carbon nanofiber composites with hollow cores for high-performance supercapacitor electrodes: Effect of poly(methyl methacrylate) concentration

Accepted Manuscript Title: Hierarchical porous MnO2 /carbon nanofiber composites with hollow cores for high-performance supercapacitor electrodes: Effect of poly(methyl methacrylate) concentration Author: Do Geum Lee Ji Hoon Kim Bo-Hye Kim PII: DOI: Reference:

S0013-4686(16)30635-1 http://dx.doi.org/doi:10.1016/j.electacta.2016.03.095 EA 26925

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

15-2-2016 11-3-2016 16-3-2016

Please cite this article as: Do Geum Lee, Ji Hoon Kim, Bo-Hye Kim, Hierarchical porous MnO2/carbon nanofiber composites with hollow cores for high-performance supercapacitor electrodes: Effect of poly(methyl methacrylate) concentration, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.03.095 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hierarchical porous MnO2/carbon nanofiber composites with hollow cores for highperformance supercapacitor electrodes: Effect of poly(methyl methacrylate) concentration

Do Geum Leea, Ji Hoon Kimb, Bo-Hye Kima,c*

a

Department of Chemistry, Graduate School, Daegu University, 201 Daegudae-ro, Gyeongsan-si, Gyeongsangbuk-do, Korea

b

Department of Polymer Engineering, Graduate School, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 500-757, Korea c

Division of Science Education, Daegu University, 201 Daegudae-ro, Gyeongsan-si, Gyeongbuk-do 712-714, Korea

1

*Tel.: +82 53-850-6982; Fax: +82 53-850-6989. E-mail address :[email protected] (Bo-Hye Kim). 1

Abstract Hierarchical porous MnO2/carbon nanofiber composites with hollow cores (MnPMCNFs) are fabricated by a simple electrospinning method using poly(methyl methacrylate) (PMMA) for electrochemical capacitor electrodes. The introduction of PMMA into the PAN solution aids the uniform dispersion of the amorphous MnO2 particles as a stabilizer and increases the specific surface area with numerous hollow cores, thereby aiding the electrolyte diffusion from the exterior into the interior of the electrode material. Electrochemical measurements of the MnPMCNFs reveal a maximum specific capacitance of 228 Fg-1 with a capacitance retention of 88 % and high energy densities of 27-18 Whkg-1 in the power density range of 400 to 10,000 Wkg-1 in aqueous KOH electrolyte. This impressive electrochemical performance of the MnPMCNF electrode highlights the importance of incorporating the MnO2 nanostructure and hierarchical structure into the composite, owing to the synergistic contribution of the redox pseudocapacitance and the electric double layer capacitance.

Keywords: MnO2, Carbon nanofiber composite, Hollow core, Hierarchical porosity, Poly(methyl methacrylate), Electrochemical performance

2

1. Introduction Supercapacitors, also known as electrical double layer capacitors, are promising power sources for electrical vehicles, portable electronic devices, and renewable energy power plants that can support the sustainable development of the global economy and society [1-3]. However, in addition to their high power density, supercapacitors also require high energy density, large specific capacitance and low production cost for such applications. Therefore, designing electrode materials with high reactivity and tailored structure is a major research objective for high-performance supercapacitors because the electrochemical performance depends heavily on the electrode materials. Among the various supercapacitor electrode materials, amorphous MnO2 is one of the most promising pseudocapacitive materials because of its high pseudocapacitance, high energy density, cost effectiveness, and absence of environmental pollution. Unfortunately, MnO2 as a pseudocapacitor electrode has the weakness of high contact resistance, poor rate capability, and insufficient capacitance display, due to its intrinsically low electrical conductivity and lack of accessible surface area [4-6]. This drawback can be overcome by using highly conductive carbon nanomaterials such as carbon nanotubes (CNTs), graphene, and carbon nanofibers (CNFs) as support materials for MnO2 to improve the charge exchange efficiency and stability during redox cycling of the MnO2 electrode, thanks to the synergistic effect of the double layer capacitance and Faradaic pseudocapacitance [113]. M. Zhang et al. have synthesized carbon nanotube/MnO2 composites by microwave-assisted method for supercapacitors and the composites have the maximum power density (45.4 kWkg-1, the energy density is 25.2 Whkg-1) when the content of MnO2 reaches 57 wt% [14]. More recently, Guan 3

