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Nitrogen-Enriched Carbon Nanofiber Aerogels Derived from Marine Chitin for Energy Storage and Environmental Remediation Beibei Ding, Shasha Huang, Kai Pang, Yongxin Duan, and Jianming Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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Nitrogen-Enriched Carbon Nanofiber Aerogels Derived from Marine Chitin for Energy Storage and Environmental Remediation Beibei Ding, Shasha Huang, Kai Pang, Yongxin Duan and Jianming Zhang* Key Laboratory of Rubber-Plastics, Ministry of Education/ Shandong Provincial Key Laboratory of Rubber-plastics, Qingdao University of Science & Technology, Qingdao 266042, China.

* To whom all correspondence should be addressed. Mailing address: No. 51-1, Wuyang Road, Qingdao 266045, China. Fax: +86-532-84022791 E-mail: : [email protected] (J. Z.)

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ABSTRACT: Nitrogen-enriched (N-enriched) carbon nanofiber aerogels (NCNAs) with ultrafine nanofiber network structure were designed and prepared by using chitin nanofibers aerogel as the precursor. Thanks to the uniform nanofibrous architecture and nitrogen-rich composition of chitin nanofibers aerogel, the NCNAs exhibited large specific surface area (490‒1597 m2 g-1) and a high nitrogen content (2.07‒7.65%). As a consequence, supercapacitor electrodes prepared from NCNA-900 showed specific capacitances as high as 221 F g-1 at the current density of 1.0 A g−1 and good capacitance retention of 92% over 8000 cycles in 6.0 mol L−1 KOH electrolyte without further activation. Moreover, the NCNA-900 could also be applied as an effective adsorbent for dye adsorption, such as Congo red (496 mg g−1) and Rhodanine B (489 mg g−1). In view of the excellent electrochemical performance and high adsorption capacities for dyes, cost-effective and eco-friendly approach, NCNAs derived from marine chitin show great potential for application in energy storage and environmental remediation. KEYWORDS: Chitin nanofibers aerogel; Carbon aerogels; Supercapacitors; Adsorption

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 INTRODUCTION Carbon nanomaterials have shown great potential for the electrochemical energy conversion and storage on account of their unique size/surface-dependent properties.[1,

2]

Tremendous progress has been made in developing carbon nanomaterials (e.g. fullerenes, carbon nanotubes and graphene) for supercapacitors with high performance.[3-5] However, the high-cost process for preparation may hinder their development and commercialization. Nitrogen-doped porous carbons are emerging as attractive materials for energy storage devices due to their outstanding characteristics with high porosity, high nitrogen content and good conductivity.[6-9] Generally, the precursors, such as polymers,[10] ionic liquid,[11] zeolitic imidazolate frameworks,[12,

13]

and biomass[14] have been successfully used to fabricate

nitrogen-doped porous carbon materials. Among those, biomass was considered to be a kind of sustainable and low-cost carbon source for their abundance and renewability in nature. Moreover, the electrode materials for supercapacitors based on biomass are not only being green non-pollution but also showing good cycling performance.[15-24] Chitin is the second most abundant natural polymer after cellulose on earth and mainly derived from marine biomass, such as shrimp and crab shells wastes (the chitin content is as high as 40%).[25-30] It’s composed of β-(1, 4)-linked chains of N-acetyl-D-glucosamine and has the potential to be a promising N-enriched porous carbon for supercapacitors without additional nitrogen source. However, there have been few reports which focused on directly using chitin as carbon sources for producing energy storage materials due to its relatively low porosity.[18, 31, 32] Several studies that have tried to design the chitin-based carbon with welldefined structure for high performance supercapacitors, such as using chitin as soft/hard template or increasing the porosity of chitin precursor for the assembly of mesoporous carbon.[33-37] Unfortunately, these methods mentioned above involved the complicated organic-inorganic hybrid procedure or the dissolution of chitin, which in turn limits the large-

