCM in hybrid electrode materials for supercapacitors. As shown in Figures 4 and S5, HCM-PANI composites with controllable PANI nanoparticles were fabricated by uniformly depositing PANI on the carbon fibers. The content and morphology of PANI could be simply adjusted through varying the amount of aniline. With an increasing of the aniline concentration from 0.03 to 0.07 M, the nanostructure of PANI on the surface of carbon nanofiber transformed from small nanoparticles to randomly stacked nanoclusters (Figure S5), and the PANI contents in the HCM-PANI composites increased from 45 wt% to 70 wt%. Specifically, for the aniline concentration of 0.05 M, the PANI nanoclusters with a diameter range from 20 to 30 nm were homogenously deposited on the nanofibers in HCM (Figure 4b, c and e). EDX elemental mappings in Figure 4g further confirmed the complete coverage of PANI on the HCM nanofibers framework with homogeneous distributions of C, N and O over the surveyed range. The well dispersed PANI nanoclusters on the hierarchically porous carbon networks is vital for the improvement of electrochemical performances.



Figure 5. FTIR spectra (a), XPS analysis (b), nitrogen adsorption/desorption isotherm (c), and pore size distribution (NLDFT) (d) of HCM-PANI samples. Figure 5a displayed the FTIR spectra of HCM and HCM-PANI. The peaks of HCM at 3432, 1639 and 1387 cm-1 were attributed to the H-O-H bending, O-H stretching and alkoxy C-O stretching, respectively. Most of the peaks for HCM were related to oxygen-containing functional groups, to be beneficial to the hydrophilia of HCM. Compared with HCM, a group of typical peaks corresponding to PANI appeared in the spectra of HCM-PANI. The peaks at 1576, 1490, 1307, and 1238 cm-1 corresponding to the C＝C, C-N, and C＝N stretching vibrations in the PANI. The XRD patterns for HCM and HCM-PANI samples were presented in Figure S7. The HCM exhibited two broader diffraction peaks at 24.2º, 43.7º, attributed to the (002) and (100) plane of the graphitic structure, respectively. For the HCM-PANI samples, the typical diffraction peaks of PANI were located at 14.8º, 20.3º, and 25.3º, ascribed to (011), (020), (200) crystal planes of PANI, respectively. Obviously, the diffraction peaks of PANI increased with increasing aniline concentration, which was caused by


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the mass increase of PANI deposition. The XPS results indicated that C, N and O elements appeared in the full survey spectra of HCM and HCN-0.05PANI (Figure 5b). The N content in HCM-0.05PANI increased to 15.8 wt% for after PANI deposition, which was consistent with N content obtained from EDX analysis (15.2 wt%, Figure 4f). However, the real N content of HCM-0.05PANI was 8.83 wt% according to elemental analysis (Table S4), because the XPS analysis focuses on the top few nanometers of a sample, so the higher N content in XPS came from the PANI. For the N 1s spectrum in HCM, it could be divided into four peaks, pyridinic N (N-6, 398.4 eV), pyrrolic N (N-5, 399.8 eV), quaternary N (N-Q, 401.2 eV) and oxidized N (N+O, 406 eV) (Figure S6a), respectively. High resolution of N 1s for HCM-0.05PANI could be deconvoluted into three peaks located at 398.8, 399.7 and 401.6 eV, attributed to the structure of -C＝N-, -NH-, and -NH+-, respectively (Figure S6b). These results further proved the uniformly deposition of PANI on the surface of HCM nanofibers, uniformly dispersed PANI nanoparticles could efficiently reduce the structural collapse of PANI resulted from the volume expansion of PANI during charge/discharge process.33 Nitrogen adsorption/desorption isothermals of HCM-PANI (Figure 5c) exhibited the characteristics of I and Ⅳ type isothermals. The N2 adsorption increased rapidly at low pressure, suggesting microporous feature (typical character of Ⅰ type isothermal)39, and the hysteresis loop appeared at high relative pressure demonstrating the existence of mesopores (typical character of Ⅳ type isothermal)46. The Ⅰ/Ⅳ type isothermal of HCM-PANI was not so obvious compared with HCM because the



deposition of PANI covered many micro-meso pores, leading to the decrease in N2 adsorption. From the analysis of their corresponding data (Table S3), the SSA of HCM-0.03PANI (476 m2/g), HCM-0.05PANI (326 m2/g) and HCM-0.07PANI (105 m2/g) were much lower than that of HCM (1450 m2/g). Which further confirmed that the PANI molecules and nanoclusters occupied partly the pores in HCM. In spite of this, the SSA value of HCM-0.05PANI was still comparable or higher than many values of the reported carbon-PANI composites.38,39 The high SSA and hierarchical nanopores (Figure 5d) of HCM-PANI are favorable to increase the specific capacitance and rate capability.

