Rational design of MnO2-nanosheets-decroated hierarchical porous carbon nanofiber frameworks as high-performance supercapacitor electrode materials

Electrochimica Acta 324 (2019) 134891

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Rational design of MnO2-nanosheets-decroated hierarchical porous carbon nanofiber frameworks as high-performance supercapacitor electrode materials Yi Yang, Bo-wen Deng, Xu Liu, Yan Li, Bo Yin*, Ming-bo Yang College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan, People's Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 July 2019 Received in revised form 8 September 2019 Accepted 14 September 2019 Available online 16 September 2019

A hierarchical structure is proved to be important for the improvement of electrochemical performances of MnO2-based electrode materials used in supercapacitors. However, the rational design of materials’ structure and its real performance served as electrode materials are still facing challenges. Herein, hierarchical porous carbon nanofibers using ZIF-8 nanoparticles as pore templates are designed to provide one-dimensional hollow frameworks for the growth of ultrathin MnO2 nanosheets (referred as HPCNF/ MnO2). The as-prepared HPCNF/MnO2 samples have a hierarchical coaxial core-shell structure, with HPCNFs as the core and MnO2 sheets as the shell. The novel nanostructure of this hybrid provides abundant electrochemical active sites for Faradic reactions and short diffusion channels for ions/electron transport. The utilization of HPCNF/MnO2 active material with such a structure can be significantly enhanced, especially at the high current density. When evaluated as electrode materials, HPCNF/MnO2 exhibits a specific capacitance of 269 F g
1 at 0.5 A g
1 (308 F g
1 for MnO2), a high capacitive retention of 98% even after 5000 cycles at 50 mV s
1 by cyclic voltammetry (86.7% capacitance retention at 1 A g
1 after 2000 cycles) and a rate capability of 58% from 0.5 to 10 A g
1. Especially at the current density of 10 A g
1, the specific capacitance of HPCNF/MnO2 has increased by 133% compared with that of nonporous carbon fiber/MnO2 composites, demonstrating a great superiority of hierarchical porous carbon nanofibers as frameworks to support pseudocapacitive materials. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Supercapacitors Hierarchical structure Porous carbon nanofiber MnO2 nanosheets

1. Introduction Supercapacitors have attracted great attentions as a promising energy storage device in recent years [1,2]. It plays an important role as the gateway to connect the capacitors and batteries, for its advantages of long cycle life, fast charge/discharge rate and high power density [3,4]. A lot of work has been carried out, mainly in the directions of energy storage theory research, electrode materials and capacitor devices [5e9]. Among them, the electrode material is the most widely talked about and studied because of its direct and decisive influence on the electrochemical performance of supercapacitors [10]. The most commonly used electrode materials include carbon materials, transition metal oxides and conducting polymers [11e16]. The former relies on the charge storage

* Corresponding author. E-mail address: [email protected] (B. Yin). https://doi.org/10.1016/j.electacta.2019.134891 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

mechanism of electrical double layer capacitors (EDLCs) which is caused by the electrostatic charge separation and accumulation at the interface of electrode and electrolyte [17]. The latter two are based on the fast and reversible Faradaic redox processes on the surface or in the bulk of active materials of pseudocapacitors [18]. Compared with the other two types of materials as electrodes, transition metal oxide usually exhibits higher specific capacitance and energy density in diverse and complex morphology and structures [19e23]. Manganese dioxide (MnO2) is one of the most widely studied transition metal oxides in view of its good prospect for potential applications, due to its moderate price, rich natural reserves and outstanding theoretical specific capacitance (1380 F g
1) [24e26]. Moreover, MnO2 also has a wide operating potential window in neutral aqueous electrolytes, with obvious advantages over the ones (e.g. NiO and Co3O4) mainly used in strong acid/alkaline solutions [27,28]. However, MnO2 suffers from a relatively poor electrical conductivity (10
5-10
6 S/cm), which would lead to

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Y. Yang et al. / Electrochimica Acta 324 (2019) 134891

unpleasant electrochemical performances when used as electrode materials [29]. Thus, to overcome the issues mentioned, two general strategies are usually considered. One is to build reasonable structures for MnO2 to provide abundant accessible pathways for ions/electron transport and additional electrochemical active sites for fast and reversible redox reactions [30e32]. For example, Liu and his coworkers designed specific heteronanostructures for high mass loading MnO2 electrodes by the interconnection of primary 2D ε-MnO2 nanosheets and secondary 1D a-MnO2 nanorod arrays to get a remarkable areal capacitance of 3.04 F cm
2 (3 mA cm
2) and an excellent rate capability [33]. Goli Nagaraju et al. constructed a core-shell-like MnO2 structure by electrochemical deposition which had an improved specific capacitance of 244.54 F g
1 at 0.5 A g
1 compared with that of bare MnO2 (112.1 F g
1 at 0.5 A g
1) [34]. The other strategy is to incorporate MnO2 with conductive materials, such as carbon materials or metals, as supporting frameworks to simultaneously improve electrical conductivity and endow specific nanostructures for MnO2 based nanocomposites [34,35]. Du et al. synthesized a kind of Ndoped porous carbon spheres/MnO2 nanocomposites using biological cell as template to get a specific capacitance of 255 F g
1 at 1 A g
1 in 1 M Na2SO4 and a cycling stability of 93.9% capacitance retention after 5000 cycles [36]. In order to improve electrochemical properties of MnO2 based electrodes, Wang's group decorated hierarchical MnO2 ultrathin nanoplates with graphene sheets using a polyaniline-assisted growth method, and the graphene/MnO2 electrode exhibited an outstanding specific capacitance of 245.0 F g
1 at a current density of 0.5 A g
1, 74.5% of retention ratio at 20 A g
1 [37]. Although electrode materials with improved electrochemical properties have been acquired, the rational design and preparation of hierarchical nanostructures for MnO2-based materials are still a big challenge, due to the difficulties in the control of structural stability and regularity, which would significantly influence the electroactive specific surface area and the effective exploitation of active materials. To address this problem, we develop a facile method to prepare a novel 1D hierarchical porous carbon nanofiber framework (HPCNF) to support the growth of MnO2 nanosheets. As a result, HPCNF/MnO2 hybrids with stable coaxial core-shell structure are obtained. In more details, regularly shaped ZIF-8 nanoparticles [38] are used as pore-forming templates to be electrospun with polyacrylonitrile (PAN) to get 1D composite precursor, which is then carbonated to form HPCNFs with rich regular pores and good electrical conductivity. Afterwards, ultrathin MnO2 nanoplates are grown on the surface of HPCNFs. The novel hierarchical structures of ultrathin MnO2 nanoplates/HPCNFs hybrids can provide additional electroactive sites, short diffusion pathways for ions/electrons transfer and maximum utilization of MnO2 materials. Thus, HPCNF/MnO2 hybrids exhibit pretty good electrochemical properties compared with that of non-porous carbon fiber/MnO2 composites as electrode materials.

