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Relationship between microstructure and electrochemical properties of 2lignin-derived carbon nanofibers prepared by thermal treatment
Synthetic Metals 260 (2020) 116287
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Relationship between microstructure and electrochemical properties of 2lignin-derived carbon nanofibers prepared by thermal treatment
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Ji Hwan Jeong, Yun Ho Lee, Bo-Hye Kim* Division of Science Education, Chemistry Education Major, Daegu University, 201 Daegudae-ro, Gyeongsan-si, Gyeongsangbuk-do, 712-714, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Lignin MnO2 Carbonization temperature Microstructure Supercapacitor
Low-cost porous carbon nanofibers with MnO2 are developed via simple and inexpensive processes by using lignin as an affordable carbon precursor and generator of porous structures through one-step electrospinning and carbonization without requiring any other activating agent and process. The lignin-based carbon nanofiber/ MnO2 composites carbonized at high temperature have a high mesoporosity induced by pore opening effect, and thus exhibit a high capacitance retention of 87 % and optimum cycling stability with 93 % retention due to the excellent chemical reversibility. Further, the large surface area with many micropores derived from the low carbonization temperature affords high capacitive performance with a maximum specific capacitance of 212 Fg−1 at low current density and a high energy density of 26.5 Whkg−1 at a power density of 400 Wkg−1. These eco-friendly, low-cost, and pore-controlled MnO2/CNF composites have been designed with optimized carbonization temperature and lignin addition to enhance their capacitive properties.
1. Introduction Supercapacitors, also called electrochemical capacitors, have received much attention due to their high power density, excellent reversibility, long cyclic life, and good rate capability. It is important to study innovative electrode materials for the development of next-generation supercapacitors that have attracted increasing attention as alternative energy storage systems for electric vehicles, portable electronics, and renewable energy systems. [1–7]. In recent years, various hybrids of carbonaceous materials and manganese oxides (MnO2) have been investigated for their synergistic effect between the electric double layer capacitance of carbon and the faradaic capacitance of MnO2, affording good capacitive behavior [8–16]. Although the MnO2 functional groups in the carbon/MnO2 composites can make a pseudocapacitance contribution through the redox reaction of electrode material and the electrolyte, the poor electronic conductivity of the MnO2 and low surface area of the carbon material, induced by pore blocking by the MnO2 particles, limit the cycling behavior and the high-rate capability. Thus, it is necessary to develop a carbon material having a large specific surface area, adjustable pore size, large mesopore volume, and good electric conductivity, because porous carbon with a large surface area and well-developed mesopores has a high charge storage capacity at the electrode-electrolyte interface and is advantageous for highspeed transport of the electrolyte ions [17–24].
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In this work, we present lignin-based carbon nanofibers (CNFs) with MnO2 fabricated by one-step electrospinning, followed by stabilization and carbonization processes. In general, the activation process, carried out after carbonization to increase the surface area, can unfortunately reduce the capacity by decreasing the proportion of heteroatoms, thereby requiring an additional process to remove the activating agent. We used lignin with large heteroatom content to develop the porous CNF structure by the thermal decomposition of organic parts in lignin without any chemical/physical activation process. Lignin is an excellent precursor for replacing polyacrylonitrile (PAN) in CNF production, because it is an abundant, naturally occurring, organic polymer compound with high carbon content and high aromaticity. In addition, in order to optimize the electrochemical properties of the carbon materials, the pore structures are tailored for the most effective utilization of the surface area by varying the carbonization temperature. Therefore, our research goal in this study is to develop low-cost porous CNFs with MnO2 via simple and inexpensive processes by using lignin as an affordable carbon precursor and generator of porous structures through one-step electrospinning and carbonization without requiring any other activating agent or process. The full relationship between the microstructure and the electrochemical properties of the lignin-based CNFs with MnO2 composites carbonized at two different heat temperatures is described in detail.
Corresponding author. E-mail address: [email protected] (B.-H. Kim).
https://doi.org/10.1016/j.synthmet.2019.116287 Received 10 June 2019; Received in revised form 13 December 2019; Accepted 23 December 2019 0379-6779/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. FE-SEM images showing the morphology of (a) PL-800, (b) PLMn-800, and (c) PLMn-900, and (d) EDS data of PLMn-800.
Fig. 2. (a) Wide-scan XPS spectrum of PLMn-800 and PLMn-900, High-resolution XPS scan for (b) Mn2p, (c) O1s core levels. (d) XRD spectra of PL-800, PLMn-800, and PLMn-900.
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Fig. 3. (a, b) TEM images of PLMn-800 under different magnifications, (c) the associated SAED pattern of the area in (b), and (d) the corresponding elemental mapping images of carbon, manganese, and oxygen of the selected area in (b).
Fig. 4. CV curves of (a) PL-800, (b) PLMn-800, (c) PLMn-900 at various scan rates from 10 to 100 mVs−1, and (d) CVs of four CNF electrodes at a scan rate of 50 mVsin 6.0 M KOH(aq) electrolyte.