Figure 6. Electrochemical performance of HCM and HCM-PANI based SSC devices: CV curves of different SSCs at a scan rate of 1000mV/s (a), EIS spectra of different SSCs (b), CV curves of HCM-0.05PANI based SSC at different scan rates (c), specific capacitance of different SSCs at different scan rate (d), Ragone plots of different SSCs (e), and Cycling stability of different SSCs under the sweep rate of 50 mV/s (f). 3.2. Electrochemical performance of HCM and HCM-PANI


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To investigate the electrochemical performance of HCM and HCM-PANI samples, symmetric supercapacitors (SSCs) based on HCM or HCM-PANI in a two-electrode system using 1 M H2SO4 aqueous solution as electrolyte under the voltage window of 0-1.0 V were constructed. The cyclic voltammetry (CV) curves of HCM based device (Figure S8a) exhibited symmetrical rectangular shape, demonstrating an ideal capacitive behavior. In the GCD plots (Figure S8b) of the HCM based SSC, there was almost no IR drop, revealing the good electrical conductivity of HCM. Figure 6a illustrated the CV curves of HCM and HCM-PANI based SSC under a high scan rate of 1000 mV/s. As expected, HCM-PANI based SSC exhibited a couple of broad redox peaks, indicating the pseudocapacitive characteristics of PANI.38 Notably, the integrated areas of CV curves for HCM-PANI were much larger than that of HCM, demonstrating a significant improvement in specific capacitance after the deposition of PANI. The gravimetric capacitance calculated from the CV dates40, 41

increased gradually from HCM-0.03PANI to HCM-0.05PANI with an increase of

PANI incorporation (Figure 6d), which attributed to the reversible redox reaction of PANI in H2SO4 electrolyte. Nevertheless, as the aniline amount increased to 0.07 M, the capacitance decreased, this primarily resulted from the excessive accumulation of PANI nanoclusters, which increased the transfer resistance as well as aggravated the volume expansion of PANI during charge-discharge process. Therefore, the HCM-0.05PANI based SSC exhibited higher gravimetric capacitance (Cg) of 88 F/g under the scan rate of 5 mV/s. Moreover, for contrast, CV curves of CCM-C-0.05PANI based SSC were investigated, the Cg of HCM-0.05PANI was



apparently higher than that of CCM-C-0.05PANI (Figure 7), suggesting a higher efficiency of the HCM than CCM-C. The higher capacitance of HCM-0.05PANI mainly attributed to two factors. Firstly, the carbon nanofibers in HCM-0.05PANI were more dispersive and wider than that in CCM-C-0.05PANI because the disturbance of nanofibers during the acid treatment process, as discussed above, obviously, more dispersive and wider carbon nanofibers could benefit the uniformly and continuously coating of PANI. The second and more important factor is that the higher SSA and rationally hierarchical pore structure of HCM-0.05PANI provided more adsorption and reaction sites for the electrolyte ions. And both of them were in favor of the improvement of specific capacitance.

Figure 7. CV curves at different scan rate for HCM-0.05PANI based SSC (a), CCM-C-0.05PANI based SSC (b), Specific capacitance based on the total mass of the two electrodes of SSC at different scan rate (c). Moreover, the shape and redox peaks of the CV curves for HCM-0.05PANI were well-maintained even under a high scan rate of 5000 mV/s (Figures 6c and S9), and the Cg remains at 56 F/g under the scan rate of 500 mV/s (Figure 6c), retaining over 64% of the capacitance at 2 mV/s, showing a high rate capability. In Figure S10a, Galvanostatic CD curves of the HCM-0.05PANI based SSC in the voltage window of


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0-1.0 V were presented. Obviously, the IR drop was ignorable, demonstrating the remarkable electrical conductivity of HCM-0.05PANI, which could be supported by the Nyquist plots in Figure 6b. The Cg at different current density obtained from the GCD curves (Figure S10b) further presented the good rate capability of HCM-0.05PANI: Cg of the SSC was 76 F/g at 0.2 A/g, and the Cg retains at 48 F/g at 10 A/g, maintaining 64% of the capacitance at 0.2 A/g. The remarkable rate capability primarily profited by the cross-linked carbon nanofiber structure and rational pore size distribution of HCM-PANI, which could provide remarkably conductive pathway and fast diffusion channels for ion transference during charge-discharge process. Electrochemical impedance spectroscopy (EIS) of HCM and HCM-PANI based device were also performed. As displayed in Figure 6b, the Nyquist patterns consisted of a nearly vertical diagonal at low frequency and a quasi-semicircle at high frequency. The intercept of the semicircle with the real axis refers to the resistance of the system primarily comes from the intrinsic resistance of the electrolyt (RS).42 The diameter of semicircle represents the interfacial charge transfer resistance (Rct).43 And the Rct values were 0.34 Ω, 0.54 Ω, 0.67 Ω, and 0.88 Ω for HCM, HCM-0.03PANI, HCM-0.05PANI, and HCM-0.07PANI, respectively. The smallest Rct for HCM resulted from its good conductivity, hierarchically porous structure, which benefited the rapid ion transference during charge-discharge process. The Rct value increased progressively as the increase of PANI amount due to the lower electroconductivity of PANI compared to carbon. Moreover, the deposition of PANI will partly occupy the pores/channels of HCM, which further increased the interfacial charge transfer