2.2. Preparation of hierarchical porous carbon nanofiber frameworks (HPCNFs) In a typical procedure, a dispersion consisting of 0.706 g ZIF-8 nanoparticles and 5 mL of N, N-dimethylformamide (DMF) was prepared by sonication, followed by the addition of 0.47 g of polyacrylonitrile (PAN, Mw ¼ 150000) with vigorous stirring for 12 h. Then the precursor solution was loaded into a plastic syringe using a stainless-steel nozzle with a feed rate of 1 mL/h. The voltage between the needle tip and aluminum foil collector was 18 kV. Afterwards, the collected electrospun PAN/ZIF-8 fabrics were peroxided in air for 30 min at 250  C, followed by carbonization process in N2 at 800  C for 1 h. Subsequently, carbonized products were washed with 1 M H2SO4 solution to remove residual Zn species completely, and then rinsed with ethanol and deionized water to neutral. Hierarchical porous carbon nanofiber frameworks (HPCNFs) were finally obtained after drying under vacuum at 60  C for 24 h. The control sample of PAN-based carbon nanofibers (P-CNFs) was prepared by the same steps and conditions. The difference was that its precursor solution for electrostatic spinning was composed of 13 wt% PAN with no ZIF-8 added to keep a similar diameter with HPCNFs after carbonation. 2.3. Synthesis of HPCNF/MnO2 hybrids 0.03 g HPCNFs (or P-CNFs) were immersed into 75 ml of 0.01 M neutral KMnO4 solution under magnetic stirring at 70  C for 24 h. After the reaction (Eq. (1)), products were obtained, and rinsed with deionized water several times. Finally, HPCNF/MnO2 hybrids were collected after drying at 60  C for 24 h in a vacuum oven. 2

4KMnO
4 þ 3C þ H2 O/4MnO2 þ CO3 þ 2HCO3

(1)

2.4. Materials characterization The crystal phase was investigated by X-ray powder diffraction (XRD; Ultima IV, Cu Ka radiation, l ¼ 1.5406 Å). The morphology and structure were examined by a transmission electron microscope (TEM; Tecnai G2 F20 S-TWIN) and a field emission scanning electron microscope (FESEM; JSM-7500 F). The surface elemental analysis was tested by X-ray photoelectron spectroscopy (XPS, XSAM800) instrument with an Al Ka excitation source (hn ¼ 1486.6 eV) and energy dispersive spectrometer (EDS). The thermogravimetric analyse (TGA) was performed at 700  C on a thermogravimetric analyzer (TGA 2, METTLER TOLEDO) at a heating rate of 10  C$min
1 under air atmosphere. N2 adsorptiondesorption isotherms were obtained on a fully automatic specific surface area and microporous physical adsorption instrument (3Flex, Mike Instruments Company) used to calculate the specific surface area (based on BarretteEmmetteTeller (BET) theory), pore volumes and average pore sizes (according to the adsorption branch using the BarretteJoynereHalenda (BJH) model).

2. Experimental section 2.5. Electrochemical measurements 2.1. Synthesis of ZIF-8 nanoparticles Typically, a solution of 2-methylimidazole (MeIM, 6.568 g) in 100 ml of methanol (MeOH) was rapidly poured into a solution of Zn(NO3)2$6H2O (2.974 g) in 200 ml of MeOH under magnetic stirring at 25  C for 2 h. Then the precipitate was collected by centrifugation and washed with methanol. The ZIF-8 nanocrystals were finally dried under vacuum at 60  C for 18 h.

All electrochemical measurements were carried out on a CHI 660 E electrochemical workstation in 1 M Na2SO4 aqueous electrolyte. The electrochemical properties of the prepared electrodes were tested using a three-electrode system. Platinum plate and saturated calomel electrode (SCE) were served as the counter electrode and the reference electrode, respectively. To prepare electrode materials, the active material, acetylene black and

Y. Yang et al. / Electrochimica Acta 324 (2019) 134891

polytetrafluoroethylene (PTFE) were milled at a weight ratio of 80:10:10 in ethanol, then the as-obtained slurry was coated onto nickel (Ni) foam and dried at 60  C under vacuum for 24 h. After that, the obtained Ni foam was pressed at 10 MPa to get the working electrode. To test the electrochemical properties, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were carried out. The specific capacitance of the electrode material was calculated from discharge curves according to the following equation (Eq. (2)):

Cm ¼ I Dt=mDV

(2)

where Cm is the specific capacitance (F$g
1), I is the constant discharge current (A), Dt is the discharge time (s), m is the total mass of the active material (g) and DV is the set potential window (v). The specific capacitance could also be calculated from the CV curves by the following equation (Eq. (3)):

H

Cm ¼

IdV S ¼ 2vmDV 2vmDV

(3)

where Cm is the specific capacitance (F$g
1), S is the enclosed area of the CV curve, v is the potential scan rate (v$s
1), m is the total mass of the active material (g) and DV is the potential window (v). 3. Results and discussion The synthetic strategy for HPCNF/MnO2 hybrids is schematically depicted in Fig. 1. At first, porous carbon nanofibers are obtained by carbonization of electrospun PAN/ZIF-8 fibers, which are served as frameworks to support the deposition of MnO2 nanoplates. During the carbonization under N2, PAN and organic ligands of ZIF-8 will decompose and turn into carbon materials. In the meantime, the Zn2þ in ZIF-8 nanoparticles is reduced to metallic Zn and part of Zn vaporizes. As a result, ZIF-8 nanoparticles which are closeconnected by the carbon from the carbonization of PAN form porous nanofiber frameworks. Whereafter, to induce the growth of ultrathin MnO2 nanosheets onto porous nanofiber frameworks, HPCNFs are dispersed into a neutral solution of KMnO4 at a specified concentration. A self-limiting reaction based on nanoscale microelectrochemical cells mechanism between KMnO4 and carbon occurs [39e41]. As a result, MnO2 nanosheets can be regulated and deposited on the surface of HPCNFs to form the hybrid of HPCNF/MnO2. The hierarchical structure of HPCNF/MnO2 constructed by porous nanofiber frameworks and homogeneous MnO2 nanoflakes will provide abundant electrochemical active sites on its surface for Faradic reactions and facilitate faster ionic and

