Tuning the microstructure and electrochemical behavior of lignin-based ultrafine carbon fibers via hydrogen-bonding interaction

Tuning the microstructure and electrochemical behavior of lignin-based ultrafine carbon fibers via hydrogen-bonding interaction

BIOMAC-14016; No of Pages 9 International Journal of Biological Macromolecules xxx (xxxx) xxx Contents lists available at ScienceDirect Internationa...

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BIOMAC-14016; No of Pages 9 International Journal of Biological Macromolecules xxx (xxxx) xxx

Contents lists available at ScienceDirect

International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac

Tuning the microstructure and electrochemical behavior of lignin-based ultrafine carbon fibers via hydrogen-bonding interaction Shichao Wang a,⁎, Mugaanire Tendo Innocent b, Jianyu Chen a, Qianqian Wang b,⁎, Wujun Ma c, Jianguo Tang a,⁎ a b c

Institute of Hybrid Materials, College of Materials Science and Engineering, Qingdao University, Qingdao 266071, China State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China School of Chemistry, Biology and Material Engineering, Suzhou University of Science and Technology, Suzhou 215009, China

a r t i c l e

i n f o

Article history: Received 8 October 2019 Received in revised form 28 November 2019 Accepted 29 November 2019 Available online xxxx Keywords: Lignin Ultrafine carbon fibers Electrochemical behavior

a b s t r a c t Hardwood Kraft lignin (HKL)-based ultrafine carbon fibers with different pore structures and properties were prepared by controlling the intermolecular interaction between HKL and incorporated poly(ethylene glycol)block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) triblock copolymer. The thermal properties of HKL-based ultrafine fibers together with the morphology and pore structures of HKL-based ultrafine carbon fibers were extensively investigated with DSC, TG, SEM, BET, DLS and HRTEM to provide comprehensive knowledge on the effect of added PEG-PPG-PEG on the properties of obtained fibers. Results suggested that addition of PEG-PPG-PEG increased the thermal stability of HKL and promoted the formation of graphite crystallites in HKL-based ultrafine carbon fibers via enhanced intermolecular hydrogen bonding interactions. The electrochemical behavior of HKL-based ultrafine carbon fibers with different PEG-PPG-PEG contents were also characterized to expand their potential application in electrochemical capacitors. All the HKL ultrafine carbon fibersbased electrodes showed good capacitive behavior and stability. Besides, the specific capacitance of HKL-based ultrafine carbon fibers can be significantly tuned by the addition of PEG-PPG-PEG. © 2019 Elsevier B.V. All rights reserved.

1. Introduction One-dimensional nanostructured carbon materials have attracted much attention due to their unique structures, functionalities, and array of intriguing applications [1–3]. Continuous ultrafine carbon fibers with large surface area, which can be prepared by electrospinning or centrifugal spinning methods followed by the stabilization and carbonization processes, are among such a structure [4,5]. It is worth mentioning that ultrafine carbon fibers with bimodal porosity containing micropores and either meso- or macro-pores present a unique advantage in flexible supercapacitors, owing to the fact that actual energy storage occurs predominately in the smaller micropores where the bulk of the surface area lies while the larger pores provide fast masstransport of electrolytes to and from the micropores [6–9]. The porous structure in ultrafine carbon fibers can be obtained through the activation process or template method. Normally, the activation process involves the high temperature treatment of fibers in the presence of an activation agent. However, surface oxygen functionalities, which are unfavorable for the conductivity, can be formed and additional steps of neutralization and washing are also needed [9]. For the

⁎ Corresponding authors. E-mail addresses: [email protected] (S. Wang), [email protected] (Q. Wang), [email protected] (J. Tang).

template method, templates can be classified into two classes, namely hard templates and soft templates. As a common template, silica presynthesized through the use of certain surfactant templates has been intensively explored for the preparation of porous carbon fibers with controlled pore structure [10,11]. However, many draw backs still exist in the nanocasting method. For example, the extra step for the preparation of scaffolds and etching of siliceous templates employed with toxic regents [12]. In addition, the mechanical properties of obtained carbon fibers deteriorate after etching. Different from hard-templating method, the one-pot soft-templating method often use thermally unstable soft templates, which can easily be removed from the starting composites. Poly(ethylene oxide) (PEO), polypropylene (PP), poly(lactic acid) (PLA), thermoplastic polyurethane (TPU) and recently bio-based poly (ethylene terephthalate) (PET) are used as sacrificial polymers to produce porous carbon fibers [13–16]. Similar with the above-mentioned polymers, the commercially available and inexpensive Pluronic block copolymers with a relatively high molecular weight are also extensively used as templates to prepare various pore structures [10,17,18]. Dai et al. examined the uses of three phenolic resins which formed single, double, and triple hydrogen bond interactions to the polyethylene oxide chains of Pluronic F127, and found that the high hydroxyl density in the oligomers formed from phloroglucinol provided a large driving force for self-assembly interaction with the PEO blocks [10]. This work paves a new way for the regulation of pore structure via hydrogenbonding interaction.