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scale applications. Recently, chitin nanofiber/nanowhisker with large surface area, high aspect ratio and good hydrophilicity has attracted the attention for fabricating hydrogels and aerogels.[38-41] The nanofibrous structure of these hydrogels and aerogels facilitated chitin nanofiber/nanowhisker utilization as catalyst supports and adsorbents. Moreover, the nanofibrillar aerogel prepared from chitin nanofiber/nanowhisker has the potential to be a promising precursor for the N-enriched mesoporous carbon. To the best of our knowledge, there was only one report (by Nogi et al) about nanofibrillar carbon from chitin nanofibers.[42] Until now, no one has made systematical research on carbon aerogels derived from chitin nanofibers aerogel for application in supercapacitors and dyeing wastewater treatment. Here, chitin nanofibers (ChNFs) isolated from marine biomass (crab shells wastes) were used to prepare chitin nanofibers aerogel by hydrogen bond crosslinking without any toxic or corrosive agents. Impressively, the chitin nanofibers aerogel with well-defined morphology was discovered to be very favorable for the formation of three-dimensional carbon aerogels with ultrafine nanofiber networks and mesoporous structure. Moreover, chitin nanofibers aerogel was naturally rich in nitrogen and oxygen, which endowed the prepared NCNAs with good electrochemical properties as an efficient electrode material for supercapacitors. In addition, the adsorption capacities of NCNAs for Congo red and Rhodanine B from water were also evaluated.  EXPERIMENTAL SECTION Materials. The raw chitin powder was purchased from Golden-Shell Biochemical Co. Ltd (Zhejiang, China). The raw chitin powder was purified according to the reported procedure[43], and finally kept in a desiccator for further study. The viscosity-average molecular weight (Mη ) of the purified chitin powder was calculated to be 10.7 × 104 in 5% (w/w) LiCl/DMAc at 25 oC and its degree of deacetylation was determined to be 2% by

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potentiometry.[44] Here, all the reagents were of analytical reagent grade and were used without further purification. Preparation of the Chitin nanofibers Hydrogel and Aerogel. Partially deacetylated chitin (DE-Chitin) were obtained from the deacetylation of α-chitin derived from crab shells wastes in 33 wt% NaOH at 90 oC for 2h.[45] The ChNFs were prepared by ultrasonic exfoliation of the DE-chitin under weak acid conditions (pH=3‒4). And the ultrasonic treatment was conducted in an ice bath for 10 min (20 kHz, JY92-IIDN, Ningbo Scientz Biotechnology Co. Ltd., China). The resulting ChNFs suspension was dialyzed against distilled water and concentrated from 0.5 to 2.5 wt% by rotary evaporation at 40 oC, poured into a 35-mm plastic petri dish, and then converted to robust hydrogel by hydrogen bond crosslinking under the gas phase coagulation of 50 % (v/v) ammonia and ethanol at ambient temperature in a sealed container. The hydrogel was cut using a round punch, 10 mm in diameter, and transformed into the gel containing t-BuOH by solvent exchange. Afterwards, a conventional freezer dryer was employed to give the chitin nanofiber aerogel. Fabrication of NCNAs. Chitin is a natural amino polysaccharide composed of Nacetyl-D-glucosamine units. Thus, the N-enriched carbon derived from chitin can be easily obtained by a simple process of carbonization. Here, chitin nanofibers aerogel with nanofibrous architecture was used as a precursor and carbonized in a quartz tube under N2 atmosphere at a flow rate of 80 cm3 min−1 with a 2 oC min−1 heating rate from 30 to 300 oC and keep the temperature constant for 1h, and then heating up at a rate of 3 oC min−1 to carbonization temperature (500, 700, 900 and 1000 oC) for 2h to obtain the NCNAs (marked as NCNA-500, NCNA-700, NCNA-900 and NCNA-1000, respectively). Characterization. The dynamic rheology experiments were carried out on an ARES G2 dynamic rheometer (TA, USA). Two parallel plates with a gap of 1 mm were used to measure dynamic viscoelastic parameters such as the storage modulus (G′) and loss modulus (G′′) as