3

electronic transport, which are highly conducive to improve the electrochemical performance. As revealed by FESEM images in Fig. 2a, the as-spun PAN/ZIF-8 nanofiber appears as 1D nanostructure with ZIF-8 nanoparticles (as indicated by the red arrows) well dispersed in the body of nanofibers (inset in Fig. 2a), which is quite different from PAN electrospun fibers with no ZIF-8 added (Fig. S1). The corresponding TEM image in Fig. 2b also clearly reveals the presence of numerous nanoparticles bonded by PAN to form a nanofiber. After carbonation, the obtained HPCNFs are still present as 1D nanofibers with a uniform diameter of about 300e400 nm shown by FESEM images in Fig. 2c, which is similar to the diameter of P-CNFs (Fig. S2). Moreover, with the decomposition of polymer and removal of metallic Zn, porous structures appear within the nanofibers. Obvious pores with diameters of dozens of nanometers can be observed both on the surface and cross section of nanofibers (Fig. 2c and the inset in it). Similarly, TEM image of HPCNFs (Fig. 2d) further shows that many hollow particle frameworks are interconnected with each other within a nanofiber, which demonstrates the essence of hierarchical porous structures of HPCNFs and is distinct from the structure of P-CNFs (Fig. S2). After the deposition of MnO2 nanosheets, the obtained HPCNF/ MnO2 hybrids still keep the 1D morphology as depicted in Fig. 3a. Besides, it is easy to observe that the as-prepared nanofiber frameworks are homogeneously covered by ultrathin MnO2 nanosheets to form a coaxial core-shell structure (Fig. 3a). The interconnected MnO2 nanosheets of the shell constitute a loose porous nanostructure which provides rich electrochemically active sites for Faradic reactions and enough space for ions transport. Thus, the structural feature of HPCNF/MnO2 hybrids will significantly increase the utilization of electrode materials and improve its electrochemical performance. The energy dispersive X-ray spectroscopy (EDS) elemental mapping images in Fig. 3b-d indicate the spatial distribution of elements C, Mn and O in HPCNF/MnO2 hybrids [42]. The diameter of HPCNF/MnO2 hybrid fiber is about 500 nm which is within the detection range of EDS. Thus, the elements Mn and O can be considered to evenly distribute in the shell of MnO2 and the element C is due to the existence of HPCNFs. To take a further insight of the morphology and structure of HPCNF/MnO2 hybrids, TEM and high resolution TEM (HRTEM) are carried out. The picture in Fig. 4a reveals the similar morphology as shown in Fig. 3a, a fiber with loose porous nanostructures of MnO2 wrapping it. We can also find out that alternating dark and bright areas are shown on the image of HPCNF/MnO2 hybrids in Fig. 3a, suggesting the existence of numerous pores in HPCNFs. Besides, ultrathin MnO2 nanosheets are visually presented by the magnified

Fig. 1. Schematic illustration of the preparation of HPCNF/MnO2 hybrids.

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Y. Yang et al. / Electrochimica Acta 324 (2019) 134891

Fig. 2. FESEM and TEM images of PAN/ZIF-8 electrospun fibers (a, b) and HPCNFs (c, d). Inset in Fig. 2a is the magnified FESEM image of PAN/ZIF-8 fibers. Inset in Fig. 2c is the cross section of HPCNFs.

TEM image in Fig. 4b. According to Mathieu Toupin's work about the charge storage mechanism of MnO2 electrodes in the aqueous electrolyte [43], only a very thin layer of MnO2 is involved in the charge storage process. Thus, the structure of ultrathin nanosheets of MnO2 is beneficial to increase the fraction of electrochemically addressable electrode active materials and make the best use of its performance. Moreover, the HRTEM image in Fig. 4c shows lattice fringes of MnO2 with an interplanar distance of 0.7 nm, which is consistent with the space of (0 0 1) planes of the brinessite-type MnO2 [44]. By FESEM, EDS Mapping, TEM and HRTEM, it is easy to confirm that the nanocomposite of hierarchical porous carbon nanofiber frameworks wrapped by ultrathin MnO2 nanosheets are successfully synthesized. XRD and XPS are employed to further investigate the crystalline phase and composition of HPCNF/MnO2 hybrids. In Fig. 5a, two diffraction peaks are seen locating at 24.62 and 43.50 (2q) for HPCNFs, which represent the typical (0 0 2) and (1 0 1) planes of amorphous carbon [45]. For HPCNF/MnO2 hybrids, four diffraction peaks at 12.64 , 25.16 , 37.02 and 66.02 indexed to the (0 0 1), (0 0 2), (1 1 1) and (0 2 0) planes of birnessite-type MnO2 (JCPDS NO. 42e1317), respectively, can be observed. The broad diffraction peaks indicate the as-synthesized MnO2 has a poor crystallinity [46]. Moreover, the purity of birnessite-type MnO2 can be affirmed by no other diffraction peaks. To further explore the composition of products and oxidation states of element Mn, high-resolution XPS spectra of O 1s, Mn 2p and Mn 3s are shown in Fig. 5bed. The Mn 2p spectrum in Fig. 5c shows a Mn 2p1/2 spin-orbit peak at 654.25 eV and a Mn 2p3/2 spin-orbit peak at 642.55 eV with a spin-energy separation of 11.7 eV, similar to those reported for MnO2 materials [33,47]. The detailed valence states of Mn can be further

studied by the core level spectrum of Mn 3s in Fig. 5d. As reported [43,48], the separation of peak energies of Mn 3s components increases linearly from 4.7 eV to 5.4 eV with the change of mean oxidation states of Mn from Mn4þ to Mn3þ. So as depicted in Fig. 5d, the separation energy is 4.7 eV and the mean manganese oxidation state can be recognized as 4.0 (Mn4þ). The O 1s spectrum of HPCNF/ MnO2 hybrids in Fig. 5b is fitted with three peaks at 529.9 eV, 531.4 eV and 432.8 eV, corresponding to MneOeMn for the tetravalent oxide, MneOH for an hydrated trivalent oxide and HeOeH for residual structure water [26,49], respectively. By the above results of XRD and XPS, the birnessite-type MnO2 of HPCNF/MnO2 hybrids is successfully prepared. To investigate the specific surface area and porous structure of HPCNF/MnO2 hybrids, the N2 adsorption-desorption isotherm is tested and a typical type IV curve with a remarkable H3-type hysteresis loop is shown in Fig. 6, indicating the presence of mesopores of the material [50,51]. Similar results of PeCNF/MnO2 can be observed in Fig. S3. Subsequently, the pore size distribution plot according to BJH model inset in Fig. 6 shows that the size of the pores has a pretty wide distribution at a range of 2e50 nm, and its average pore width is calculated about 9.9 nm according to the adsorption branch using BJH method in Fig. 6. Moreover, the pore volume of HPCNF/MnO2 material is about 0.4 cm3 g
1, and the BET specific surface area is 148 m2 g
1, both of which are much higher than those of PeCNF/MnO2 (0.12 cm3 g
1 and 43 m2 g
1, Table S1). Thus, the HPCNF/MnO2 electrode material with such high specific surface area and abundant pore structures not only potentially offers rich electrochemical active sites, but also is conducive to the penetration of electrolyte and transport of ion. In more detail, mesopores with wide distribution of pore sizes (2e50 nm) provide

Y. Yang et al. / Electrochimica Acta 324 (2019) 134891

5

Fig. 3. FESEM image (a) and EDS mapping images (b, c, d) of HPCNF/MnO2 hybrids.