https://doi.org/10.1016/j.ijbiomac.2019.11.235 0141-8130/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: S. Wang, M.T. Innocent, J. Chen, et al., Tuning the microstructure and electrochemical behavior of lignin-based ultrafine carbon fibers via h..., , https://doi.org/10.1016/j.ijbiomac.2019.11.235

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Similar with phloroglucinol, amounts of hydroxyl groups exist in the chemical structure of lignin [19]. As the largest biomass source with aromatic properties and a carbon content above 60%, lignin is considered as the most attractive sustainable precursor for carbonaceous material [20]. Low cost, bio-renewable and minimal environmental effects make lignin even attractive in the field of carbon fibers [4,21,22]. The chemical structure of lignin is complex with p-coumaryl (H), coniferyl (G), and sinapyl alcohol (S) as the basic units and arylglycerol-β-aryl ether (β-O-4), pinoresinol (β-β), phenylcoumaran (β-5), and diphenylethane (β-1) as the main linkages capable of forming numerous hydrogen-bonds with poly(ethylene glycol) (PEG) chains of triblock copolymer [23]. In addition, the residual carbohydrates in lignin can also form microporous structures during the carbonization process. In this work, we combined the advantage of lignin in pore formation with the self-assembly of triblock copolymer in an effort to prepare ultrafine carbon fibers with controlled pore structure, and further explore their potential application in energy storage field. The effects of incorporated triblock copolymer with a low molecular weight on the thermal properties of lignin-based ultrafine fibers together with the morphology and pore structures of lignin-based ultrafine carbon fibers were extensively investigated by controlling the intensity of the formed hydrogen-bonding interactions. 2. Material and methods

into the solution for the formation of consistent ultrafine fibers [25]. After PEO was well dispersed in HKL solution, triblock copolymer PEGPPG-PEG with different contents (0 wt%, 0.5 wt%, 1.0 wt% and 1.5 wt% based on the solid weight of HKL) was incorporated into the mixed solution and further stirred for another 12 h. After being cooled to room temperature, the solution mixtures were placed in a 1 mL cylinder fitted with a 25 G needle as a spinneret and electrospun using a horizontal electrospinning device under 15 kV operating voltage with a propulsion speed of 0.3 mm/min and a collecting distance of 15 cm. The electrospinning temperature was 40 °C with a relative humidity of 60%. Each sample was electrospun for 4 h. The collected ultrafine fibers were named as HKL/PEO, HKL/PEO/PEG-PPG-PEG0.5, HKL/PEO/PEGPPG-PEG1.0 and HKL/PEO/PEG-PPG-PEG1.5, respectively. 2.3. Stabilization and carbonization of HKL-based ultrafine fibers HKL-based ultrafine carbon fibers were prepared after the preoxidation and carbonization processes of HKL/PEO/PEG-PPG-PEGn (n = 0, 0.5, 1.0 or 1.5) electrospun fibers in a tube furnace (OTF1200X-II, HF-Kejing, China). The as-spun HKL-based fibers were thermally stabilized from 60 °C to 270 °C with a heating rate of 0.15 °C/ min and held isothermally for 1 h under air atmosphere. Afterward, the stabilized HKL-based ultrafine fibers were further heated to 900 °C with a heating rate of 3 °C/min and carbonized for another 1 h under Ar gas atmosphere.

2.1. Materials 2.4. Property and morphology characterization Hardwood Kraft lignin (HKL, Klason lignin content, 97.1%; Ash content, 1.1%) was kindly provided by Suzano Papel e Cellulose S.A. Corp. (Brazil) and used after the purification through a 15 kDa ceramic membrane [24]. N, N-Dimethylformamide (DMF) was supplied by Sinopharm Chemical Reagent Co., Ltd. (China). Polyethylene oxide (PEO, Mv ~1 × 106 g/mol) and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG, Mn ~2900) were purchased from Sigma-Aldrich. All chemicals were used as received without further purification. 2.2. Preparation of HKL/PEO/PEG-PPG-PEG ultrafine fibers The entire preparation procedure of HKL/PEO/PEG-PPG-PEG ultrafine fibers is illustrated in Fig. 1. HKL (2.0 g) was first dissolved in 4 mL of DMF under magnetic stirring with a speed of 1000 rpm at 80 °C. Then 2.0 wt% PEO on the basis of HKL solid weight was added