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2.2. Characterization The surface morphology of the nano-structured materials was examined by field emission scanning electron microscopy (FE-SEM, Hitachi, S-4700) equipped with an energy dispersive X-ray spectroscopy (EDS) detector. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) micrographs were obtained with a Tecnai-F20 system operated at 200 kV. Samples for analysis were prepared on a carbon-coated Cu grid by dip coating in appropriately dilute solutions (∼1.0 wt% solid content). The chemical state of the surface was characterized by X-ray photoelectron spectroscopy (XPS) on a VG Scientific ESCALAB 250 spectrometer with an Al Kα X-ray source (15 mA, 14 kV). X-ray diffraction (XRD) analysis was conducted with ground-up samples of the fibers using a DMax-2400 diffractometer and CuKα radiation (λ = 0.15418 nm). Porosity was analyzed by the Brunauer–Emmett–Teller (BET) and Barrett-JoynerHalenda (BJH) methods (MicrotracBEL, Corp. BELSORP-max, Osaka, Japan). Electrochemical tests were performed in a two-electrode symmetric cell using Ni foil as the current collector in 6.0 M KOH aqueous electrolyte. The unit cell was tested with cyclic voltammetry (CV) in the potential range of -0.2-0.8 V at a scan rate of 10-100 mVs−1. The charge/discharge properties of the electrodes were galvanostatically measured with a VSP multi potentiostat/galvanostat system at a current density of 1–20 mAcm-2. The impedance of the electrodes was measured over the frequency range of 10 MHz to 100 kHz 10 MHz using an electrochemical impedance analyzer. 3. Results and discussion The morphologies of PL-800, PLMn-800, and PLMn-900 are firstly presented by SEM, as shown in Fig. 1a-c. All three composites present continuous cylindrical morphologies with interlinked structure between the NFs without identifiable beaded-NFs. The average fiber diameter of the two PLMn composites with a mean diameter of 310−470 nm was thicker than that of the PL composite within an average diameter of 280 nm. The addition of MnCl2 increased the viscosity and reduced the electrical conductivity of the spinning solution, which afforded thick fibers due to insufficient elongation of the jet due to electrical forces [25–27]. The corresponding EDX spectrum of an individual fiber of PLM-800 (Fig. 1d) provides information on the distribution of C, O, and Mn at 71.07, 21.44, and 7.48 atomic percent, respectively. The XPS spectra of PLMn-800 and PLMn-900 in the range 0 eV–1150 eV shows four distinct peaks, indicating the existence of C, O, N, and Mn atoms, as shown in Fig.2a. The XPS spectrum of PLMn-800 and PLMn-900 with corresponding spin-orbit components of the Mn2p core levels, depicted in Fig. 2b, was collected to clarify the valence state of Mn. The XPS peaks of Mn2p indicate that two strong peaks of Mn2p3/ 2 and Mn2p1/2 are centered at about 642.1 eV and 653.8 eV, respectively. Thus, the predominant oxidation state of Mn was +4 by the spin energy separation of 11.7 eV, suggesting that MnO2 had been introduced into the PLMn-800 composite [28]. In addition to the Mn(IV), weak peaks of the Mn(III) state are observed in separated peaks of the Mn2p3/2 and Mn2p1/, respectively, suggesting that the main oxidation of the Mn element is +4. The O1s spectra (Fig. 2c) was indicative of Mn-O-Mn (530.8 eV), hydroxyl or ether oxygen (531.2 eV), and CeOHe of alcohol and carboxyl bonds (532.8 eV). The structures of carbon and MnO2 in the various composites were confirmed by the XRD patterns as a function of the carbonization temperature and MnCl2 addition, as shown in Fig. 2d. One broad and strong diffraction peak centered at a 2θ value of between 20° and 30° implies (002) lattice planes of the disordered carbon structure. Moreover, the diffraction peaks of PLMn-900 (inset Fig. 2c) at 36.82°, 39.73°, 57.69°, and 61.48° are attributed to the (211), (330), (600), (521) crystal planes, respectively, of the tetragonal α-MnO2 (JCPDS No. 44-0141) [29,30]. The XRD result agrees well with the XPS result confirming the presence of the MnO2 particles with an amorphous structure of carbon. The
Fig. 5. Complex-plane impedance plots at a perturbation amplitude of 10 mV and (d) Bode phase plot obtained with the AC impedance method for the two PLMn composites and the PL composite.
2. Experimental 2.1. Materials and fabrication Hardwood lignin (GS Caltex, Korea) was desalted five times using 1 N hydrochloric solution, washed with distilled water several times until pH = 7, and then vacuum-dried to obtain powder-type lignin. The elemental analysis of lignin using the Mettler method (Mettler-Toledo AG, Switzerland) showed 56.06 % carbon, 5.75 % hydrogen, 0.18 % nitrogen, and 23.10 % oxygen. PAN, manganese(II) chloride (MnCl2), and dimethylformamide (DMF) were used to prepare the experimental composites. Electrospinning solutions were prepared by dispersing an appropriate amount of MnCl2 (5 wt % relative to PAN and lignin) in PAN/lignin at a weight ratio of 90/10 wt % in a DMF solution. The PAN/lignin/MnCl2 (PLMn) blend was electrospun onto nanofibers (NFs) using an electrospinning machine (NTPS-35 K, NTSEE Co., Korea). Electrospinning was conducted by applying a high positive voltage (20 kV) to the polymer solution via the tip of the syringe needle. The electrospun NFs were collected as a thin web on an aluminum foil wrapped on a metal drum rotating at ∼300 rpm. The NFs were stabilized in air at 280 °C for 1 h in an air atmosphere and then carbonized in an inert atmosphere at 800 and 900 °C for 1 h at a heating rate of 5 °C min−1 using an electrical furnace. The resulting samples were denoted as PLMn-800 and PLMn-900, respectively, with the carbonization temperature being indicated. PL-800 and P-800, both without MnCl2 and the latter without lignin, respectively, were also carbonized at 800 °C as control samples to compare the electrochemical properties. 4
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Fig. 6. (a) Nitrogen adsorption-desorption isotherms at 77 K, (b) BJH pore size distributions of the four CNF composites, and (c) specific surface area and micropore/ mesopore volume fractions for the four composites.