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functions of angular frequency (ω). The frequency sweep measurement was conducted in the range from 0.1 to 100 rad s−1 at 20 °C. And the value of the strain amplitude was set at 10%, which is within the linear viscoelastic region. The Zeta potential measurement of ChNFs suspension was examined using a Nano-ZS ZEN3600 (Malvern Instruments, UK) at 25 oC. Transmission electron microscopy (TEM) image of ChNFs was conducted on a JEOL JEM2200 FS instrument with an accelerating voltage of 200 kV. A tiny falling drop of the 0.2% (w/w) ChNFs/water dispersion was mounted on a carbon coated copper grid and the excess liquid was removed by filter paper, following negatively stained with 1% (w/w) sodium phosphotungstate and finally dried naturally by evaporation. Atomic force microscopy (AFM) image of ChNFs was collected by AFM (MultiMode 8, Bruke, U.S.A) with RTESP probes in ScanAsyst mode. The Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a Nicolet 6700 FTIR spectrometer in the region of 400–4000 cm−1 with a 2 cm−1 resolution and an accumulation of 32 scans. Wide angle X-ray diffraction (WAXD) patterns were collected from a Rigaku Ultima Ⅳ diffractometer with Cu Kα radiation (λ = 0.1542 nm). The proportional counter detector was set to collect data at a rate of 1°/min over the 2θ range from 5 to 40°. Morphology of chitin nanofibers aerogel and NCNAs was observed by a JEOL SEM 6700 operating at a voltage of 3 kV. The samples were sputtered with gold for better observation. Nitrogen physisorption measurements were performed on a static volumetric sorption analyzer (ASAP2020, Micrometrics, and USA) at 77 K. The chitin nanofibers aerogel and NCNAs were degassed at 90 oC in vacuum for 6 hours to remove the adsorbed species before the test. The Brunauer–Emmett–Teller (BET) and Non Localized Density Functional Theory (DFT) method were done for analysis of the specific surface area and pore size distribution. The Raman spectra of NCNAs were obtained at a laser wavelength of 532

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nm using a DXR Microscope. And the X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB 250 (Thermo Fisher Scientific, USA). And the peak resolution and fitting were performed by the software of XPSPEAK41. Electrochemical Measurement. The electrochemical measurements were carried out on a CHI 660e electrochemical workstation (Shanghai Chenhua Instruments Co., Ltd) at normal temperature. In a three-electrode system, platinum foil was used as the counter electrode, Hg/HgO electrode as reference electrode, the sample as the working electrode and 6 mol L‒1 KOH was the electrolyte solution. The active mass was about 1.8‒2.0 mg per electrode. Before testing, the prepared electrodes were soaked overnight in the electrolyte. Cyclic voltammetry measurement was performed in the potential range of 0‒1.0 V vs. Hg/HgO by varying the scan rate from 5 to 400 mV s‒1. The galvanostatic charge/ discharge was used to estimate the specific capacitances (Cs) according to the formula of Cs = I × ∆t/(m × ∆V), where I is the constant discharge current, ∆t is the discharge time, ∆V is the potential window during discharge, and m is the active mass. EIS measurements were recorded at 10 mV of amplitude in a frequency range from 0.1 to 100 kHz. Dye adsorption tests. The adsorption tests of NCNAs were carried out for removal of dyes from aqueous solution. Here, Congo red and Rhodanine B supplied by Aldrich were employed for the tests. In each experiment, 1.0 mg of the NCNA-900 was mixed with 10 mL dye solution in a 20 ml glass reagent bottle at 25 oC with an intermittent shake. The initial concentration of Congo red and Rhodanine B was 50 mg L−1. At certain time interval, 2 mL solution was analysed for dye concentration on a spectrophotometer at the wavelength of 497 and 553 nm for Congo red and Rhodanine B, respectively. 