The thermal properties of HKL-based ultrafine fibers were characterized using differential scanning calorimetry (DSC, TA Q20, USA) and thermogravimetric analyzers (TG, 209F1 Iris, Germany). DSC experiments were conducted in nitrogen atmosphere with a flow of 50 mL/ min. Samples were first heated from 30 °C to 250 °C with a heating rate of 10 °C/min and kept isothermally for 2 min prior to being quenched to −30 °C. DSC thermograms were recorded by further increasing the temperature to 250 °C at a heating rate of 10 °C/min. For TG analysis, the samples were heated from 30 °C to 1000 °C at 10 °C/ min under continuous nitrogen flow rate of 50 mL/min. The morphologies of HKL-based ultrafine carbon fibers were characterized using field emission scanning electron microscopy (FESEM, Sigma 500, Germany). The diameters of HKL-based ultrafine carbon fibers were reported as the average of 100 measurements per sample. The pore structure and graphitization degree of prepared ultrafine

Fig. 1. Schematic for the preparation of HKL-based ultrafine carbon fibers.

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carbon fibers were measured using transmission electron microscope (TEM, FEI Talos F200i, USA). The presence and distribution of C, N, O, S and Na elements were also measured by TEM mapping. The pore textural characteristics of all the HKL-based ultrafine carbon fibers were measured volumetrically with a Micromeritics ASAP 2460 analyzer by N2 adsorption/desorpotion at liquid-N2 temperature (77 K). The specific surface area (SBET) and total volumes (Vtot) were calculated by the Brunauer-Emmett-Teller (BET) and BJH method. The micropore surface areas (Smicro) and micropore volumes (Vmicro) were obtained by the t-plot method. Dynamic light scattering (DLS) (BI-200SM, Brookhaven) was performed to determine the mean size and size distribution of the formed micelles. Each measurement was carried out in triple at 25 °C using a laser at 532 nm, and an average value was reported. Electrochemical properties of HKL-based ultrafine carbon fibers were measured in a three-electrode cell using a CHI 660E electrochemical workstation (Shanghai Chenhua, China). Each fiber mat with a dimension about 0.5 cm × 1.0 cm was clapped to a Pt electrode and was used as free-standing working electrode. Pt sheet and Hg/Hg2SO4 electrode were used as counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) tests were conducted at scan rates of 5 mV s−1, 10 mV s−1, 20 mV s−1, 50 mV s−1 and 100 mV s−1 in a 0.5 M H2SO4 electrolyte. The capacitance of the working electrode was calculated from a full cycle of the CV curve by Eq. (1) [26]: C¼

1 2νΔU

Z j I j ⅆU

decreased to 152.3 °C when only 0.5 wt% of PEG-PPG-PEG was incorporated, and the value further dropped to 143.7 °C when the content of PEG-PPG-PEG in HKL-based ultrafine fibers reached to 1.5 wt%. The relationship between Tg values of HKL-based ultrafine fibers and PEGPPG-PEG contents is depicted in Fig. 2(b). It can be found that a near linear relationship existed between Tg values of HKL/PEO/PEG-PPG-PEGn fibers and the content of incorporated triblock copolymer within a certain range. This phenomenon suggests that the amount of movable molecular segments of HKL at a relatively low temperature increased with the increasing content of PEG-PPG-PEG. The addition of triblock copolymer PEG-PPG-PEG decreased the steric hindrance and intramolecular hydrogen-bonding interaction of HKL and acted as a plasticizer. Thus, with the increasing content of PEG-PPG-PEG, more HKL molecular segments become moveable. And when the number of moveable segments increased to a certain quantity, the Tg value of HKL/PEO/PEG-PPG-PEGn ultrafine fibers at a relatively low temperature was detected. The TG and DTG curves of HKL/PEO/PEG-PPG-PEGn ultrafine fibers are shown in Fig. 3(a) and (b), respectively. The thermal decomposition temperature (Td, 5% weight loss temperature), maximum decomposition temperature (Tmax, DTG peak temperature), residual char at 1000 °C (Wf), and activation energy of decomposition (Ea) obtained from these curves are calculated to estimate the influence of triblock copolymer on the thermal stability of HKL, as listed in Table 1. The Ea values of HKL-based ultrafine fibers were calculated by the integral method proposed by Horowitz and Metzger, as follows [27]: 