et al. [2] reported honeycomb MnO2 nanospheres/carbon nanoparticles/graphene composite electrodes that showed 255 Fg-1 at a current density of 0.5 Ag-1. Z.-J. Zhang, et al. [1] investigated porous manganese dioxide/graphene composite films for high performance supercapacitors, resulting in an electrode with a high specific capacitance of 266.3 Fg-1 at the density of 0.2 Ag-1. However, the fabrication processes was a complicated, energy-consuming process, and relatively high cost due to the multistep by oxidation and reduction of graphene. In the present work, we report the fabrication of hierarchical porous MnO2/CNF composites with hollow cores (MnPMCNFs) in the form of a web by the one-step electrospinning of two immiscible polymer solutions, polyacrylonitrile/poly(methyl methacrylate) (PAN/PMMA), followed by carbonization process for electrochemical capacitor electrodes. Controllable and hierarchical porous composites can be produced by using PMMA as sacrificial templates, which is a key factor affecting the formation of many hollow cores [15-18]. Such hollow cores in MnPMCNFs are helpful for electrolyte diffusion from the exterior into the interior of the electrode material, leading to rapid ion transport and low resistance for charge diffusion in the electrolyte [19-20]. In addition, amorphous MnO2 loaded on CNFs can enhance the specific capacitance and energy density through fast and reversible Faradic reactions, which store the charge in the bulk of the amorphous materials from redox reactions. Herein, the hierarchical porous structure of the MnPMCNFs is controlled by varying the PMMA content. The composites are then morphologically and electrochemically characterized to evaluate their electrochemical performance in the aqueous electrolyte on the basis of the surface area, pore size, and fraction of carbon mesopores. 4

2. Experimental 2.1. Materials and Fabrication PAN, PMMA, manganese(II) chloride (MnCl2), and dimethylformamide (DMF) were purchased from Aldrich Chemical Co. (USA) and used as received. Electrospinning solutions were prepared by dispersing an appropriate amount of MnCl2 (3 wt%) in PAN/PMMA with different blend ratios (PAN:PMMA = 7:3 and 8:2) in a DMF solution. This mixture was continuously stirred at 60 C until a homogeneous solution formed, after which it was cooled to room temperature. This solution was then spun into nanofiber (NF) webs using an electrospinning apparatus (NTPS-35K, Ntsse Co., Korea) operating at 20 kV. Spinning solutions were fed through a capillary tip (diameter = 0.5 mm) using a syringe (10 ml). The anode of the high voltage power supply was clamped to a syringe needle tip and the cathode was connected to a metal collector. During electrospinning, the distance between the tip and collector was 13 cm, and the flow rate of the spinning solution was 2 mlh-1. The electrospun NF webs were stabilized in flowing air at 280 C to induce thermal stability and then held at this temperature for 1 h in an air atmosphere. The stabilized NFs were carbonized at 800C in a horizontal furnace under a flow of nitrogen at a heating rate of 5 C min-1. The carbonized samples at 800 C were termed MnPM(3)CNF, MnPM(2)CNF, and MnPM(0)CNF with PAN/PMMA blend ratios of 7:3, 8:2, and 10:0 respectively. To compare the electrochemical properties, CNF without MnO2 was also prepared as control sample. 5

2.2. Characterization The surface morphology of the nano-structured materials was examined by field emission scanning electron microscopy (FE-SEM, Hitachi, S-4700). Transmission electron microscopy (TEM) images equipped with energy dispersive X-ray spectroscopy (EDS) 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 Xray photoelectron spectroscopy (XPS) on a VG Scientific ESCALAB 250 spectrometer with an Al K X-ray source (15 mA, 14 kV).The specific surface area and the pore volume fraction of the samples were evaluated by using the Brunauer-Emmett-Teller (BET) method.