Fig. 4. (a) TEM image of HPCNF/MnO2 hybrids. (b) Magnified TEM image of the selected region in Fig. 3a. (c) HRTEM image of MnO2 nanosheet of HPCNF/MnO2 hybrids.

fast ionic transportation, and micropores (~2 nm) provide a high surface area for the adsorption/desorption of electrolyte ions. The hierarchical nanostructure of electrode materials composed by numerous poles plays a vital role in the improvement of capacitance and rate performances [20,52e55]. Next, to investigate electrochemical properties of obtained HPCNF/MnO2 hybrids, especially to assess the role of hierarchical porous carbon nanofiber frameworks (HPCNFs) compared with PAN-based carbon nanofiber frameworks (PeCNF) in the hybrids, cyclic voltammetry and galvanostatic charge-discharge tests are carried out in a three-electrode system using 1 M Na2SO4 as electrolyte. As a typical way of CV to characterize the capacitive behavior of an electrode material, Fig. 7a shows CV curves of HPCNF/MnO2 hybrids at different scan rates of 10, 20, 50, 70 and 100 mV s
1 in a potential window of
0.1 Ve0.8 V versus SCE. And

the CV curves exhibit a symmetric and quasi-rectangular shape which suggests ideal capacitive behaviors from fast, successive and reversible redox reactions of Mn4þ/Mn3þ [48,55,56]. In detail, the charge storage mechanism of MnO2 is mainly based on the surface adsorption of electrolyte cations (Naþ) as well as proton (Hþ) incorporation as shown in the following Eq. (4) and Eq. (5) [34,43,57].

MnO2 þ Naþ þ e
4MnOONa

(4)

MnO2 þ Hþ þ e
4MnOOH

(5)

Subsequently, with the increase of scan rates from 10 mV s
1 to 100 mV s
1, the quasi-rectangular shape of CV curves is well retained, and the current response shows corresponding increases

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Y. Yang et al. / Electrochimica Acta 324 (2019) 134891

Fig. 5. XRD patterns (a), XPS spectra of O 1s (b), Mn 2p (c) and Mn 3s (d) of HPCNF/MnO2 hybrids.

Fig. 6. N2 adsorption-desorption isotherm and the corresponding BJH pore size distribution plot (Inset in the picture) of HPCNF/MnO2 hybrids.

as well. It reveals a good rate capability and a small equivalent series resistance of HPCNF/MnO2 hybrids as electrode active materials. The specific capacitance of the HPCNF/MnO2 electrode calculated from CV curves from 100 mV s
1 to 10 mV s
1 are 115, 132, 147, 182 and 204 F g
1, respectively, as shown in Fig. S4.

Galvanostatic charge-discharge is also used to investigate the specific capacitance and the rate capability of HPCNF/MnO2 hybrids. GCD curves of HPCNF/MnO2 at different current densities from 0.5 to 15 A g
1 are depicted in Fig. 7b. All GCD curves are in an almost symmetric triangular shape, indicating a good capacitive characteristic and excellent electrochemical reversibility of HPCNF/ MnO2 materials. No obvious IR drops are found in charge/discharge curves (Fig. 7b), indicating a low internal series resistance of electrode materials. In addition, specific capacitances of HPCNF/MnO2 hybrids at different current densities estimated from discharge curves are shown in Fig. 7c, and the capacitance retention at each current density is calculated. With the increase of current density, the specific capacitances of HPCNF/MnO2 electrodes reach 269, 242, 188, 156 and 137 F g
1 at 0.5, 1, 5, 10 and 15 A$g
1, respectively. The corresponding capacitance retentions are 100%, 90%, 70%, 58% and 51% of the initial specific capacitance at the current density of 0.5 A g
1 (Fig. 7c). Compared with the sample of PeCNF/MnO2 which is synthesized based on CNF frameworks (Fig. S5), we can find a significant increase of specific capacitance for HPCNF/MnO2 at different current densities (Fig. 8a). An increase of 57% at 0.5 A g
1 and even 133% at 10 A g
1 can be seen. Similarly, according to the inset in Fig. 8a, as the current density changes from 0.5 to 10 A g
1, HPCNF/MnO2 exhibits a much better rate capability, which is 58% of the initial specific capacitance, while the sample of PeCNF/MnO2 remains only 39%. Thus, a great improvement of the specific capacitance and rate performance can be observed on HPCNF/MnO2 hybrids, owing to the introduction of hierarchical porous carbon nanofiber frameworks (HPCNFs). Moreover, to further highlight the advantage of hierarchical porous carbon

Y. Yang et al. / Electrochimica Acta 324 (2019) 134891

7

Fig. 7. Electrochemical performances of HPCNF/MnO2 hybrids (a) CV curves at different scan rates, (b) Charge-discharge curves at different current densities, (c) Specific capacitances and corresponding rate capabilities at different current densities.

Fig. 8. (a) Comparison of specific capacitances of HPCNF/MnO2 and PeCNF/MnO2 at different current densities. Inset: Capacitance retentions of HPCNF/MnO2 and PeCNF/MnO2 at 10 A g
1. (b) Schematic illustration of the electrolyte ions/electrons transport through nanostructures of HPCNF/MnO2 hybrids.

nanofiber frameworks, the specific capacitance of MnO2 in the HPCNF/MnO2 and PeCNF/MnO2 hybrids, excluding carbon, is calculated, which is 200 and 308 Fg
1 (Fig. S6), respectively, at a current density of 0.5 A g
1. The result also turns out that the incorporation of HPCNF can make better use of MnO2 and improve its electrochemical performance much more compared with the incorporation of PeCNF. To get deeper insight into the relationship between the novel structure of HPCNF/MnO2 and the improvement of the specific

capacitance and rate performance compared with PeCNF/MnO2, the electrolyte ions/electron transport through hierarchical structures of HPCNF/MnO2 is depicted as the schematic illustration in Fig. 8b. Because of the hierarchical structure of HPCNF/MnO2 attributed to interconnected MnO2 nanosheets in the shell and the porous nature of HPCNFs core, short and unimpeded diffusion channels are prepared conducive to the electrolyte penetration and fast transport of ions [58,59]. The good conductivity of HPCNFs itself (green paths depicted in Fig. 8b) also serves the electron

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Y. Yang et al. / Electrochimica Acta 324 (2019) 134891

Table 1 Electrochemical performances of C/MnO2-based electrode materials reported and in this work. Materials

Electrolyte

Specific capacitance

Refs.