ð1Þ

where C, υ, U, ΔU and I are the capacitance, scan rate, potential, potential window and current, respectively. 3. Results and discussion 3.1. Thermal properties of HKL/PEO/PEG-PPG-PEGn ultrafine fibers The thermal properties of HKL-based ultrafine fibers, which should be fully considered during the thermal stabilization and carbonization processes, are largely affected by the content of incorporated triblock copolymer. Fig. 2 (a) shows the DSC curves of HKL-based ultrafine fibers with different content of PEG-PPG-PEG. Due to the fact that HKL is associated with an amorphous structure, aromatic in nature with complex connectivity, no melting peak was identified during the heating process. Instead, each sample exhibited a distinctive glass transition temperature (Tg), which varied with the content of incorporated triblock copolymer. For HKL/PEO fibers, the large steric hindrance caused by aromatic structure and strong intramolecular hydrogen bonding interaction led to a relatively higher Tg (158.6 °C). However, the Tg of HKL/PEO fibers

3

ln

 ln

W0 WT

 ¼

Ea θ RT 2max

ð2Þ

where W0 and WT are the initial and residual weights of the samples at a temperature T respectively. R is the gas constant, and θ = T − Tmax. The activation energy was obtained from the slope of the plot of ln [ln (W0/ WT)] versus θ for the main stage of thermal degradation. It can be found that all the samples presented a similar degradation process, and the incorporation of PEG-PPG-PEG improved the thermal stability of HKL/PEO ultrafine fibers to some extent. For example, after 1.5 wt% of PEG-PPGPEG was added into HKL/PEO composites, the Td, Tmax and Ea increased by 4.4 °C, 4.0 °C and 5.3 kJ/mol, respectively. The thermal decomposition of HKL-based ultrafine fibers started with a slight weight loss between 100 °C and 150 °C, which was assigned to the loss of bound water [28]. When the temperature reached to 300 °C, the residue mass of all the samples almost remained constant regardless of the PEG-PPG-PEG contents. Thermal decomposition after 300 °C involved scission of interunit linkages between HKL monomers and the main molecular chains of PEG-PPG-PEG, leading to the release of monomeric phenols, carbon dioxide and water, as well as the methanol caused by the cleavage of methyl-aryl ether bonds

Fig. 2. (a) DSC curves of HKL-based ultrafine fibers with different PEG-PPG-PEG contents. (b) Relationship between Tg values of HKL/PEO/PEG-PPG-PEGn fibers and PEG-PPG-PEG contents.

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Fig. 3. TG (a) and DTG (b) curves of HKL/PEO/PEG-PPG-PEGn ultrafine fibers.

[29,30]. Therefore, the residual mass of HKL/PEO fibers at 400 °C was higher than that of HKL/PEO/PEG-PPG-PEGn fibers due to the degradation of PEG-PPG-PEG. Degradation continued due to breakdown of methoxyl groups, decomposition, and condensation of aromatic rings at temperatures higher than 400 °C. It is interesting to note that the residual weights of HKL/PEO/PEG-PPG-PEGn fibers at 1000 °C were higher than that of HKL/PEO fibers. Take HKL/PEO/PEG-PPG-PEG1.5 for example, the Wf was 36.6%, which was higher than 33.3% of HKL/PEO fibers. Additionally, the degraded weights between 500 °C and 1000 °C for HKL-based ultrafine fibers with the triblock copolymer content of 0 wt %, 0.5 wt%, 1.0 wt% and 1.5 wt% were 14.3%, 12.9%, 9.8%, and 9.0%, respectively. This phenomenon suggests that incorporation of PEG-PPGPEG increased the thermal stability of HKL, which can be explained as follows. When PEG-PPG-PEG was added into HKL, hydrogen bonds were formed between -OH from HKL and -C-O-C- from PEG-PPG-PEG thereby increasing the amounts of formed hydrogen bonds as the content of incorporated triblock copolymer increased. The strong hydrogen bond interactions closed the distance between HKL molecular chains, leading to a more condensed structure and a relatively high thermal stability. 3.2. Morphology and pore structure of HKL-based ultrafine carbon fibers Due to the low molecular of HKL, the spinnability of HKL was relatively poor, leading to the fact that electrospun fibers from HKL are difficult to form without the assistance of PEO. After the incorporation of PEG-PPG-PEG triblock copolymer, the spinnability of HKL/PEO composite significantly decreased. As the content of added triblock copolymer exceeded 2 wt%, the fibers could not be formed. After thermal stabilization and carbonization processes, HKL-based ultrafine carbon fibers were obtained (Fig. 4). As shown in Fig. 4(a), (d), (g) and (j), all the obtained fibers were not stuck together regardless of the PEG-PPG-PEG content. However, the surface morphology and diameter of HKL-based ultrafine carbon fibers were different. For HKL/PEO-based carbon fibers, the surface was relatively smooth (inset in Fig. 4(b)) and the diameter was normally distributed with an average diameter of 1034 ±