2.3. Cell fabrication and measurement Two-electrode supercapacitor cells (two identical carbon electrodes, no reference electrode present) were fabricated with two symmetric MnPMCNF composite electrodes (1.5cm×1.5 cm) as the counter and reference electrode using Ni foil as the current collector. All samples used as electrodes were cut only without adding any polymer binder, such as poly(vinyliene fluoride), or conducting agent, such as super-p, because they were fabricated as a web suitable for good contact between the sample and current collector. The capacitor consisted of a couple of electrodes which were arranged face to face and a separator (glass paper) was inserted between the two-electrodes. 6

The electrolyte used in this work was an aqueous electrolyte, i.e., 6.0 M KOH and voltage difference was limited to 1.0 V in order to inspect the stability in voltage regime. Cyclic voltammetry (CV) of the unit cell was performed between -0.2 and -0.8 V (vs. standard hydrogen electrode) at different scan rates varying from 10 to 100 mVs-1 for the aqueous electrolyte. The charge/discharge properties of the samples were measured using a WBCS 3000 battery cycler system (Won-A Tech. Co., Korea) at a current density of 1-20 mAcm-2. In a symmetrical system, the specific capacitance [21] of the single electrode in Farad per gram (Fg-1) is calculated from the discharge slope during galvanic cycling in terms of Eq. (1); Cm = 4It/Vm

(1)

where Cm is the specific capacitance, I is the discharge current in amps, Dt is the discharge time in seconds, DV is the discharge voltage in volts, and m is the weight (g) per electrode of samples. The energy density was measured as a function of constant power discharge in the range of 400-10,000 Wkg-1. Energy density in Watt-hours per kilogram (Whkg-1) and power density in Watts per kilogram (Wkg-1) are calculated using the following Eq. (2 and 3): E = (Cm*V2)/2

(2)

P = [I*V)/2*m]

(2)

The ac impedance measurements of the cell were carried out over the frequency range of 100 kHz to 10MHz using an electrochemical impedance analyzer (Jahner Electrik IM6e, Germany).

3. Results and discussion 7

Scheme 1 shows the preparation of the hierarchical porous structure of MnPMCNFs. The type of PMMA-Mn2+ is caused by the hydrophobic interaction between the carbonyl oxygen of PMMA and the Mn2+in the non-woven MnCl2/PAN/PMMA electrospun NFs. Particle sintering was prevented by adding a critical dosage of PMMA as a stabilizer agent whose function is to cover the particles, thus effectively preventing aggregation of the nanoparticles (NPs) [22-23]. The CNFs with well distributed MnO2 were then formed by optimum thermal process. XPS was employed to evaluate the chemical bonding states for MnPM(2)CNF and confirm the introduction of MnO2 to the composite. The XPS spectra of MnPM(2)CNF shows four distinct peaks, indicating the existence of C, O, N, and Mn atoms, as shown in Fig. 1a. In the Mn2p XPS spectrum shown in Fig. 1b, two strong peaks centered at about 654.0 and 642.3 eV, can be assigned to Mn2p1/2 and Mn2p3/2 of Mn4+ in MnO2, respectively. The spin energy separation was 11.7 eV, indicating that the predominant oxidation sate of Mn in MnPM(2)CNF was +4. Thus, this result suggested that MnO2 had been introduced into this composites [24]. The morphologies of the MnCNF, MnPM(2)CNF, and MnPM(3)CNF composites with PMMA contents were characterized by FESEM. Fig. 2a shows that diameters of the MnCNF are distributed within average diameter of 700 nm and smooth and uniform surface. The average fiber diameter of MnPM(2)CNF is thinner than that of MnPM(0)CNF at about 490 nm and MnPM(2)CNF retains a smooth surface, as shown in Fig. 2b. Interestingly, measuring the fiber diameter of MnPM(3)CNF (Fig. 2c) became difficult, because some fiber-fiber interconnections occurred at the intersection areas and the fibers were merged around the fiber-fiber intersection areas. This deformed inter8

