MnO2 nanosheets decorated porous active carbon derived from wheat bran for high-performance asymmetric supercapacitor

Journal of Electroanalytical Chemistry 850 (2019) 113412

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MnO2 nanosheets decorated porous active carbon derived from wheat bran for high-performance asymmetric supercapacitor Shuying Kong a, Binbin Jin a, Xin Quan a, Guoqing Zhang a,⁎, Xiaogang Guo a, Qiuyin Zhu a, Fan Yang b, Kui Cheng c,d,⁎, Guiling Wang c, Dianxue Cao c a

Chongqing Key Laboratory of Inorganic Special Functional Materials, College of Chemistry and Chemical Engineering, Yangtze Normal University, Chong Qing 408000, China College of Water Resources and Civil Engineering, Northeast Agricultural University, Harbin 150030, China Key Laboratory of Superlight Material and Surface Technology of Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China d Colleage of Engineering, Northeast Agricultural University, Harbin 150030, China b c

a r t i c l e

i n f o

Article history: Received 31 March 2019 Received in revised form 18 July 2019 Accepted 27 August 2019 Available online 27 August 2019 Keywords: Molten salt Porous carbon MnO2 Asymmetric supercapacitor Energy density

a b s t r a c t MnO2 is regarded as an ideal material of supercapacitor since its low-cost, environment friendly and high specific capacitance but hindered by its poor electrical conductivity. Developing a composite electrode that combines nano-structure MnO2 with a conductive skeleton such as carbon materials could make up for the shortcomings. Here, porous activated carbon (PAC) is synthesized by using low-cost wheat bran as biomass carbon precursor and a mixture of NaCl/ZnCl2 as combined solvent-porogen. The resultant PAC sample presents a hierarchical porous structure and large specific surface area up to 1058 m2 g−1. Afterwards, MnO2 nanosheets decorated PAC (MnO2@PAC) is prepared via an in-situ hydrothermal deposition. It is a key finding that the ion/electron transfer kinetics of MnO2@PAC could be effectively improved by the addition of hierarchical porous carbon. Thus, the MnO2@PAC electrode displays a high specific capacitance (258 F g−1 at 1 A g−1) and superior rate performance (82.8% capacitance retention with the current density ranging from 1 A g−1 to 20 A g−1). Furthermore, an asymmetric supercapacitor is assembled by employing the MnO2@PAC as the positive electrode and PAC as negative electrode, which exhibits a high energy density of 32.6 Wh kg−1 and as well as 93.6% capacity retention at over 10,000 charge/discharge cycles. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, the grand challenges facing our society result from fossil fuels excessive consumption and severe environmental pollution, thus, the development of green, sustainable, high-performance energy storage devices have become extremely urgent. In this context, supercapacitors have attracted significant attention due to their high specific power, rapid charging/discharging rate and long cycle life, and as well as their wide range of practical applications, such as electronic devices, electrical vehicles and industrial equipment [1,2]. However, challenge still remains in increasing the rather moderate energy density of supercapacitors. According to the equation E = 1/2CV2, the enlargement of energy density is dependent on increasing the specific capacitance and/or the operation voltage. Compared with traditional carbonbased materials only store limited chargers function on the double layer capacitance, the pseudocapacitive type electrode materials could ⁎ Corresponding authors. E-mail addresses: [email protected] (G. Zhang), [email protected] (K. Cheng).

https://doi.org/10.1016/j.jelechem.2019.113412 1572-6657/© 2019 Elsevier B.V. All rights reserved.

supply more capacitances result from the charge stored mechanism combined ion adsorption and near surface redox reactions [3–6]. Therefore, construct asymmetric supercapacitors with pseudocapacitive materials as positive electrode and carbon materials as negative electrode could inherit the advantage of two different type materials, in which enhancing the special capacitance and as well as enlarging the voltage window in neutral aqueous electrolytes, and thus improving the energy density [7–11]. Among different pseudocapacitive-type materials, MnO2 is regarded as an ideal material since its low-cost, environment friendly and high specific capacitance [12–15]. However, its poor electronic conductivity becomes an obstacle that impedes its wide application. Thus, combine nano-structure MnO2 with a conductive substrate such as carbon materials could make up for the shortcomings of the poor electrical conductivity, and as well as increase the effective active surface area. As a result, carbon materials, such as carbon aerogels [16,17], carbon nanotubes [18,19], carbon nanofibers [20,21] and graphene [22–25], etc., have been widely researched as substrates to support MnO2 growth. However, the practical specific capacitance is still far from the theoretical capacity result from the generating stacking and agglomeration leading to reducing electrochemical reaction activity