RESULTS AND DISCUSSION

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Scheme 1. Schematic illustration for the preparation of ChNFs, chitin nanofibers hydrogel, chitin nanofibers aerogel and N-enriched carbon nanofiber aerogel (NCNA). The overall fabrication process of ChNFs, chitin nanofibers hydrogel, chitin nanofibers aerogel and NCNAs are illustrated in Scheme 1. ChNFs were obtained by ultrasonic exfoliation of DE-chitin derived from crab shells wastes. The prepared ChNFs suspension was dialyzed against distilled water and concentrated to about 2.5 wt%, and then converted to robust hydrogel by hydrogen bond crosslinking under the gas phase coagulation of 50 % (v/v) ammonia and ethanol. Subsequently, the aerogel was freeze-dried from chitin nanofibers gel containing t-BuOH. Here, the formation of nanofibrous architecture composed of ChNFs in the hydrogel could largely avoid collapse during drying process. Thus, the resulting aerogel maintained the nanofibrillar network structure of hydrogel with a high specific surface area of 308 m2 g-1, which is higher than reported values in the literature for chitin nanofiber/nanowhisker aerogels so far.[38-40, 42] It's worth mentioning that the degree of Nacetylation of purified chitin, DE-chitin and chitin nanofibers aerogel were evaluated from the ratio of A1560/1030 in the FT-IR spectra to be 96%, 90% and 88% (Figure S1), which were higher than those reported by Fan.[46] Hence, the deacetylation and hydrogen bond crosslinking process had little effect on the chemistry and crystal structure of α-chitin, which

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is also confirmed by XRD analysis (Figure S2). Finally, the chitin nanofibers aerogel was employed as the precursor to fabricate NCNAs by a simple process of carbonization without additional nitrogen source. The NCNAs consisted of ultrafine nanofiber network are benefit for electron transport with a reduced internal resistance.

Figure 1. TEM image (a) and AFM image (b) of individual ChNFs by the ultrasonic exfoliation. The morphology of ChNFs was characterized by TEM and AFM (Figure 1a and b), showing that the individual ChNFs had a high aspect ratio, with a uniform width of 10 to 20 nm and about 510 nm in length. Moreover, ChNFs was positively charged, and therefore, showing good stability dispersed in water as a result of electrostatic repulsions. The Zeta potential of ChNFs suspension under neutral condition was determined to be +30 mV. Further, the frequency sweep measurement for viscoelastic properties of ChNFs suspensions and chitin nanofibers hydrogel was conducted at 20 oC. As shown in Figure 2, the storage modulus (G') was higher than the loss modulus (G'') for ChNFs suspension even at a low concentration of 0.5 wt% in spite of good fluidity, which indicated a typical feature of gels.[40, 41]

And the storage modulus (G') increased apparently when the suspension had a high ChNFs

content and with further cross-linking treatment. The values of G' at low frequency were 0.1 and 40 Pa with the ChNFs concentration of 0.5 and 2.5 wt%, and then reached 1050 Pa for the chitin nanofibers hydrogel. The schematic diagram for the formation of chitin nanofibers

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hydrogel was illustrated in Scheme 2. The positively charged ChNFs suspension was neutralized by dialysis and then producing hydrogen-bonding crosslinks among ChNFs. Afterwards, ChNFs were deprotonated and cross-linked under the gas phase coagulation of 50 % (v/v) ammonia and ethanol to give a robust hydrogel with more cross-linking points.

G' G'', Pa

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10

1

10

0

10

-1

10

-2

o

G' G'' 20 C a b c 0.1

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-1

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ω, rad s

Figure 2. Storage modulus G' and loss modulus G'' as a function of angular frequency ω at 20 oC for ChNFs suspension with the concentration of 0.5 wt% (a), 2.5 wt% (b) and the chitin nanofibers hydrogel with further cross-linked by hydrogen-bonding (c).

Scheme 2. Schematic diagram for the formation of robust chitin nanofibers hydrogel from ChNFs by hydrogen bond crosslinking. Chitin as a renewable amino polysaccharide can be a promising precursor for costeffective and pure N-enriched carbon without cumbersome pretreatment to introduce heteroatoms. Here, chitin nanofibers aerogel with nanofibrous architecture freeze-dried from