bonded fibrous structure may have caused the curled morphology and the rough surface of the composites. In the cross sectional field-emission SEM image, MnPM(0)CNF without PMMA has no hollow cores, while the hollow cores in a single fiber of MnPM(3)CNF are clearly visible in Fig. 2(a-1) and (c-1). In this work, two immiscible polymer solutions (PAN/PMMA) induced the phase separation due to their different surface tension. The PAN part in the continuous phase stabilized the original morphology and was easily transformed into carbon during the oxidization and cyclization to form a three-dimensional chemical ladder structure. Whereas the PMMA solution induced the formation of many hollow cores, because the elongated PMMA phase in the discontinuous phase decomposes for pore-creating material via thermal decomposition [16, 25-27]. The microstructure and porous structure of the MnPMCNF composites was further characterized by TEM. TEM images for these fibers show the same morphology as observed in SEM. The MnPM(0)CNF composites had a smooth surface with one phase (Fig. 3a), while MnPM(2)CNF and MnPM(3)CNF NFs (Fig. 3b-c) became rougher with striped patterns, indicating the long hollow cores were well developed along the fiber length. The SAED pattern for MnPM(3)CNF (inset Fig. 3c) exhibits only two broad diffraction circles, which is indexed to the [111] and [311] diffraction rings; this pattern is suggestive of crystal planes of MnO2 with a relatively poor crystallinity (JCPDS File Card No. 42-1169) [9]. To confirm the distributed composition of the MnO2 particles, dark-field TEM images were obtained, as shown in the insets of Fig. 3b and Fig. 3d. They further confirm the crystalline MnO2 NPs (bright spot) and amorphous nature of the MnO2 (black spot), indicating that most of the amorphous MnO2 NPs were uniformly distributed and embedded in the CNF matrix. 9

PMMA with oxygen functional groups is capable of anchoring MnO2 NPs without aggregation effects such that they are preserved inside the CNF surface by the discontinuous and long, rod-like PMMA phase. The results are in good agreement with the SAED and SEM data. In the corresponding EDX spectrum (the inset of Fig. 3d), the individual fibers consist of C, O and Mn and the atomic percentages of C, O, and Mn in MnPM(3)CNF are estimated to be 81.09, 11.92, and 6.11%, respectively, indicating the formation of MnO2 particles. The

nitrogen

adsorption/desorption

isotherms

of

MnPM(0)CNF,

MnPM(2)CNF,

and

MnPM(3)CNF are shown in Fig. 4a. The adsorption isotherms of MnPM(0)CNF showed typical type I behavior representing the microporous adsorption, while MnPM(2)CNF and MnPM(3)CNF exhibited type IV isotherm curves and showed hysteresis loops at a relative pressure P/P0 of 0.45-1.0, which is a feature of capillary condensation occurring in mesoporous carbon. As shown in Fig. 4b, the porous composites exhibit an improved microstructure in terms of increased surface area and large mesopore volume with increasing PMMA content, due to the numerous hollow cores created within CNFs, suggesting that the PMMA phase plays a key role in developing mesopores during thermal decomposition. The electrochemical performance of the free-standing composites with large accessible surface areas and pore structures was tested in a two-electrode configuration with 6 M KOH as the electrolyte. All samples were cut into pieces of the web and directly used for the electrode, without the addition of any polymer binder, such as poly(vinylidene fluoride), or conducting agent, such as super-p, because their web fabrication offered adequate contact between the sample and the current 10