Carbon spheres/MnO2 composites Well-matched graphene@MnO2 Coreeshell-like MnO2@MnO2 on carbon fibers MnO2 on carbon nanotubes Hollow carbon spheres/MnO2 nanofibers MnO2/R-GO@Ni-foam Hollow MnO2 nanofibers HPCNF/MnO2 hybrids

1M 1M 1M 1M 1M 1M 1M 1M

255 F g
1 at 1 A g
1 245 F g
1 at 0.5 A g
1 244.54 F g
1 at 0.5 A g
1 247.9 F g
1 at 0.15 A$g
1 190 F g
1 at 0.1 A$g
1 267 F g
1 at 0.25 A g
1 291 F g
1 at 1 A g
1 269 F g
1 at 0.5 A g
1

[36] [37] [34] [31] [61] [62] [63] This work

Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4 Na2SO4

transport and improves the conductivity of the bulk of HPCNF/ MnO2 compared with pure MnO2. Besides, the increased surface area of HPCNF/MnO2 compared with PeCNF/MnO2 provides richer electrochemical active sites for reversible redox reactions and more contacting interfaces between the electrode and electrolyte [44]. Thus, fast electrochemical kinetics during charging-discharging process is facilitated, and the rate performance and specific capacitance are greatly improved [60]. To further visualize the superior specific capacitance of HPCNF/MnO2 hybrids in this work, results of some C-based/MnO2 composites in recent papers are summarized in Table 1. To gain further insight into the conductive and ions/electron diffusive behaviors of HPCNF/MnO2 based electrode and identify the advantage of porous CNF frameworks (HPCNFs) used in the composites, electrochemical impedance spectroscopy (EIS) is examined. Firstly, by comparing Nyquist plots of HPCNF/MnO2 and PeCNF/MnO2 electrodes shown in Fig. 9a, it's easy to find out that both impendence spectra are consisted of a semicircular arc at high-frequency region whose diameter is corresponding to the charge transfer resistance (Rct) at the electrode-electrolyte interface and a nearly vertical line at low frequency related to the diffusive resistance (Zw) in electrode materials [64,65]. From the magnified high frequency domain inset in Fig. 9a, a much smaller semicircle of the HPCNF/MnO2 electrode can be observed, which means a smaller charge transfer resistance and an easier faradaic reaction compared with the PeCNF/MnO2 electrode [66]. The line with a higher slope of HPCNF/MnO2 electrode in the high-tomedium frequency range indicates a lower diffusion resistance [67]. In addition, Bode plots of phase angle versus frequency are shown in Fig. 9b. The corresponding frequency (fo) at the phase angle of
45 relates the equilibrium point of the resistive and capacitive impedances [17,65]. The magnitudes of fo for PeCNF/ MnO2 and HPCNF/MnO2 electrodes are 0.1 Hz and 0.24 Hz, respectively, corresponding to a characteristic relaxation time constant to (to ¼ 1/fo, the minimum time needed with an efficiency of greater than 50%) of 10.00 s and 4.24 s in Fig. 9b [68]. The rapid frequency response of the HPCNF/MnO2 electrode (4.24 s) indicates a much better ion transport rate of it brought by porous structures of HPCNFs which is consistent with the result of rate performance (Fig. 8a). Moreover, the characteristic relaxation time constant of HPCNF/MnO2 (4.24 s) is even lower than that of conventional activated carbon-based ECs (10 s) [69]. Subsequently, the magnitude of the Z0 -intercept in Fig. 9a attributes to the equivalent series resistance of the system (Rs), which includes the ionic resistance of electrolyte, intrinsic resistance of the active materials and contact resistance caused by the current collector and the electrode interface [44,66]. It shows a lower value of 2.351 U for the HPCNF/MnO2 electrode compared with that of 2.688 U for the PeCNF/MnO2 electrode as present in the inset of Fig. 9a, consistent with IR drops of galvanostatic CD curves at 0.5 A g
1 for HPCNF/MnO2 (9.5 mV) and PeCNF/MnO2 (24.4 mV) (Fig. 9c). Thus, by detailed analysis of electrochemical impedance spectroscopy, we can find that Rct, Zw

and Rs of HPCNF/MnO2 electrodes are obviously lowered and its ion transport rate is significantly improved, which can be well explained by the construction of the hierarchical porous structure of HPCNFs/MnO2 composites (depicted as Fig. 8b). Finally, based on the well-designed nanostructure and better ions/electron diffusive behaviors of HPCNF/MnO2 electrode materials, the obvious improvement of the specific capacitance and rate performance compared with PeCNF/MnO2 can be easily understood. Moreover, the cycling test of HPCNF/MnO2 electrode is carried out with galvanostatic charge-discharge and it shows a good capacitance retention of 86.7% over 2000 cycles at a current density of 1 A g
1 (Fig. 10b). Besides, the cycling performance of HPCNF/ MnO2 electrodes is also examined by cyclic voltammetry for 5000 cycles at the scan rate of 50 mV s
1 as displayed in Fig. S7. It reveals that the HPCNF/MnO2 electrode keeps a high capacitive retention of 98% even after 5000 cycles. Both tests exhibit remarkable cyclic stability of HPCNF/MnO2 electrodes. The galvanostatic CD curves after 2000 cycles (Fig. S8a) still keep an almost symmetric triangular shape and no obvious IR drops can be observed, and the equivalent series resistance (Rs, inset in Fig. 10a) of HPCNF/MnO2 electrodes after 2000 cycles is 2.435 U, just with a slight increase compared with the initial sample (2.351U), indicating an outstanding stability behavior and a superior electrochemical reversibility. Besides, the HPCNF/MnO2 composites still keep a good morphology of a fiber (Fig. S8b) homogeneously covered by ultrathin MnO2 nanosheets after 2000 cycles, indicating a superior stability of its structure during galvanostatic charge-discharge cycles. The HPCNF/MnO2 electrode material exhibits remarkable electrochemical performances and great improvement compared with PeCNF/MnO2, ascribed to the unique nanostructure of HPCNF/ MnO2, especially the introduction of porous carbon nanofiber frameworks. The hierarchical pore structure of HPCNF can significant increase the specific surface for electrochemical active sites, and create paths for ion transport and electrolyte penetration in the main body of electrode material. Similarly, a loose porous nanostructure covered by interconnected MnO2 nanosheets also greatly enhances the electrode/electrolyte contact area and the intercalation/deintercalation of ions. It is worth mentioning that the ultrathin sheet structure of MnO2 is conducive to maximum utilization of electrode materials [43]. Thus, remarkable specific capacitance and rate capability of HPCNF/MnO2 electrodes are obtained. Moreover, the hierarchical structure can effectively accommodate the volumetric expansion and the collapse of active materials during charge/discharge cycles, resulting a good cycling stability. 4. Conclusion In conclusion, a hierarchical porous carbon nanofiber framework prepared from the electrospun PAN/ZIF-8 fiber is introduced to support ultrathin MnO2 nanosheets in this work. The obtained HPCNF/MnO2 nanocomposites with the novel structure can offer a high specific surface area up to 148 m2 g
1. When used as electrode

Y. Yang et al. / Electrochimica Acta 324 (2019) 134891

9

Fig. 9. (a) Nyquist plots. Inset: High-frequency domain of Nyquist plots, (b) Bode plots of phase angle versus frequency and (c) IR drops of galvanostatic charge-discharge curves at 0.5 A g
1 of HPCNF/MnO2 and PeCNF/MnO2 as electrode materials.