resistance. In spite of this, the Rct values of all HCM-PANI samples were small enough, and the slant line at low frequency region for all HCM-PANI samples were almost vertical just like the HCM, indicating a low ionic diffusion resistance in the electrode materials44, which ensured the realization of high rate capability. The Ragone plots of HCM and HCM-PANI based SSCs were shown in Figure 6e, and the energy density and power density were calculated from the CV dates according to previously reported methods.40, 41 The HCM-0.05PANI based SSC exhibited a high energy density of 8.9 Wh/kg at a power density of 1644 W/kg. The cycling stability of these SSCs were evaluated under the sweep rate of 50 mV/s to be 10000 cycles. As displayed in Figure 6f, the HCM-0.05PANI based SSC retained about 98.5% of the initial specific capacitance after 3000 cycles and a gentle reduction of the capacitance to 90.6% after 10000 cycles, just a little bit less than that of HCM, indicating an excellent cycling stability. The cycling stability of HCM-PANI was comparable or better than many reported carbon-PANI composite based cells.45-47 The improved cycling stability mainly benefited from three factors: continuously interconnected carbon nanofiber structure allows fasting ions inside the whole electrode; homogeneous PANI nanoclusters uniformly deposited over the carbon nanofibers efficiently reduced the volume expansion of PANI caused by doping/dedoping process; uniformly doped nitrogen and oxygen of HCM offered favorable hydrophilicity to benefit the stable deposition of PANI and prevented the PANI from falling off during charge/discharge process. SEM images of the HCM-0.05MPANI after 10000 cycling were presented in Figure S12. The


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morphology of HCM-0.05PANI remained well and the PANI nanoclusters still tightly coated on the carbon nanofiber, while the PANI nanocluster becoming shorter and aggregated because of the depletion under long cycling process. The well-maintained morphology of PANI directly proved the good cycling stability of HCM-0.05PANI. The above results indicated that the HCM-0.05PANI composite materials exhibited good rate capability, long cycling life and high energy density when used as the electrode material of SSC, demonstrating the high efficiency of HCM in the application for hybrid electrode materials of supercapacitors.

Conclusion The hierarchically porous carbon microspheres were constructed by pyrolyzing the chitin nanofibrous microspheres fabricated via a facile and innovative method, which could be employed for the manufacture of different porous materials. By the combining of the compatibility between chitin and chitosan and the dissolubility of chitosan in acid solution, the micro-mesopores generated in the chitin nanofibers of the microspheres, in which chitosan as a pore-forming agent. After carbonization, the HCM with high SSA and hierarchically porous structure was obtained, and the interpenetrating hierarchical pore structure could promote the fast movements of electrolyte ions. When HCM served as the substrate of PANI to construct the HCM-PANI composites for symmetric supercapacitor (SSC), the SSC exhibited remarkable rate capability and cycling durability of 90.6% capacitance retention after 10000 cycles, confirming the high efficiency of HCM in constructing hybrid electrode materials for supercapacitors. Moreover, the method established in this study to obtain



carbon microspheres with porous structure and improved surface area was facile and efficient, which would be applied in the manufacture of other porous materials such as porous membrane and fibers.

Associated Content Supporting Information. Picture and properties of chitin/chitosan solution; SEM images of CCM-A, HCM, and HCM-PANI samples; surface area and pore volumes of CCM-A, HCM, and HCM-PANI samples; High resolution of N 1s; XRD patterns; CV curves and GCD curves of HCM and HCM-PANI; Picture of electrodes; SEM images of HCM-PANI after cycling; elemental analysis.

Acknowledgements This work was financially supported by the Major Program of National Natural Science Foundation of China (No.21334005), the Major International (Regional) Joint Research Project of National Natural Science Foundation of China (No. 21620102004), and the National Natural Science Foundation of China (51602115, and 21620102004). The authors thank to the facility support of the Center for Nanoscale Characterization & Devices, WNLO-HUST and the Analysis and Testing Center, HUST.

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Distinctive Construction of Chitin Derived Hierarchically Porous

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