collector. The capacitive properties of the MnPM(0)CNF, MnPM(2)CNF, and MnPM(3)CNF electrodes was studied by CV technique at different scan rates ranging from 10 to 100 mVs-1, as shown in Fig. 5a-c. The CV curves of all electrodes at the low scan rate of 10 mVs-1 exhibit almost ideal rectangle shape, suggesting good electrochemical reversibility and capacitive behavior. In particular, the CV curves of MnPMCNF retained their rectangular shape until the scan rate reached 100 mVs-1with increasing the PMMA concentration, indicating that MnPM(3)CNF exhibited good rate capability. Furthermore, the MnPM(3)CNF electrode shows a much larger quasi-rectangular shape than the other samples at the scan rate of 50 mVs-1, as shown in Fig. 5d. This result represents the expansion of the electrical double-layer region and rapid chemisorption/desorption reaction rate with a low equivalent series resistance because the MnPM(3)CNF composite has a large surface area with high mesoporosity [6]. Galvanostatic charge-discharge properties were measured in the voltage range of 0-1.0 V with increasing current density to investigate the capacitance and rate behavior (Fig. 6a). The specific capacitances of the MnPM(0)CNF, MnPM(2)CNF, and MnPM(3)CNF electrodes were calculated to be 190, 209, and 228 Fg-1, which is consistent with the CV data. The reduction of specific capacitance was less than 17% of the initial value at a discharge current of 20mAcm-2 for MnPM(2)CNF and MnPM(3)CNF, but not for the MnPM(0)CNF electrode (~37%). This result demonstrates that the MnPM(2)CNF and MnPM(3)CNF electrodes continued to deliver specific capacitance at the highest discharge current density by efficient ion diffusion and continuous electron pathways, whereas the CNF electrode without MnO2 showed the lowest specific 11

capacitance of 60 Fg-1 at the initial value and capacitance retention of 16 %. Thus, the specific capacitance and capacitance retention at high discharge rates of the MnPMCNF composites may be further enhanced by the incorporation of the redox-type capacitor of MnO2 in the electric doublelayer capacitance of CNF matrix with a large surface area. The energy density and power density are the crucial parameters for supercapacitor applications. The Ragone plot calculated from the galvanostatic charge-discharge data is shown in Fig. 6b. The energy densities stored in the three capacitors decreased in the order MnPM(3)CNF>MnPM(2)CNF>MnPM(0)CNF> CNF and were 27, 24, 20, and 6 Whkg-1, respectively, at a power density of 400 Wkg-1. The MnPM(3)CNF and MnPM(2)CNF electrodes possessed an energy density of 18 and 14 Whkg-1 as the power density was increased to 10 kWkg-1, providing good rate performance, while the energy density of the MnPM(0)CNF electrode rapidly dropped to 7 Whkg-1 at the same power density. Furthermore, the CNF electrodes showed even lower energy density of 3.5 Whkg-1 at power densities of 4,000 Wkg-1. The Ragone plot reveals that the MnPM(3)CNF and MnPM(2)CNF electrodes exhibit higher power capability than that of MnPM(0)CNF and CNF, because a large fraction of mesopores (60-80% of total pore volume fraction) are beneficial for providing quick pathways for electrolyte transportation and maintaining high specific energy at high power densities [28]. Electrochemical impedance spectroscopy (EIS) measurements were used to assess the performance of the supercapacitors. The impedance spectra consist of a vertical line at the lowfrequency and a quasi-semicircle at high-frequency. As shown in the Nyquist plots in Fig. 7, the nearly straight line represented the diffusive resistance and the mass transfer rate of the electrolyte in 12