at the double-layer interfaces. The same capacitive performance as these CV results is demonstrated by Nyquist plots in the frequency range of 100−10 MHz. The electrochemical impedance spectra of PL-800, PLMn-800, and PLMn-900 are shown in Fig. 5a as Nyquist plots to investigate the conductive behavior of the electrodes. The charge transfer resistance (Rf) at high-frequency range represents the movement of ions at the interface between the electrode and electrolyte. The Rf value decreases from 1.8 Ω for PL-800, to 1.6 Ω for PLMn-800 and to 0.4 Ω for PLMn900. This indicates that the PLMn-900 electrode has a lower internal resistance and relatively better conductivity than the other electrodes. Furthermore, the steepness of the slope of the straight line at low-frequency range increases in the following order: PL-800 < PLMn800 < PLMn-900; which means that the resistance to the kinetics of ion diffusion is reduced. Fig. 5b shows a bode-angle plot of the three electrodes as the function of different frequencies. The response frequency (f0) at a phase angle of 45° is used to characterize low electrontransfer resistance for high capacitive performance. The f0 values of PL800, PLMn-800, and PLMn-900 are approximately 0.07, 0.3, and 0.56 Hz, respectively. The corresponding response time defined as the inverse of f0 is reduced from 14.3 s for PL-800 to 3.33 s for PLMn-800 and to 1.78 s for PLMn-900. Moreover, the PLMn-900 electrode is the best electrode material for capacitor applications because it shows the ideal capacitor closer to the phase angle of 90°. This result means that the PLMn-900 electrode has rapid ion transport and low resistance for charge diffusion in the electrolyte, which demonstrates the good rate capability [31–34]. Therefore, the PLMn-900 electrode with low resistance for better accessibility of electrolyte ions into pores shows a perfect-rectangular shape in the CV results, which demonstrates the good electrochemical reversibility arising from the fast electro-adsorption/desorption of the electrolyte ions. These results can be
reflection peak of crystalline MnO2 is also seen to remarkably increase with increasing carbonization temperature. Fig. 3a-b show TEM images of the PLMn-800 mats, indicating that the MnO2 particles are dispersed throughout the single fibers of PLMn800 under different magnifications. The selected region electron diffraction (SAED) pattern further presents well-defined lattice fringes with diffraction spots confirming the polycrystalline structure of the MnO2 particles in Fig. 3c. Furthermore, the corresponding elemental mapping images (Fig. 3d) of the selected area in Fig. 3b show that the C, Mn, and O elements are uniformly distributed within the single fiber. The MnO2 crystals are clearly distributed on the NF, which is consistent with the XRD and XPS observations. The electrochemical measurements based on a two-electrode system were performed in 6.0 M KOH aqueous electrolyte to explore the electrochemical properties of various composites as electrochemical capacitor electrodes. The CV curves of all CNF electrodes at different scan rates ranging from 10 to 100 mVs−1 were obtained in the voltage range of -0.2-0.8 V. The CV curves (Fig. 4a and c) of the PL-800 and PLMn-900 electrodes exhibit a symmetrical box-like shape in the aqueous electrolyte, indicative of their ideal electrical double layer capacitor behavior with excellent reversibility under quick charge-discharge operations. However, the CV curve of the PLMn-800 electrode (Fig. 4b) exhibits a non-rectangular, sweet potato shape with a large distortion as the scan rate increases, indicating low mass transfer capability with large equivalent series resistance. The induced current of the CV curves at scan rate of 50 mVs−1 over the entire potential range increased in the order of PL-800 < PLMn-900 < PLMn-800 in Fig. 4d, which demonstrated the good capacitance capability of PLMn-800. The CV results showing the nearly perfect-rectangular shaped PLMn-900 electrode are indicative of fast mass transfer capability and the much wider CV curve of the PLMn-800 electrode represents significant charge accumulation 5
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Fig. 7. Electrochemical test of four CNF electrodes in aqueous electrolyte: (a) specific capacitances at various current densities and (b) the energy density vs. power density of the CNF symmetric supercapacitor in a Ragone plot.
Fig. 8. (a) Variation of specific capacitance for 4000 cycles of PLMn-800 and PLMn-900 at a constant current density of 1 mAcm−2 in 6 M KOH aqueous electrolyte and (b) complex-plane impedance plots at a perturbation amplitude of 10 mV acquired for both devices after running for 4000 cycles.