Fig. 10. (a) Nyquist plots of the HPCNF/MnO2 electrode before and after 2000 CD cycles. Inset: High-frequency domain of Nyquist plots. (b) Cycling performance of the HPCNF/MnO2 electrode at a current density of 1 A g
1.

material for supercapacitors, HPCNF/MnO2 exhibits a maximum specific capacitance of 269 F g
1 at 0.5 A g
1 (308 F g
1 for MnO2), 58% of the initial specific capacitance at 10 A g
1 and a high capacitive retention of 98% even after 5000 cycles at 50 mV s
1 by cyclic voltammetry (a good capacitance retention of 86.7% after 2000 cycles at 1 A g
1). More importantly, the specific capacitance of HPCNF/MnO2 has a 133% increase compared with that of non-

porous carbon fiber/MnO2 composites at 10 A g
1, which demonstrates the important role of hierarchical porous carbon nanofiber frameworks to increase the electrochemical performances of electrode materials, especially under high current density. Such a welldesigned conductive carbon framework can play a bigger role to support pseudocapacitive materials in the supercapacitor field.

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Y. Yang et al. / Electrochimica Acta 324 (2019) 134891

Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Contract No. 51573106 and 51721091).

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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134891. References [1] L. Zhang, X. Hu, Z. Wang, F. Sun, D.G. Dorrell, A review of supercapacitor modeling, estimation, and applications: a control/management perspective, Renew. Sustain. Energy Rev. 81 (2018) 1868e1878. [2] M. Huang, F. Li, F. Dong, Y.X. Zhang, L.L. Zhang, MnO2-based nanostructures for high-performance supercapacitors, J. Mater. Chem. 3 (2015) 21380e21423. [3] P. Huang, C. Lethien, S. Pinaud, K. Brousse, R. Laloo, V. Turq, M. Respaud, A. Demortia Re, B. Daffos, P.L. Taberna, On-chip and freestanding elastic carbon films for micro-supercapacitors, Science 351 (2016) 691e695. [4] S.F. Tie, C.W. Tan, A review of energy sources and energy management system in electric vehicles, Renew. Sustain. Energy Rev. 20 (2013) 82e102. [5] K. Fic, A. Platek, J. Piwek, E. Frackowiak, Sustainable materials for electrochemical capacitors, Mater. Today 21 (2018) 437e454. [6] F. Li, Z. Zhou, Micro/nanostructured materials for sodium ion batteries and capacitors, Small 14 (2018) 1702961. [7] N. Choudhary, C. Li, J. Moore, N. Nagaiah, L. Zhai, Y. Jung, J. Thomas, Asymmetric supercapacitor electrodes and devices, Adv. Mater. 29 (2017) 1605336. [8] Y. Gogotsi, P. Simon, Materials science. True performance metrics in electrochemical energy storage, Science 334 (2011) 917e918. [9] O. Munteshari, J. Lau, A. Likitchatchawankun, B.-A. Mei, C.S. Choi, D. Butts, B.S. Dunn, L. Pilon, Thermal signature of ion intercalation and surface redox reactions mechanisms in model pseudocapacitive electrodes, Electrochim. Acta 307 (2019) 512e524. [10] G. Wang, L. Zhang, J. Zhang, ChemInform Abstract: a review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797e828. [11] G.A. Snook, P. Kao, A.S. Best, Conducting-polymer-based supercapacitor devices and electrodes, J. Power Sources 196 (2011) 1e12. [12] L.F. Chen, Y. Lu, Y.U. Le, X.W.D. Lou, Designed formation of hollow particlebased nitrogen-doped carbon nanofibers for high-performance supercapacitors, Energy Environ. Sci. 10 (2017) 1777e1783. [13] H. Jia, Q. Li, C. Li, Y. Song, H. Zheng, J. Zhao, W. Zhang, X. Liu, Z. Liu, Y. Liu, A novel three-dimensional hierarchical NiCo2O4/Ni2P electrode for high energy asymmetric supercapacitor, Chem. Eng. J. 354 (2018) 254e260. [14] A. Mondal, K. Kretschmer, Y. Zhao, H. Liu, B. Sun, G. Wang, C.W. Wang, Nitrogen doped porous carbon nanosheets from eco-friendly eucalypt leaves as high performance electrode materials for supercapacitors and lithium ion batteries, Chem. Eur J. 23 (2017) 3683e3690. [15] A.K. Mondal, K. Kretschmer, Y. Zhao, H. Liu, H. Fan, G. Wang, Naturally nitrogen doped porous carbon derived from waste shrimp shells for highperformance lithium ion batteries and supercapacitors, Microporous Mesoporous Mater. 246 (2017) 72e80. [16] J. Hao, J. Wang, S. Qin, D. Liu, Y. Li, W. Lei, B/N co-doped carbon nanosphere frameworks as high-performance electrodes for supercapacitors, J. Mater. Chem. 6 (2018) 8053e8058. [17] Y. Xu, Z. Lin, X. Zhong, X. Huang, N.O. Weiss, Y. Huang, X. Duan, Holey graphene frameworks for highly efficient capacitive energy storage, Nat. Commun. 5 (2014) 4554. [18] Y. Xiao, S. Liu, L. Feng, A. Zhang, J. Zhao, S. Fang, D. Jia, 3D hierarchical Co3O4 twin-Spheres with an urchin-like structure: large-scale synthesis, multistepsplitting growth, and electrochemical pseudocapacitors, Adv. Funct. Mater. 22 (2012) 4052e4059. [19] J.J. Zhou, X. Han, K. Tao, Q. Li, Y.L. Li, C. Chen, L. Han, Shish-kebab type MnCo2O4@Co3O4 nanoneedle arrays derived from MnCo-LDH@ZIF-67 for high-performance supercapacitors and efficient oxygen evolution reaction, Chem. Eng. J. 354 (2018) 875e884. [20] L.L. Xing, X. Wu, K.J. Huang, High-performance supercapacitor based on threedimensional flower-shaped Li4Ti5O12-graphene hybrid and pine needles derived honeycomb carbon, J. colloid interf. Sci. 529 (2018) 171e179. [21] G. Saeed, S. Kumar, N.H. Kim, J.H. Lee, Fabrication of 3D graphene-CNTs/aMoO3 hybrid film as an advance electrode material for asymmetric supercapacitor with excellent energy density and cycling life, Chem. Eng. J. 352 (2018) 268e276. [22] P. Zhao, M. Yao, H. Ren, N. Wang, S. Komarneni, Nanocomposites of hierarchical ultrathin MnO2 nanosheets/hollow carbon nanofibers for highperformance asymmetric supercapacitors, Appl. Surf. Sci. 463 (2019) 931e938. [23] L. Zhang, Z. Chen, S. Zheng, S. Qin, J. Wang, C. Chen, D. Liu, L. Wang, G. Yang, Y. Su, Z.-S. Wu, X. Bao, J. Razal, W. Lei, Shape-tailorable high-energy