the electrode pores, and the slope linearly increases with increasing PMMA concentration. The charge transfer resistance (Rf) is related to the reversibility of the electrochemical reaction caused by the double layer capacitance and redox reactions [13]. The Rf of the MnPM(0)CNF, MnPM(2)CNF, and MnPM(3)CNF electrodes was calculated as 7.02, 2.72, and 0.65 Ω, respectively. Thus, the MnPM(3)CNF electrode with small charger transfer resistance and low diffusive resistance could improve the accessibility of ions on the pore surface and in the rapid ion channels, thereby enhancing the capacitance and energy/power capabilities. Fig. 7b exhibits the behavior of the resistance of the supercapacitors as a function of frequency. For decreasing frequencies from 1 KHz to 0.01 Hz, the Re[Z] decreases in the order of MnPM(0)CNF > MnPM(2)CNF > MnPM(3)CNF. Therefore, the MnPM(3)CNF electrode with the large mesopore volume shows a lower resistance than the other electrodes, indicating rapid ion transport and low resistance for charge diffusion in the electrolyte. Fig. 8 shows the cycling stability of MnPM(3)CNF electrode measured by constant current charging/discharging at 1 mAcm-2 for 1000 cycles in 6.0 M KOH aqueous electrolyte. The MnPM(3)CNF electrodes show high electrochemical reversibility and lost only 8% after 1000 cycles. The CV curves acquired at a scan rate of 50 mVs-1 for the MnPM(3)CNF electrode are shown in inset fig. 8. The CV curves for both electrodes are rectangular after 1000 cycles, revealing the longterm charge-discharge behavior of the MnPM(3)CNF electrodes as a supercapacitor. The superior electrochemical behavior of the MnPMCNF electrode highlights the importance of incorporating the MnO2 nanostructure and hierarchical structure into the composite, resulting in a synergistic 13

contribution from the redox pseudocapacitance and the electric double layer capacitance.

4. Conclusions Hierarchical porous MnO2/CNF composites were fabricated using PMMA as a sacrificial template, which is a key factor affecting the formation of many hollow cores. The MnPMCNF composites showed variable porous textures and remarkably enhanced specific surface area and volume of both mesopores and micropores by varying the PMMA content to optimize the electrochemical performance in an aqueous electrolyte. With its oxygen functional groups, PMMA was capable of anchoring MnO2 NPs without aggregation effects such that they were preserved inside the CNF surface. The superior electrochemical performance of MnPMCNF was mainly attributed to the interaction between the amorphous MnO2 and the many hollow cores within a single CNF, which can store energy by redox reactions on the electrode surface and enhance the ion transfer rate into the pores. Hence, these MnPMCNF composites have a very promising potential as electrode materials for energy storage applications due to their high capacitance, high energy/power efficiency, and high rate capability. Acknowledgment This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF- 2014R1A1A3053053).

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Scheme 1.Schematic llustration for the preparation of MnPMCNF composites

19

Fig. 1. (a) Wide-scan XPS spectrum of MnPM(2)CNF, and (b) deconvolution of Mn 2p core levels.

20

Fig. 2. FE-SEM images showing the morphology of (a) MnCNF, (b) MnPM(2)CNF, and (c) MnPM(3)CNF, and cross-sectional field-emission SEM images for (a-1) MnCNF, and (c-1) MnPM(3)CNF.

21

Fig. 3. TEM images of (a)MnCNF, (b) MnPM(2)CNF, and (c) MnPM(3)CNF, and (d) dark-field TEM image of MnPM(3)CNF. The inset shows the SAED pattern EDX data of MnPM(3)CNF.

22

Fig. 4. Nitrogen adsorption-desorption isotherms at 77 K, and (b) specific surface area and micropore/mesopore volumes.

23

Fig. 5. CV curves of (a)MnPM(0)CNF, (b) MnPM(2)CNF, and (c) MnPM(3)CNF at different scan rates, and (d) CVs of the three composites at a scan rate of 50 mVs-1 in 6.0 M KOH aqueous solution.

24

Fig. 6. Electrochemical tests of various electrodes in 6.0 M KOH (aq) electrolyte: (a) specific capacitance as a function of a various current densities, and (b) Ragone plots,

25

Fig. 7. (a) Complex-plane impedance plots at a perturbation amplitude of 10 mV, (b) Re[Z] vs. frequency plot for the different electrodes.

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Fig. 8. (a) The variation of specific capacitance of MnPM(3)CNF over 1000 cycles at a constant current density of 1 mAcm-2 in 6 M KOH aqueous electrolyte (the inset figures show CV curves at scan rate of 50 mVs-1 acquired for both devices after running for 1000 cycles.

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