explained by the pore characteristics shown in Fig. 6, such as specific surface area and micropore/mesopore volume fraction. The nitrogen adsorption/desorption isotherms of P-800, PL-800, PLMn-800, and PLMn-900 are displayed in Fig. 6a. The PL-800, PLMn800, and PLMn-900 composites demonstrate the co-existence of microand meso-pores by exhibiting type IV isotherm behavior with capillary condensation at high relative pressures. The mesopore size distributions in the range of 2.0−50 nm as determined by the BJH method are shown in Fig. 6b. PLMn-900 exhibits many mesopores over the entire pore width, compared to the PL-800 and PLMn-800 electrodes. For P800, the nitrogen adsorption isotherm (Fig. 6a) and the mesopore volume (Fig. 6b) show typical type I curves with micropores where nitrogen adsorption occurs predominantly. Furthermore, the fraction of micropores/mesopores and the BET surface area of all CNFs are summarized in Fig. 6c. Lignin increases the specific surface area and the mesopore volume fraction, indicating that it induces the porous structure by its pyrolysis without any chemical/physical activation process. Thus, the use of lignin eliminates the need for additional activation processes and replaces expensive PAN materials, thus simplifying the process and reducing costs. Of the PLMn composites, PLMn-800 has the largest surface area with many micropores, while the PLMn-900 composite shows the best-developed mesopores. This indicates that the pore widening effect appears at high carbonization temperature, which decreases the specific surface areas and increases the mesopore volume [35]. These pore characteristics explain the CV and impedance results well. The PLMn-900 composite electrode has a high mesoporosity and
thus exhibits an ideal-rectangular shape with excellent chemical reversibility arising from rapid chemical adsorption/desorption reaction rates due to low equivalent series resistance. PLMn-800, with the largest CV integrated area, shows improved capacitance capability, which is attributed to the electric double layer capacitance occurring at the large surface area [36]. However, PLMn-800 with many micropores exhibits a non-rectangular, sweet potato shape because of its high resistance to migration into pores at high scan rates. Fig. 7a and b show the change in specific capacitance at discharge current density from 1 to 20 mAcm−2 and power/energy performance evaluated with a Ragone plot in aqueous electrolytes. The specific capacitance decreased in the order PLMn-800 > PLMn-900 > PL800 > P-800 at low discharge current density of 1mAcm-2 in Fig. 7a. In the Ragone plot, the energy density value of the PLMn-800 composite peaks at 26.5 Whkg-1 at a power density of 400 Wkg-1, which is enhanced compared with that of PLMn-900 (21.7 Whkg-1), PL-800 (19.5 Whkg-1), and P-800 (6.0 Whkg-1). The PLMn-800 electrode exhibits a much higher specific capacitance and energy density than P-800 at all current densities and power densities, due to the synergistic effect between the double-layer capacitance, which is ascribed to the large surface area induced by lignin and the pseudo-capacitive character of MnO2 NPs. In addition, the specific capacitance of the PLMn-900 electrode is reduced by approximately 13 % at high current density of 20 mAcm-2, compared to a 26 %-reduction for PLMn-800. In other words, the PLMn-900 electrode with large mesopore volume shows the 6
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the Ministry of Science, 2017R1A2B1009073).
best rate capability at a high rate by improving the charge diffusion for ion transport and power capability of the supercapacitor electrode material. This is because many of the mesopores of PLMn-900 help the solvated ions to diffuse into pores during high rate charge and discharge. Compared to the PLMn-900 electrode at low current density, the large surface area of PLMn-800 is advantageous for achieving high capacitive performance by gathering a large charge at the electrodeelectrolyte interface during the charging process. The cycling stability of the PLMn-800 and PLMn-900 electrodes was measured by constant current charging/discharging at 1 mAcm−2 for 4000 cycles in the 6 M KOH aqueous electrolyte. The initial capacitance decreased by ∼21 % for PLMn-800 and 7 % for PLMn-900 after 4000 cycles, demonstrating the better cycling stability of PLMn-900, as shown in Fig. 8a. The Nyquist plot of the two electrodes (Fig. 8b) maintains a good shape after the 4000 charge-discharge cycles, showing a nearly straight line and semicircles over the entire frequency. Even after 4000 cycles, the PLMn-900 electrode exhibits a low charge transfer resistance and an ideal straight line, resulting in more efficient power density and excellent rate capability. Thus, the long-term stability of the PLMn-900 electrode is due to its high reversibility in the welldeveloped mesopores during the repeated charge-discharge cycles. We conclude that the high surface area with many micropores helps increase the capacitance by the high adsorption efficiency of the electrolyte ions, but it is difficult to obtain good capacity retention and cycling stability due to the high internal resistance for charge diffusion into the micropores during high rate charge-discharge processes. On the other hand, the many mesopores of the carbon materials are beneficial for providing rapid ion migration in the electrodes, which leads to good retention of the high capacitance. As a consequence, eco-friendly, lowcost, and pore-controlled PLMn composites have been designed with optimized carbonization temperature and lignin addition to optimize and enhance their capacitive properties.