[26]

[27]

[28]

[29] [30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39] [40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

asymmetric micro-supercapacitors based on plasma reduced and nitrogendoped graphene oxide and MoO2 nanoparticles, J. Mater. Chem. 7 (2019) 14328e14336. Q.Y. Lv, S. Wang, H. Sun, J. Luo, J. Xiao, J. Xiao, F. Xiao, S. Wang, Solid-state thinfilm supercapacitors with ultrafast charge/discharge based on N-doped-carbon-tubes/Au-nanoparticles-doped-MnO2 nanocomposites, Nano Lett. 16 (2015) 40. K. Zhang, X. Han, Z. Hu, X. Zhang, Z. Tao, J. Chen, Nanostructured Mn-based oxides for electrochemical energy storage and conversion, Chem. Soc. Rev. 44 (2015) 699e728. D. Banerjee, H.W. Nesbitt, S.L.S. Stipp, P.V. Brady, K.V. Ragnarsdottir, L. Charlet, XPS study of reductive dissolution of birnessite by oxalate; rates and mechanistic aspects of dissolution and redox processes, Geochem. Cosmochim. Acta 63 (1999) 3025e3038. L. Seung Woo, K. Junhyung, C. Shuo, P.T. Hammond, S.H. Yang, Carbon nanotube/manganese oxide ultrathin film electrodes for electrochemical capacitors, ACS Nano 4 (2010) 3889e3896. Phosphate ion functionalized Co3O4 ultrathin nanosheets with greatly improved surface reactivity for high performance pseudocapacitors, Adv. Mater. 29 (2017) 1604167. H. Ming, L. Fei, D. Fan, X.Z. Yu, L.Z. Li, MnO2-based nanostructures for highperformance supercapacitors, J. Mater. Chem. 3 (2015) 21380e21423. Z. Ma, G. Shao, Y. Fan, G. Wang, J. Song, D. Shen, Construction of hierarchical aMnO2 nanowires@ultrathin d-MnO2 nanosheets core-shell nanostructure with excellent cycling stability for high-power asymmetric supercapacitor electrodes, ACS Appl. Mater. Interfaces 8 (2016) 9050e9058. H. Huang, W. Zhang, Y. Fu, X. Wang, Controlled growth of nanostructured MnO2 on carbon nanotubes for high-performance electrochemical capacitors, Electrochim. Acta 152 (2015) 480e488. F. Grote, R.S. Kuhnel, A. Balducci, Y. Lei, Template assisted fabrication of freestanding MnO2 nanotube and nanowire arrays and their application in supercapacitors, Appl. Phys. Lett. 104 (2014), 053904. Z.H. Huang, Y. Song, D.Y. Feng, Z. Sun, X. Sun, X.X. Liu, High mass loading MnO2 with hierarchical nanostructures for supercapacitors, ACS Nano 12 (2018) 3557e3567. G. Nagaraju, Y.H. Ko, S.M. Cha, S.H. Im, J.S. Yu, A facile one-step approach to hierarchically assembled coreeshell-like MnO2@MnO2 nanoarchitectures on carbon fibers: an efficient and flexible electrode material to enhance energy storage, Nano Res 9 (2016) 1507e1522. M. Pang, G. Long, S. Jiang, Y. Ji, W. Han, B. Wang, X. Liu, Y. Xi, One pot lowtemperature growth of hierarchical d-MnO2 nanosheets on nickel foam for supercapacitor applications, Electrochim. Acta 161 (2015) 297e304. W. Du, X. Wang, J. Zhan, X. Sun, L. Kang, F. Jiang, X. Zhang, Q. Shao, M. Dong, H. Liu, V. Murugadoss, Z. Guo, Biological cell template synthesis of nitrogendoped porous hollow carbon spheres/MnO2 composites for highperformance asymmetric supercapacitors, Electrochim. Acta 296 (2019) 907e915. L. Wang, Y. Ouyang, X. Jiao, X. Xia, W. Lei, Q. Hao, Polyaniline-assisted growth of MnO2 ultrathin nanosheets on graphene and porous graphene for asymmetric supercapacitor with enhanced energy density, Chem. Eng. J. 334 (2018) 1e9. J. Sanchez-Lainez, B. Zornoza, S. Friebe, J. Caro, S. Cao, A. Sabetghadam, B. Seoane, J. Gascon, F. Kapteijn, C. Le Guillouzer, G. Clet, M. Daturi, C. Tellez, J. Coronas, Influence of ZIF-8 particle size in the performance of polybenzimidazole mixed matrix membranes for pre-combustion CO2 capture and its validation through interlaboratory test, J. Membr. Sci. 515 (2016) 45e53. X. Jin, W. Zhou, S. Zhang, G.Z. Chen, Nanoscale microelectrochemical cells on carbon nanotubes, Small 3 (2007) 1513e1517. D. Zhou, H. Lin, F. Zhang, H. Niu, L. Cui, Q. Wang, F. Qu, Freestanding MnO2 nanoflakes/porous carbon nanofibers for high-performance flexible supercapacitor electrodes, Electrochim. Acta 161 (2015) 427e435. A.E. Fischer, K.A. Pettigrew, D.R. Rolison, R.M. Stroud, J.W. Long, Incorporation of homogeneous, nanoscale MnO2 within ultraporous carbon structures via self-limiting eectroless deposition: Implications for electrochemical capacitors, Nano Lett. 7 (2007) 281e286. H.H. Chang, C.L. Cheng, P.J. Huang, S.Y. Lin, Application of scanning electron microscopy and X-ray microanalysis: FE-SEM, ESEM-EDS, and EDS mapping for studying the characteristics of topographical microstructure and elemental mapping of human cardiac calcified deposition, Anal. Bioanal. Chem. 406 (2014) 359e366. langer, Charge storage mechanism of MnO2 M. Toupin, T. Brousse, D. Be electrode used in aqueous electrochemical capacitor, Chem. Mater. 16 (2004) 3184e3190. P. Liu, Y. Zhu, X. Gao, Y. Huang, Y. Wang, S. Qin, Y. Zhang, Rational construction of bowl-like MnO2 nanosheets with excellent electrochemical performance for supercapacitor electrodes, Chem. Eng. J. 350 (2018) 79e88. A. Cuesta, P. Dhamelincourt, J. Laureyns, A. Martínezalonso, J.M.D. Tascon, Comparative performance of X-ray diffraction and Raman microprobe techniques for the study of carbon materials, J. Mater. Chem. 8 (1998) 2875e2879. L. Tao, C. Jiang, Y. Wei, J. Yu, Hierarchical porous C/MnO2 composite hollow microspheres with enhanced supercapacitor performance, J. Mater. Chem. 5 (2017) 8635e8643. Y. Dai, L. Chen, V. Babayan, Q. Cheng, P. Saha, H. Jiang, C. Li, Ultrathin MnO2 nanoflakes grown on N-doped carbon nanoboxes for high-energy asymmetric supercapacitors, J. Mater. Chem. 3 (2015) 21337e21342.