146 nm. When 0.5 wt% of PEG-PPG-PEG was incorporated into HKL/ PEO, the shape of prepared fibers became irregular, and the average diameter changed to 1008 ± 235 nm. Besides, many voids caused by the volatilization of PEG-PPG-PEG appeared on the surface of carbonized fibers, as shown in the inset of Fig. 4(e). Further increase in the content of PEG-PPG-PEG to 1.0 wt%, larger pores began to appear on the surface of obtained carbon fibers, as marked by red circles in Fig. 4(h). The surface outside the pores was as smooth as that of HKL/PEO-based ultrafine carbon fibers. When the content of added triblock copolymer was up to 1.5 wt%, more voids with large size combined with micropores appeared on the surface of HKL/PEO/PEG-PPG-PEG1.5- based ultrafine carbon fibers. Although various pores with varying sizes in HKL-based ultrafine carbon fibers were observed in the SEM micrographs, their effective surface area and pore volumes could not be calculated. To further investigate the effects of PEG-PPG-PEG on the pore structure of HKL-based ultrafine carbon fibers, N2 adsorption/desorption isotherms of various HKL/PEO/PEG-PPG-PEGn-based ultrafine carbon fibers were tested and shown in Fig. 5(a). The corresponding porous structure parameters obtained from adsorption/desorption isotherms are listed in Table 2. For samples with the PEG-PPG-PEG content below 1.0 wt%, the N2 sorption-desorption isotherms of HKL-based ultrafine carbon fibers were of type I, which was typical of a microporous structure. However, when the content of PEG-PPG-PEG was increased to 1.5 wt%, hysteresis appeared near P/P0 = 0.4, suggesting that some mesopores were formed in HKL/PEO/PEG-PPG-PEG1.5-based ultrafine carbon fibers. The specific surface (SBET), micropore surface areas (Smicro), total pore volumes (Vtot) and micropore volumes (Vmicro) of HKL-based ultrafine carbon fibers were increased after the incorporation of PEG-PPG-PEG, and the ideal values were obtained at 0.5 wt% of PEG-PPG-PEG. At this condition, the values of SBET, Smicro, Vtot and Vmicro were 903.0 m2/g, 850.1 m2/g, 0.35 cm3/g and 0.32 cm3/g, respectively. To further explain the relationship between the pore size and the content of incorporated triblock copolymer, the aggregation structures of PEG-PPG-PEG in DMF were characterized using dynamic light scattering method, as shown in Fig. 5(b). It can be found that many micelles

Table 1 Thermal data obtained from TG and DTG curves.

Td (°C) Tmax (°C) Mass at 300 °C (%) Mass at 400 °C (%) Mass at 500 °C (%) Wf (%) Ea (kJ/mol)

HKL/PEO

HKL/PEO/PEG-PPG-PEG0.5

HKL/PEO/PEG-PPG-PEG1.0

HKL/PEO/PEG-PPG-PEG1.5

248.4 368.4 90.0 58.9 47.6 33.3 56.3

249.8 364.2 89.7 58.0 47.0 34.1 58.4

250.0 371.5 90.0 55.6 45.4 35.6 61.7

252.8 372.4 90.1 55.5 45.6 36.6 61.6

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Fig. 4. SEM images of HKL-based ultrafine carbon fibers with different PEG-PPG-PEG contents. (a), (d), (g), (j) and (b), (e), (h), (k) are the overall morphology and corresponding enlarged view of HKL/PEO/PEG-PPG-PEGn-based ultrafine carbon fibers with the PEG-PPG-PEG content of 0 wt%, 0.5 wt%, 1.0 wt% and 1.5 wt%, respectively. (c), (f), (i) and (l) are the statistical diameter distributions of (a), (d), (g) and (j), respectively.