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4. Conclusions We successfully fabricated lignin-based CNFs with MnO2 (PLMn) for electrochemical capacitor electrodes through one-step electrospinning and thermal treatment. The conversion of biomass lignin to high-performance carbon materials is very economical because lignin is naturally abundant with high contents of carbon, aromatics, and heteroatoms. Furthermore, in order to optimize their electrochemical properties, the pore structure of PLMn can be tailored for the most effective utilization of the surface area by varying the carbonization temperature. The PLMn-900 electrode carbonized at 900 °C exhibits a large mesopore volume by the pore opening effect, which promotes excellent rate capability at high speeds by enhancing the charge diffusion for ion transport. On the other hand, PLMn-800 with its large surface area and high micoporosity achieves high capacitive performance by sufficiently accumulating charge at the electrode-electrolyte interface during the charging process at low current densities. Thus, these eco-friendly, low-cost, and pore-controlled PLMn composites have been designed with optimized carbonization temperature and lignin content for high performance as supercapacitor electrode materials with high specific capacitance, high energy/power efficiency, high rate capability, and excellent cycling stability. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by 7
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Relationship between microstructure and electrochemical properties of 2lignin-derived carbon nanofibers prepared by thermal treatment
T
Ji Hwan Jeong, Yun Ho Lee, Bo-Hye Kim* Division of Science Education, Chemistry Education Major, Daegu University, 201 Daegudae-ro, Gyeongsan-si, Gyeongsangbuk-do, 712-714, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Lignin MnO2 Carbonization temperature Microstructure Supercapacitor
Low-cost porous carbon nanofibers with MnO2 are developed via simple and inexpensive processes by using lignin as an affordable carbon precursor and generator of porous structures through one-step electrospinning and carbonization without requiring any other activating agent and process. The lignin-based carbon nanofiber/ MnO2 composites carbonized at high temperature have a high mesoporosity induced by pore opening effect, and thus exhibit a high capacitance retention of 87 % and optimum cycling stability with 93 % retention due to the excellent chemical reversibility. Further, the large surface area with many micropores derived from the low carbonization temperature affords high capacitive performance with a maximum specific capacitance of 212 Fg−1 at low current density and a high energy density of 26.5 Whkg−1 at a power density of 400 Wkg−1. These eco-friendly, low-cost, and pore-controlled MnO2/CNF composites have been designed with optimized carbonization temperature and lignin addition to enhance their capacitive properties.
1. Introduction Supercapacitors, also called electrochemical capacitors, have received much attention due to their high power density, excellent reversibility, long cyclic life, and good rate capability. It is important to study innovative electrode materials for the development of next-generation supercapacitors that have attracted increasing attention as alternative energy storage systems for electric vehicles, portable electronics, and renewable energy systems. [1–7]. In recent years, various hybrids of carbonaceous materials and manganese oxides (MnO2) have been investigated for their synergistic effect between the electric double layer capacitance of carbon and the faradaic capacitance of MnO2, affording good capacitive behavior [8–16]. Although the MnO2 functional groups in the carbon/MnO2 composites can make a pseudocapacitance contribution through the redox reaction of electrode material and the electrolyte, the poor electronic conductivity of the MnO2 and low surface area of the carbon material, induced by pore blocking by the MnO2 particles, limit the cycling behavior and the high-rate capability. Thus, it is necessary to develop a carbon material having a large specific surface area, adjustable pore size, large mesopore volume, and good electric conductivity, because porous carbon with a large surface area and well-developed mesopores has a high charge storage capacity at the electrode-electrolyte interface and is advantageous for highspeed transport of the electrolyte ions [17–24].
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In this work, we present lignin-based carbon nanofibers (CNFs) with MnO2 fabricated by one-step electrospinning, followed by stabilization and carbonization processes. In general, the activation process, carried out after carbonization to increase the surface area, can unfortunately reduce the capacity by decreasing the proportion of heteroatoms, thereby requiring an additional process to remove the activating agent. We used lignin with large heteroatom content to develop the porous CNF structure by the thermal decomposition of organic parts in lignin without any chemical/physical activation process. Lignin is an excellent precursor for replacing polyacrylonitrile (PAN) in CNF production, because it is an abundant, naturally occurring, organic polymer compound with high carbon content and high aromaticity. In addition, in order to optimize the electrochemical properties of the carbon materials, the pore structures are tailored for the most effective utilization of the surface area by varying the carbonization temperature. Therefore, our research goal in this study is to develop low-cost porous CNFs with MnO2 via simple and inexpensive processes by using lignin as an affordable carbon precursor and generator of porous structures through one-step electrospinning and carbonization without requiring any other activating agent or process. The full relationship between the microstructure and the electrochemical properties of the lignin-based CNFs with MnO2 composites carbonized at two different heat temperatures is described in detail.
Corresponding author. E-mail address: [email protected] (B.-H. Kim).
https://doi.org/10.1016/j.synthmet.2019.116287 Received 10 June 2019; Received in revised form 13 December 2019; Accepted 23 December 2019 0379-6779/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. FE-SEM images showing the morphology of (a) PL-800, (b) PLMn-800, and (c) PLMn-900, and (d) EDS data of PLMn-800.
Fig. 2. (a) Wide-scan XPS spectrum of PLMn-800 and PLMn-900, High-resolution XPS scan for (b) Mn2p, (c) O1s core levels. (d) XRD spectra of PL-800, PLMn-800, and PLMn-900.
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Fig. 3. (a, b) TEM images of PLMn-800 under different magnifications, (c) the associated SAED pattern of the area in (b), and (d) the corresponding elemental mapping images of carbon, manganese, and oxygen of the selected area in (b).
Fig. 4. CV curves of (a) PL-800, (b) PLMn-800, (c) PLMn-900 at various scan rates from 10 to 100 mVs−1, and (d) CVs of four CNF electrodes at a scan rate of 50 mVsin 6.0 M KOH(aq) electrolyte.