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the prepared hydrogel was used as a precursor for fabricating NCNAs. In view of the importance of carbonization temperature of chitin nanofibers aerogel on the nitrogen contents and pore structures, it’s necessary to optimize the carbonization process.[6, 47] Thus, the chitin nanofibers aerogel was carbonized at 500, 700, 900, and 1000 oC in N2 atmosphere. The SEM images of NCNAs (NCNA-500, NCNA-700, NCNA-900 and NCNA-1000) are shown in Figure 3a and Figure S3. It was clearly to observe that the NCNAs maintained the original nanofibrous architecture of chitin nanofibers aerogel and showed a large amount of interstitial pores. The obtained TEM image of the NCNA-900 was consistent with the observation from SEM, demonstrating that the nanofibrillar network of NCNAs was composed of ultrafine nanofiber network less than 30 nm in width (Figure 3b). As known, a large accessible surface area and suitable pore size play important roles in improving the capacitive properties. The nitrogen adsorption/desorption isotherms and pore size distribution of chitin nanofibers aerogel and NCNAs were studied and the results are shown in Figure 3c, d. Apparently, chitin nanofibers aerogel exhibited a typical type IV isotherm, whereas the NCNAs showed the isotherms close to type IV with unclosed curves mainly on account of insufficient pressure to achieve microporous adsorption/desorption. The pore size distribution was calculated using NLDFT model. It is observed that NCNA-900 and NCNA-1000 presented the multistage aperture distribution composed of abundant mesoporous structures and a certain amount of micropores. In comparison, chitin nanofibers aerogel, NCNA-500 and NCNA-700 contained relatively less micropores and mesopores. To further demonstrate the existence of micropores in the NCNA with a higher carbonization temperature, the nitrogen adsorption/desorption analysis of NCNA-900 was performed under higher pressure. As shown in Figure S4, a sharp rise of isotherm at low relative pressure (p/p00.999) suggest that the adsorption processes conform to the pseudo second-order kinetic model. And the equilibrium adsorption quantities of NCNA-900 for Congo red and Rhodanine B removal were estimated to be 496 mg g−1 and 489 mg g−1, respectively, which were comparable to the commercial activated carbon and higher than most carbons derived from various biomass resources in the literature.[54-56] The high adsorption capacities for Congo red and Rhodanine B of the NCNA was mainly because of the large surface area with hierarchical porous systems that can provide more available adsorption sites for dyes. Additionally, good wettability of the NCNA by the contribution of heteroatom doping is also essential to improve the adsorption capacity for water-soluble dyes. And the comparison of adsorbent

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capacity of Congo red and Rhodanine B on various carbons are listed in Table S1 and Table S2. Moreover, chitin is the most widespread amino polysaccharide and mainly exists in shrimp and crab shell wastes. Thus, the NCNAs derived from chitin nanofibers aerogel have attractive applications as highly efficient adsorbents in printing and dyeing wastewater

-1

treatment. Adsorption capacity, mg g

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500 Congo red Rhodamine B

400 300 200 100 0

0

10

20 30 Time, h

40

50

Figure 6. Time dependent adsorption capacities (a) and the pseudo-second-order plots (b) of the NCNA-900 for Congo red and Rhodamine B. The insert shows the photographs before and after adsorption. 

CONCLUSIONS In summary, the chitin nanofibers aerogel with the nanofibrous architecture fabricated

by hydrogen bond crosslinking of ChNFs suspension was a promising precursor for the NCNAs. The unique traits of chitin nanofibers aerogel with high specific surface area (308 m2 g−1) and being naturally rich in nitrogen endowed the NCNA-900 with ultra-thin nanofiber network and large specific surface area over 1121 m2 g−1. Thus, the NCNA-900 showed high electrochemical capacitance of 221.0 F g−1 at a current density of 1.0 A g−1, an outstanding cycling stability over 8000 cycles and good retention capability of 92% at a current density of 5.0 A g−1. Moreover, the NCNA-900 also exhibits good adsorption capacities for Congo red (496 mg g−1) and Rhodanine B (489 mg g−1). Hence, the facile

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preparation of NCNAs with good electrochemical performance and high adsorption capacities for dyes has provided a new and powerful way to construct the promising materials from chitin wastes for energy storage and environmental remediation.

 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

 AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax: +86 532 84022791; Tel: +86 532 84022604 Notes The authors declare no competing financial interest.

 ACKNOWLEDGEMENTS This work was supported by the financial support from National Natural Science Foundation of China (51573082) and China Postdoctoral Science Foundation (04000623) is greatly appreciated.

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Nitrogen-enriched carbon nanofiber aerogels derived from marine chitin with good electrochemical performance and high adsorption capacities for dyes.

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