with an average diameter of 42.9 nm and a polydispersity index (PDI) of 0.856 were formed when only 0.5 wt% of PEG-PPG-PEG based on the solid weight of HKL was incorporated into DMF. And the mean diameter of formed micelles increased to 598.2 nm (PDI = 0.411) when 1 wt% of PEO was added to the solution. Further increase the content of PEGPPG-PEG to 1.0 wt%, the average diameter of formed micelles increased to 1458.0 nm (PDI = 0.323), which was 2.4 times compared with that of PEG-PPG-PEG0.5/PEO solution. Due to the fact that the color of HKL was relatively dark, which can affect the results of measured size, the DLS results of HKL/PEO/PEG-PPG-PEG composites were not presented here. Based on the results of SEM, BET and DLS, we found that the content of PEG-PPG-PEG had a great influence on the pore size of HKL-based ultrafine carbon fibers. Despite the influence of PEO added in HKL, we explained the possible reason as follows: when the concentration of PEGPPG-PEG in HKL solution is relatively low, the PEG-PPG-PEG molecular chains dispersed in HKL molecular chains volatized after carbonization process, leading to the formation of micropores in HKL-based ultrafine carbon fibers. Further increase in the content of triblock copolymer, the PEG-PPG-PEG chains self-assembled into micelles with relatively large size, leading to the formation of mesopores after the carbonization process, as illustrated in Fig. 5(c). When the content of PEG-PPG-PEG

reached to a certain level, the formed micelles began to aggregate and large voids appeared via the volatilization of aggregated micelles. The pore size and crystal structure of HKL-based ultrafine carbon fibers were also investigated using TEM to give a better understanding of the effect of PEG-PPG-PEG on the properties of HKL-based ultrafine carbon fibers. The elemental mappings of HKL/PEO-based ultrafine carbon fibers are depicted in Fig. 6(a). It can be found that many elements, including N, O, S and Na, still existed in the carbonized fibers with relatively high contents even after the carbonization process, suggesting that HKL ultrafine fibers were not fully carbonized. Normally, the not fully carbonized fibers are hard to form graphitic structure. Fig. 6 (b) and (c) show the TEM images of HKL/PEO-based ultrafine carbon fibers and HKL/PEO/PEG-PPG-PEG1.5-based ultrafine carbon fibers, respectively. Comparing Fig. 6(b) and (c), we found that pores with a relatively large size (white dots in Fig. 6(c)) appeared when 1.5 wt% of PEG-PPG-PEG was added into HKL/PEO composite, and the distribution of observed pores was relatively uniform. In addition, many dark areas were found in the TEM images of HKL/PEO/PEG-PPG-PEG1.5based ultrafine carbon fibers. To find out the exact reason, we enlarged the region marked with a red dotted line and showed the HRTEM images in Fig. 6(d). The HRTEM image indicated a lattice distance of

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Fig. 5. (a) N2 adsorption/desorption isotherms of HKL/PEO/PEG-PPG-PEGn-based ultrafine carbon fibers. (b) Dynamic light scatting size distribution of PEG-PPG-PEG/PEO composites in DMF at 25 °C. (c) Schematic of the formation of pore structures.

0.342 nm, corresponding to the interplanar spacing of (002) plane for hexagonal graphite. This result suggests that the addition of PEG-PPGPEG triblock copolymer promoted the formation of graphite crystallites for HKL-based ultrafine carbon fibers. Raman spectroscopy was further performed to provide crystallographic information on the HKL/PEO/PEG-PPG-PEGn-based ultrafine carbon fibers as a function of precursor composition, as shown in Fig. 7(a). Two characteristic peaks with intensity maxima at ~1345 cm−1 (D-band, in the hybridized sp3 valence state) and ~1585 cm−1 (G-band, in the hybridized sp2 valence state) were found in all the spectra. Ratios of the intensities of D-band to G-band (ID/IG) are shown in Table 3. Although very similar values were obtained for HKL/PEO/PEG-PPG-PEGn-based ultrafine carbon fibers, the addition of triblock copolymer decreased the value of ID/IG. Lower ID/IG values suggest higher graphitization and alignment of the graphitic planes in carbonaceous material. The graphitic crystallite domain size (La) can be estimated by the following empirical formula proposed by Knight and White [15,31], and the calculated La are also listed in Table 3. La ¼ 4:4=ðID =IG Þ

ð3Þ

It can be found that the La values were slightly increased with the increasing content of PEG-PPG-PEG, indicating larger graphite crystallites were formed after the addition of triblock polymer, which was

Table 2 Porous structure parameters of HKL/PEO/PEG-PPG-PEGn-based ultrafine carbon fibers. Samples

SBET (m2/g)

Smicro (m2/g)

Vtot (cm3/g)

Vmicro (cm3/g)

D (nm)