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2.2. Characterization The surface morphology of the nano-structured materials was examined by field emission scanning electron microscopy (FE-SEM, Hitachi, S-4700) equipped with an energy dispersive X-ray spectroscopy (EDS) detector. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) micrographs were obtained with a Tecnai-F20 system operated at 200 kV. Samples for analysis were prepared on a carbon-coated Cu grid by dip coating in appropriately dilute solutions (∼1.0 wt% solid content). The chemical state of the surface was characterized by X-ray photoelectron spectroscopy (XPS) on a VG Scientific ESCALAB 250 spectrometer with an Al Kα X-ray source (15 mA, 14 kV). X-ray diffraction (XRD) analysis was conducted with ground-up samples of the fibers using a DMax-2400 diffractometer and CuKα radiation (λ = 0.15418 nm). Porosity was analyzed by the Brunauer–Emmett–Teller (BET) and Barrett-JoynerHalenda (BJH) methods (MicrotracBEL, Corp. BELSORP-max, Osaka, Japan). Electrochemical tests were performed in a two-electrode symmetric cell using Ni foil as the current collector in 6.0 M KOH aqueous electrolyte. The unit cell was tested with cyclic voltammetry (CV) in the potential range of -0.2-0.8 V at a scan rate of 10-100 mVs−1. The charge/discharge properties of the electrodes were galvanostatically measured with a VSP multi potentiostat/galvanostat system at a current density of 1–20 mAcm-2. The impedance of the electrodes was measured over the frequency range of 10 MHz to 100 kHz 10 MHz using an electrochemical impedance analyzer. 3. Results and discussion The morphologies of PL-800, PLMn-800, and PLMn-900 are firstly presented by SEM, as shown in Fig. 1a-c. All three composites present continuous cylindrical morphologies with interlinked structure between the NFs without identifiable beaded-NFs. The average fiber diameter of the two PLMn composites with a mean diameter of 310−470 nm was thicker than that of the PL composite within an average diameter of 280 nm. The addition of MnCl2 increased the viscosity and reduced the electrical conductivity of the spinning solution, which afforded thick fibers due to insufficient elongation of the jet due to electrical forces [25–27]. The corresponding EDX spectrum of an individual fiber of PLM-800 (Fig. 1d) provides information on the distribution of C, O, and Mn at 71.07, 21.44, and 7.48 atomic percent, respectively. The XPS spectra of PLMn-800 and PLMn-900 in the range 0 eV–1150 eV shows four distinct peaks, indicating the existence of C, O, N, and Mn atoms, as shown in Fig.2a. The XPS spectrum of PLMn-800 and PLMn-900 with corresponding spin-orbit components of the Mn2p core levels, depicted in Fig. 2b, was collected to clarify the valence state of Mn. The XPS peaks of Mn2p indicate that two strong peaks of Mn2p3/ 2 and Mn2p1/2 are centered at about 642.1 eV and 653.8 eV, respectively. Thus, the predominant oxidation state of Mn was +4 by the spin energy separation of 11.7 eV, suggesting that MnO2 had been introduced into the PLMn-800 composite [28]. In addition to the Mn(IV), weak peaks of the Mn(III) state are observed in separated peaks of the Mn2p3/2 and Mn2p1/, respectively, suggesting that the main oxidation of the Mn element is +4. The O1s spectra (Fig. 2c) was indicative of Mn-O-Mn (530.8 eV), hydroxyl or ether oxygen (531.2 eV), and CeOHe of alcohol and carboxyl bonds (532.8 eV). The structures of carbon and MnO2 in the various composites were confirmed by the XRD patterns as a function of the carbonization temperature and MnCl2 addition, as shown in Fig. 2d. One broad and strong diffraction peak centered at a 2θ value of between 20° and 30° implies (002) lattice planes of the disordered carbon structure. Moreover, the diffraction peaks of PLMn-900 (inset Fig. 2c) at 36.82°, 39.73°, 57.69°, and 61.48° are attributed to the (211), (330), (600), (521) crystal planes, respectively, of the tetragonal α-MnO2 (JCPDS No. 44-0141) [29,30]. The XRD result agrees well with the XPS result confirming the presence of the MnO2 particles with an amorphous structure of carbon. The
Fig. 5. Complex-plane impedance plots at a perturbation amplitude of 10 mV and (d) Bode phase plot obtained with the AC impedance method for the two PLMn composites and the PL composite.
2. Experimental 2.1. Materials and fabrication Hardwood lignin (GS Caltex, Korea) was desalted five times using 1 N hydrochloric solution, washed with distilled water several times until pH = 7, and then vacuum-dried to obtain powder-type lignin. The elemental analysis of lignin using the Mettler method (Mettler-Toledo AG, Switzerland) showed 56.06 % carbon, 5.75 % hydrogen, 0.18 % nitrogen, and 23.10 % oxygen. PAN, manganese(II) chloride (MnCl2), and dimethylformamide (DMF) were used to prepare the experimental composites. Electrospinning solutions were prepared by dispersing an appropriate amount of MnCl2 (5 wt % relative to PAN and lignin) in PAN/lignin at a weight ratio of 90/10 wt % in a DMF solution. The PAN/lignin/MnCl2 (PLMn) blend was electrospun onto nanofibers (NFs) using an electrospinning machine (NTPS-35 K, NTSEE Co., Korea). Electrospinning was conducted by applying a high positive voltage (20 kV) to the polymer solution via the tip of the syringe needle. The electrospun NFs were collected as a thin web on an aluminum foil wrapped on a metal drum rotating at ∼300 rpm. The NFs were stabilized in air at 280 °C for 1 h in an air atmosphere and then carbonized in an inert atmosphere at 800 and 900 °C for 1 h at a heating rate of 5 °C min−1 using an electrical furnace. The resulting samples were denoted as PLMn-800 and PLMn-900, respectively, with the carbonization temperature being indicated. PL-800 and P-800, both without MnCl2 and the latter without lignin, respectively, were also carbonized at 800 °C as control samples to compare the electrochemical properties. 4
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Fig. 6. (a) Nitrogen adsorption-desorption isotherms at 77 K, (b) BJH pore size distributions of the four CNF composites, and (c) specific surface area and micropore/ mesopore volume fractions for the four composites.