Y. Yang et al. / Electrochimica Acta 324 (2019) 134891 [48] M. Toupin, T. Brousse, D. Belanger, Influence of microstucture on the charge storage properties of chemically synthesized manganese dioxide, Chem. Mater. 14 (2002) 3946e3952. [49] K.W. Nam, G.K. Min, K.B. Kim, In situ Mn K-edge X-ray absorption spectroscopy studies of electrodeposited manganese oxide films for electrochemical capacitors, J. Phys. Chem. C 111 (2007) 749e758. [50] P. Gao, P. Metz, T. Hey, Y. Gong, D. Liu, D.D. Edwards, J.Y. Howe, R. Huang, S.T. Misture, The critical role of point defects in improving the specific capacitance of delta-MnO2 nanosheets, Nat. Commun. 8 (2017) 14559. [51] S.M. Paek, H. Jung, Y.J. Lee, P. Man, S.J. Hwang, J.H. Choy, Exfoliation and reassembling route to mesoporous titania nanohybrids, Chem. Mater. 18 (2006) 1134e1140. [52] L. Celine, P. Cristelle, C. John, T. Pierre-Louis, G. Yury, S. Patrice, Relation between the ion size and pore size for an electric double-layer capacitor, J. Am. Chem. Soc. 130 (2008) 2730e2731. [53] S. Zhu, H. Tian, N. Wang, B. Chen, Y. Mai, X. Feng, Patterning graphene surfaces with iron-oxide-embedded mesoporous polypyrrole and derived N-doped carbon of tunable pore size, Small 14 (2018). [54] Y. Yanfeng, A.J. Binder, G. Bingkun, Z. Zhiyong, Q. Zhen-An, T. Chengcheng, D. Sheng, Mesoporous prussian blue analogues: template-free synthesis and sodium-ion battery applications, Angew. Chem. Int. Ed. 53 (2014) 3134e3137. [55] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845e854. [56] J. Zhang, X. Zhao, Z. Huang, T. Xu, Q. Zhang, High-performance all-solid-state flexible supercapacitors based on manganese dioxide/carbon fibers, Carbon 107 (2016) 844e851. [57] H.Y. Lee, J.B. Goodenough, Supercapacitor behavior with KCl electrolyte, J. Solid State Chem. 144 (2015) 220e223. [58] A.G. Tabrizi, N. Arsalani, A. Mohammadi, L.S. Ghadimi, I. Ahadzadeh, Highperformance asymmetric supercapacitor based on hierarchical nanocomposites of polyaniline nanoarrays on graphene oxide and its derived Ndoped carbon nanoarrays grown on graphene sheets, J. Colloid Interface Sci. 531 (2018) 369e381. [59] Y. Hu, H. Liu, Q. Ke, J. Wang, Effects of nitrogen doping on supercapacitor

[60]

[61]

[62]

[63] [64]

[65]

[66]

[67]

[68]

[69]

11

performance of a mesoporous carbon electrode produced by a hydrothermal soft-templating process, J. Mater. Chem. 2 (2014) 11753e11758. C. Zhu, M. Takata, Y. Aoki, H. Habazaki, Nitrogen-doped porous carbon asmediated by a facile solution combustion synthesis for supercapacitor and oxygen reduction electrocatalyst, Chem. Eng. J. 350 (2018) 278e289. Z. Lei, J. Zhang, X.S. Zhao, Ultrathin MnO2 nanofibers grown on graphitic carbon spheres as high-performance asymmetric supercapacitor electrodes, J. Mater. Chem. 22 (2012) 153e160. Y. Li, G. Wang, K. Ye, K. Cheng, Y. Pan, P. Yan, J. Yin, D. Cao, Facile preparation of three-dimensional multilayer porous MnO2/reduced graphene oxide composite and its supercapacitive performance, J. Power Sources 271 (2014) 582e588. K. Xu, S. Li, J. Yang, J. Hu, Hierarchical hollow MnO2 nanofibers with enhanced supercapacitor performance, J. Colloid Interface Sci. 513 (2018) 448e454. Q. Wang, Y. Ma, X. Liang, D. Zhang, M. Miao, Flexible supercapacitors based on carbon nanotube-MnO2 nanocomposite film electrode, Chem. Eng. J. 371 (2019) 145e153. P.S.P.L. Taberna, P. Simon, J.F. Fauvarque, Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors, J. Electrochem. Soc. 150 (2003) A292eA300. J. Chang, M. Jin, F. Yao, T.H. Kim, V.T. Le, H. Yue, F. Gunes, B. Li, A. Ghosh, S. Xie, Y.H. Lee, Asymmetric supercapacitors based on graphene/MnO2 nanospheres and graphene/MoO3 nanosheets with high energy density, Adv. Funct. Mater. 23 (2013) 5074e5083. B. Xu, H. Wang, Q. Zhu, N. Sun, B. Anasori, L. Hu, F. Wang, Y. Guan, Y. Gogotsi, Reduced graphene oxide as a multi-functional conductive binder for supercapacitor electrodes, Energy Storage Mater 12 (2018) 128e136. C. Portet, P.L. Taberna, P. Simon, E. Flahaut, C. Laberty-Robert, High power density electrodes for Carbon supercapacitor applications, Electrochim. Acta 50 (2005) 4174e4181. M.F. El-Kady, S. Veronica, D. Sergey, R.B. Kaner, Laser scribing of highperformance and flexible graphene-based electrochemical capacitors, Science 335 (2012) 1326.