HKL/PEO HKL/PEO/PEG-PPG-PEG0.5 HKL/PEO/PEG-PPG-PEG1.0 HKL/PEO/PEG-PPG-PEG1.5

779.5 903.0 883.9 860.5

731.2 850.1 837.6 813.2

0.30 0.35 0.34 0.35

0.28 0.32 0.32 0.31

1.56 1.55 1.52 1.63

consistence with the result of HRTEM. A schematic diagram is illustrated in Fig. 7(b) to explain the possible reason. The addition of PEG-PPG-PEG decreased the distance between HKL molecular chains via the hydrogen-bonding interaction. During the carbonization process, the molecular chains of PEG-PPG-PEG volatilized and the aromatic rings from adjacent HKL molecular chains combined together via free radicals. Thus, a relatively condensed structure was formed, which attributed to the formation of graphite crystallites during the carbonization process. 3.3. Electrochemical behavior of HKL-based ultrafine carbon fibers The electrochemical performances of HKL/PEO/PEG-PPG-PEGnbased ultrafine carbon fibers were studied using a three-electrode electrochemical system with 0.5 M H2SO4 as electrolyte, and their representative CV curves recorded at scan rates of 5 mV s−1 to 100 mV s−1 with a potential window of 0.3 V to 0.8 V are shown in Fig. 8(a–d). As can be seen, all samples exhibited typical quasi-rectangular shapes, indicating a typical characteristic of a double-layer charge/discharge. The gravimetric capacitances of the electrodes as a function of scan rates are presented in Fig. 8(e). Contrary to what we have expected, the addition of PEG-PPG-PEG played an adverse effect on the electrochemical performance of HKL/PEO/PEG-PPG-PEGn ultrafine carbon fibers based electrodes. With the increase of PEG-PPG-PEG concentration, the capacitances of HKL/PEO/PEG-PPG-PEGn ultrafine carbon fibers based electrodes showed a tendency to decrease rapidly and then increase slightly. HKL/PEO-based ultrafine carbon fibers achieved the highest gravimetric capacitances of 10.2 F g−1 under the scan rate of 5 mV s−1, which was 4.2 times larger than that of HKL/PEO/PEG-PPGPEG0.5 ultrafine carbon fibers based electrode. The unexpected results can be attributed to the change of pore structures of HKL-based ultrafine carbon fibers. Usually, the addition of PEG-PPG-PEG increased the pore volumes of HKL/PEO-based ultrafine carbon fibers, which should improve their capacitances. However, things changed when the pore

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Fig. 6. (a) Elemental mapping images of HKL/PEO-based ultrafine carbon fiber. HRTEM images of HKL/PEO (b) and HKL/PEO/PEG-PPG-PEG1.5-based ultrafine carbon fibers (c). (d) Partial enlargement of (c).

sizes decreased to a certain degree. As proposed by Simon and Gogotsi et al., the capacitance showed an abnormal increase in the double layer capacitance when the pore diameter was less than a critical value (approximately 1.5 nm) [32], which challenged the long-held

axiom that pores smaller than the size of solvated electrolyte ions were incapable of contributing to charge storage. For HKL/PEO-based ultrafine carbon fibers, lots of micropores formed due to the volatilization of PEO and carbohydrates in HKL. Normally, strong potential existed

Fig. 7. Raman spectra of HKL/PEO/PEG-PPG-PEGn-based ultrafine carbon fibers (a) and schematic of the formation of graphite crystallites (b).

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S. Wang et al. / International Journal of Biological Macromolecules xxx (xxxx) xxx

Table 3 Data obtained from Raman spectra of HKL/PEO/PEG-PPG-PEGn-based carbon ultrafine fibers. Sample

D-band (cm−1)

G-band (cm−1)

ID/IG

La (nm)

HKL/PEO HKL/PEO/PEG-PPG-PEG0.5 HKL/PEO/PEG-PPG-PEG1.0 HKL/PEO/PEG-PPG-PEG1.5

1345.5 1344.9 1344.7 1345.9

1588.0 1585.7 1588.6 1587.2

1.06 1.05 1.05 1.04

4.15 4.19 4.19 4.23

inside the micropores, which strengthened the adsorption of electrolyte ions, leading to a relatively high specific capacitance of HKL/PEO ultrafine carbon fibers based electrode [33,34]. When PEG-PPG-PEG was incorporated into HKL/PEO composites, amounts of hydrogen bonds formed between the molecular chains of HKL and triblock copolymer, which increased the content of local volatiles. As a result, some pore

size increased to a certain degree, and the capacitances decreased significantly. Further increase the content of incorporated triblock copolymer, many mesopores and large voids appeared, and the measured capacitances in turn began to increase. The electrode performance in long term cycling was also investigated at a scan rate of 100 mV s−1 for 1000 cycles. Impressively, the specific capacitances of all the samples increased with the cycle numbers, which could reach almost 1.5 times of its original capacitance after 1000 cycles. The increased specific capacitance can be ascribed to the fact that more holes inside the electrode were opened as the increase of cycle number, which increased the available specific surface area. 4. Conclusion HKL-based ultrafine carbon fibers with different pore structures and properties were prepared by controlling the intensity of hydrogen-

Fig. 8. CV curves (a-d), specific capacitance (e) under different scan rates, and relationships between specific capacitance (scan rate:100 mv s−1) and cycle number (e) of HKL/PEO/PEGPPG-PEGn ultrafine carbon fibers based electrodes.