at the double-layer interfaces. The same capacitive performance as these CV results is demonstrated by Nyquist plots in the frequency range of 100−10 MHz. The electrochemical impedance spectra of PL-800, PLMn-800, and PLMn-900 are shown in Fig. 5a as Nyquist plots to investigate the conductive behavior of the electrodes. The charge transfer resistance (Rf) at high-frequency range represents the movement of ions at the interface between the electrode and electrolyte. The Rf value decreases from 1.8 Ω for PL-800, to 1.6 Ω for PLMn-800 and to 0.4 Ω for PLMn900. This indicates that the PLMn-900 electrode has a lower internal resistance and relatively better conductivity than the other electrodes. Furthermore, the steepness of the slope of the straight line at low-frequency range increases in the following order: PL-800 < PLMn800 < PLMn-900; which means that the resistance to the kinetics of ion diffusion is reduced. Fig. 5b shows a bode-angle plot of the three electrodes as the function of different frequencies. The response frequency (f0) at a phase angle of 45° is used to characterize low electrontransfer resistance for high capacitive performance. The f0 values of PL800, PLMn-800, and PLMn-900 are approximately 0.07, 0.3, and 0.56 Hz, respectively. The corresponding response time defined as the inverse of f0 is reduced from 14.3 s for PL-800 to 3.33 s for PLMn-800 and to 1.78 s for PLMn-900. Moreover, the PLMn-900 electrode is the best electrode material for capacitor applications because it shows the ideal capacitor closer to the phase angle of 90°. This result means that the PLMn-900 electrode has rapid ion transport and low resistance for charge diffusion in the electrolyte, which demonstrates the good rate capability [31–34]. Therefore, the PLMn-900 electrode with low resistance for better accessibility of electrolyte ions into pores shows a perfect-rectangular shape in the CV results, which demonstrates the good electrochemical reversibility arising from the fast electro-adsorption/desorption of the electrolyte ions. These results can be
reflection peak of crystalline MnO2 is also seen to remarkably increase with increasing carbonization temperature. Fig. 3a-b show TEM images of the PLMn-800 mats, indicating that the MnO2 particles are dispersed throughout the single fibers of PLMn800 under different magnifications. The selected region electron diffraction (SAED) pattern further presents well-defined lattice fringes with diffraction spots confirming the polycrystalline structure of the MnO2 particles in Fig. 3c. Furthermore, the corresponding elemental mapping images (Fig. 3d) of the selected area in Fig. 3b show that the C, Mn, and O elements are uniformly distributed within the single fiber. The MnO2 crystals are clearly distributed on the NF, which is consistent with the XRD and XPS observations. The electrochemical measurements based on a two-electrode system were performed in 6.0 M KOH aqueous electrolyte to explore the electrochemical properties of various composites as electrochemical capacitor electrodes. The CV curves of all CNF electrodes at different scan rates ranging from 10 to 100 mVs−1 were obtained in the voltage range of -0.2-0.8 V. The CV curves (Fig. 4a and c) of the PL-800 and PLMn-900 electrodes exhibit a symmetrical box-like shape in the aqueous electrolyte, indicative of their ideal electrical double layer capacitor behavior with excellent reversibility under quick charge-discharge operations. However, the CV curve of the PLMn-800 electrode (Fig. 4b) exhibits a non-rectangular, sweet potato shape with a large distortion as the scan rate increases, indicating low mass transfer capability with large equivalent series resistance. The induced current of the CV curves at scan rate of 50 mVs−1 over the entire potential range increased in the order of PL-800 < PLMn-900 < PLMn-800 in Fig. 4d, which demonstrated the good capacitance capability of PLMn-800. The CV results showing the nearly perfect-rectangular shaped PLMn-900 electrode are indicative of fast mass transfer capability and the much wider CV curve of the PLMn-800 electrode represents significant charge accumulation 5
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Fig. 7. Electrochemical test of four CNF electrodes in aqueous electrolyte: (a) specific capacitances at various current densities and (b) the energy density vs. power density of the CNF symmetric supercapacitor in a Ragone plot.
Fig. 8. (a) Variation of specific capacitance for 4000 cycles of PLMn-800 and PLMn-900 at a constant current density of 1 mAcm−2 in 6 M KOH aqueous electrolyte and (b) complex-plane impedance plots at a perturbation amplitude of 10 mV acquired for both devices after running for 4000 cycles.
explained by the pore characteristics shown in Fig. 6, such as specific surface area and micropore/mesopore volume fraction. The nitrogen adsorption/desorption isotherms of P-800, PL-800, PLMn-800, and PLMn-900 are displayed in Fig. 6a. The PL-800, PLMn800, and PLMn-900 composites demonstrate the co-existence of microand meso-pores by exhibiting type IV isotherm behavior with capillary condensation at high relative pressures. The mesopore size distributions in the range of 2.0−50 nm as determined by the BJH method are shown in Fig. 6b. PLMn-900 exhibits many mesopores over the entire pore width, compared to the PL-800 and PLMn-800 electrodes. For P800, the nitrogen adsorption isotherm (Fig. 6a) and the mesopore volume (Fig. 6b) show typical type I curves with micropores where nitrogen adsorption occurs predominantly. Furthermore, the fraction of micropores/mesopores and the BET surface area of all CNFs are summarized in Fig. 6c. Lignin increases the specific surface area and the mesopore volume fraction, indicating that it induces the porous structure by its pyrolysis without any chemical/physical activation process. Thus, the use of lignin eliminates the need for additional activation processes and replaces expensive PAN materials, thus simplifying the process and reducing costs. Of the PLMn composites, PLMn-800 has the largest surface area with many micropores, while the PLMn-900 composite shows the best-developed mesopores. This indicates that the pore widening effect appears at high carbonization temperature, which decreases the specific surface areas and increases the mesopore volume [35]. These pore characteristics explain the CV and impedance results well. The PLMn-900 composite electrode has a high mesoporosity and
thus exhibits an ideal-rectangular shape with excellent chemical reversibility arising from rapid chemical adsorption/desorption reaction rates due to low equivalent series resistance. PLMn-800, with the largest CV integrated area, shows improved capacitance capability, which is attributed to the electric double layer capacitance occurring at the large surface area [36]. However, PLMn-800 with many micropores exhibits a non-rectangular, sweet potato shape because of its high resistance to migration into pores at high scan rates. Fig. 7a and b show the change in specific capacitance at discharge current density from 1 to 20 mAcm−2 and power/energy performance evaluated with a Ragone plot in aqueous electrolytes. The specific capacitance decreased in the order PLMn-800 > PLMn-900 > PL800 > P-800 at low discharge current density of 1mAcm-2 in Fig. 7a. In the Ragone plot, the energy density value of the PLMn-800 composite peaks at 26.5 Whkg-1 at a power density of 400 Wkg-1, which is enhanced compared with that of PLMn-900 (21.7 Whkg-1), PL-800 (19.5 Whkg-1), and P-800 (6.0 Whkg-1). The PLMn-800 electrode exhibits a much higher specific capacitance and energy density than P-800 at all current densities and power densities, due to the synergistic effect between the double-layer capacitance, which is ascribed to the large surface area induced by lignin and the pseudo-capacitive character of MnO2 NPs. In addition, the specific capacitance of the PLMn-900 electrode is reduced by approximately 13 % at high current density of 20 mAcm-2, compared to a 26 %-reduction for PLMn-800. In other words, the PLMn-900 electrode with large mesopore volume shows the 6
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the Ministry of Science, 2017R1A2B1009073).
best rate capability at a high rate by improving the charge diffusion for ion transport and power capability of the supercapacitor electrode material. This is because many of the mesopores of PLMn-900 help the solvated ions to diffuse into pores during high rate charge and discharge. Compared to the PLMn-900 electrode at low current density, the large surface area of PLMn-800 is advantageous for achieving high capacitive performance by gathering a large charge at the electrodeelectrolyte interface during the charging process. The cycling stability of the PLMn-800 and PLMn-900 electrodes was measured by constant current charging/discharging at 1 mAcm−2 for 4000 cycles in the 6 M KOH aqueous electrolyte. The initial capacitance decreased by ∼21 % for PLMn-800 and 7 % for PLMn-900 after 4000 cycles, demonstrating the better cycling stability of PLMn-900, as shown in Fig. 8a. The Nyquist plot of the two electrodes (Fig. 8b) maintains a good shape after the 4000 charge-discharge cycles, showing a nearly straight line and semicircles over the entire frequency. Even after 4000 cycles, the PLMn-900 electrode exhibits a low charge transfer resistance and an ideal straight line, resulting in more efficient power density and excellent rate capability. Thus, the long-term stability of the PLMn-900 electrode is due to its high reversibility in the welldeveloped mesopores during the repeated charge-discharge cycles. We conclude that the high surface area with many micropores helps increase the capacitance by the high adsorption efficiency of the electrolyte ions, but it is difficult to obtain good capacity retention and cycling stability due to the high internal resistance for charge diffusion into the micropores during high rate charge-discharge processes. On the other hand, the many mesopores of the carbon materials are beneficial for providing rapid ion migration in the electrodes, which leads to good retention of the high capacitance. As a consequence, eco-friendly, lowcost, and pore-controlled PLMn composites have been designed with optimized carbonization temperature and lignin addition to optimize and enhance their capacitive properties.
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4. Conclusions We successfully fabricated lignin-based CNFs with MnO2 (PLMn) for electrochemical capacitor electrodes through one-step electrospinning and thermal treatment. The conversion of biomass lignin to high-performance carbon materials is very economical because lignin is naturally abundant with high contents of carbon, aromatics, and heteroatoms. Furthermore, in order to optimize their electrochemical properties, the pore structure of PLMn can be tailored for the most effective utilization of the surface area by varying the carbonization temperature. The PLMn-900 electrode carbonized at 900 °C exhibits a large mesopore volume by the pore opening effect, which promotes excellent rate capability at high speeds by enhancing the charge diffusion for ion transport. On the other hand, PLMn-800 with its large surface area and high micoporosity achieves high capacitive performance by sufficiently accumulating charge at the electrode-electrolyte interface during the charging process at low current densities. Thus, these eco-friendly, low-cost, and pore-controlled PLMn composites have been designed with optimized carbonization temperature and lignin content for high performance as supercapacitor electrode materials with high specific capacitance, high energy/power efficiency, high rate capability, and excellent cycling stability. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by 7
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