Please cite this article as: S. Wang, M.T. Innocent, J. Chen, et al., Tuning the microstructure and electrochemical behavior of lignin-based ultrafine carbon fibers via h..., , https://doi.org/10.1016/j.ijbiomac.2019.11.235

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bonding interactions formed between the molecular chains of HKL and PEG-PPG-PEG triblock copolymer. The addition of PEG-PPG-PEG decreased the Tg but increased the thermal stability of HKL. The morphology and pore size of the prepared ultrafine carbon fibers were also affected by the content of incorporated triblock copolymer, and the ideal pore structure was obtained in HKL/PEO/PEG-PPG-PEG0.5-based ultrafine carbon fibers with a Smicro of 850.1 m2/g and a Vmicro of 0.32 cm3/g. In addition, the appearance of PEG-PPG-PEG promoted the formation of graphite crystallites in HKL-based ultrafine carbon fibers by decreasing the distance between HKL molecular chains via the intermolecular hydrogen-bonding interactions. The electrochemical behavior of HKL-based ultrafine carbon fibers with different pore structures were comparatively investigated to give a comprehensive knowledge on the potential application as electrodes. Although the measured specific capacitance was relatively low, HKL ultrafine carbon fiber-based electrochemical capacitor showed a good capacitive behavior. Further work is ongoing to regulate the pore size and increase the pore volume of HKL-based ultrafine carbon fibers. CRediT authorship contribution statement Shichao Wang: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft. Mugaanire Tendo Innocent: Methodology, Writing - review & editing. Jianyu Chen: Investigation. Qianqian Wang: Resources, Validation. Wujun Ma: Writing - review & editing. Jianguo Tang: Supervision. Acknowledgements This work was supported by National Natural Science Foundation of China [51903128], Shandong Provincial Natural Science Foundation, China [ZR2018BEM028], the Open Project of State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University [KF1807], and the China Postdoctoral Science Foundation [2018M632620]. Declaration of competing interest None. References [1] L. Ji, Z. Lin, A.J. Medford, X. Zhang, Porous carbon nanofibers from electrospun polyacrylonitrile/SiO2 composites as an energy storage material, Carbon 47 (14) (2009) 3346–3354. [2] W. Zhao, S. Ci, X. Hu, J. Chen, Z. Wen, Highly dispersed ultrasmall NiS2 nanoparticles in porous carbon nanofiber anodes for sodium ion batteries, Nanoscale 11 (11) (2019) 4688–4695. [3] M.-L. Song, H.-Y. Yu, L.-M. Chen, J.-Y. Zhu, Y.-Y. Wang, J.-M. Yao, Z. Zhu, K.C. Tam, Multibranch strategy to decorate carboxyl groups on cellulose nanocrystals to prepare adsorbent/flocculants and pickering emulsions, ACS Sustain. Chem. Eng. 7 (7) (2019) 6969–6980. [4] S.D. Mustafov, A.K. Mohanty, M. Misra, M.O. Seydibeyoglu, Fabrication of conductive lignin/PAN carbon nanofibers with enhanced graphene for the modified electrodes, Carbon 147 (2019) 262–275. [5] E. Stojanovska, M. Kurtulus, A. Abdelgawad, Z. Candan, A. Kilic, Developing ligninbased bio-nanofibers by centrifugal spinning technique, Int. J. Biol. Macromol. 113 (2018) 98–105. [6] D. Chen, K. Jiang, T. Huang, G. Shen, Recent advances in fiber supercapacitors: materials, device configurations, and applications, Adv. Mater. (2019) https://doi.org/10. 1002/adma.201901806. [7] C. Ma, Z. Li, J. Li, Q. Fan, L. Wu, J. Shi, Y. Song, Lignin-based hierarchical porous carbon nanofiber films with superior performance in supercapacitors, Appl. Surf. Sci. 456 (2018) 568–576.

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Please cite this article as: S. Wang, M.T. Innocent, J. Chen, et al., Tuning the microstructure and electrochemical behavior of lignin-based ultrafine carbon fibers via h..., , https://doi.org/10.1016/j.ijbiomac.2019.11.235