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sites for charge storage. More important, the preparation process of these carbon substrates is always complex and involve the use of environmentally harmful agents and/or high-cost catalysts. Natural biomaterials previously considered as waste or utilizing cheap, with a unique 3D microstructures, therefore have be wildly used as precursor to prepare hierarchical porous carbon [26,27]. Such a 3D hierarchical porous structure combines macropores, mesopores and micropores is very favorable for promoting the performance of supercapacitor, as a result of the macropores provide a reservoir of ion buffer and a short ion diffusion path to the interior surfaces, mesopores form a channel to interconnect ion-buffering reservoirs and micropores supply the locations for charge accommodation. At present, the common method to synthesize porous carbon materials is the activation and carbonization treatment of biomass precursors (bagasse [28], lotus pollen [29], willow catkin [30], rice husk [31], straw [32]) with KOH and/or NaOH at high temperatures. Unfortunately, the activator exhibits highly corrosive for the industrial equipment that thus drive the expense for scale-up. Recently, low melting point molten media (metals and salts), have been extensively explored for the synthesis of porous materials with unique nanostructures [33–35]. Our previous works have demonstrated that the ZnCl2 molten salt can act as a template for the formation of the 1D nanobelts [36]. Deng et al. reported the successful synthesis of nitrogen-doped hierarchically porous carbon material via a facile molten salt synthesis method in which a relatively high ZnCl2 to chitosan ratio of 10:1 [33]. However, the ZnCl2 has a rather high vapor pressure that will result in a serious health consequences and safety issue, therefore alternative salt mixtures may be appealing, especially in the context of upscaling. NaCl is a salt with low-cost, nonvolatile, abundant, non-toxic, and it combined with zinc chloride can be obtained lower melting points (250 °C). Thus, the NaCl/ZnCl2 eutectic salt mixture as a combined solvent-porogen to prepare porous carbon materials, which may lead to outstanding electrochemical performance.

Herein, a 3D porous active carbon (denoted as PAC) has been obtained by one-step synthesis with the low-cost and sustainable wheat bran as the carbon precursor and the eutectic salt mixture NaCl/ZnCl2 as a combined solvent-porogen. Then, MnO2 nanosheets were anchored on PAC (denoted as MnO2@PAC) via a one-step hydrothermal reaction. The resultant PAC sample presents a hierarchical porous structure and large specific surface area up to 1058 m2 g−1. The MnO2@PAC electrode displays high specific capacitance (258 F g−1 at 1 A g−1). Moreover, asymmetric supercapacitor based on PAC as a negative electrode material and MnO2@PAC as a positive electrode material delivers high energy density of 32.6 Wh kg−1 and excellent cycle stability with 93.6% of initial capacitance retained after 10,000 cycles. 2. Experimental As shown in Scheme 1, MnO2@PAC have been successfully synthesized by two-step process. Firstly, PAC is synthesized by using lowcost wheat bran as biomass carbon precursor and a mixture of NaCl/ ZnCl2 as combined solvent-porogen. Afterwards, MnO2 nanosheets loading on PAC is prepared via an in-situ hydrothermal deposition. Finally, an asymmetric supercapacitor is assembled by employing the MnO2@PAC as the positive electrode and PAC as negative electrode. The detailed experiment process is presented in the following sections. 2.1. Synthesis of PAC The fabrication process of PAC through a facile one-step pyrolysis which the wheat bran is used as precursor and NaCl/ZnCl2 as combined solvent-porogen. In a typical approach, 5 g of NaCl and ZnCl2 with mass ratios of 1/1 was added into 0.5 g of wheat bran. Then the mixture was carefully ground and transferred to a quartz boat. The samples were heated to 900 °C for 2 h under nitrogen atmosphere in a tube furnace.

Scheme 1. Schematic illustration of MnO2@PAC fabrication.

S. Kong et al. / Journal of Electroanalytical Chemistry 850 (2019) 113412

After cooling to room temperature, the obtained black material was grinded, vacuum filtration and washed with DI water, then dried at 70 °C for 12 h in a vacuum oven. As a comparison, the same quality of wheat bran without eutectic salt mixture was calcined according to the above experimental process, and the obtained product was denoted as AC. 2.2. Synthesis of MnO2@PAC composite The as-prepared PAC material (0.4 g) was added into 40 mL of 0.03 mol L−1 KMnO4 solution. After stirred for 1 h, the mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave. The autoclave was heated to 140 °C and kept for 4 h. After cooling to room temperature, the obtained composite was washed with distilled water, vacuum filtration and then dried at 70 °C in a vacuum oven. 2.3. Materials characterizations The surface structure and morphology of as-prepared samples were observed by field-emission scanning electron microscopy (FESEM, Zeiss Supra 40VP). The microstructure was obtained on transmission electron microscopy (TEM, JEM-2010FEF). Powder X-ray diffraction was operated on the Rigaku TTR III using Cu Kα radiation (λ = 0.1514178 nm). Nitrogen adsorption–desorption analysis was measured at 77 K using a Micromeritics ASAP 2020 physisorption analyzer. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250) was used to analyze the surface chemical elemental species of the samples. Raman spectra were performed using a Jobin-Yvon HR800 Raman spectrometer. Fourier transforms infrared ray (FTIR) spectra were recorded on a Varian 600 Spectromete. The MnO2 mass in the MnO2@PAC composite was tested by an inductive coupled plasma emission spectrometer (ICP, Thermo Scientific). For the ICP test, the MnO2@PAC composite was dissolved in 10 mL aqua-regia solution and then diluting to a 1 L solution to measure the amount of Mn. 2.4. Electrochemical measurements Cycling voltammetry (CV), galvanostatic charging-discharging (GCD) and electrochemical impedance spectroscopy (EIS) measurements of the electrodes were performed on a using a computerized potentiostat (VMP3/Z Bio-Logic) controlled by the EC-lab software. The tests of cycle life were performed on a NEWARE battery programcontrol test system (CT-3008W). The working electrodes were fabricated by pressing the slurry containing 80 mw% active materials, 10 mw% acetylene black and 10 mw % polytetrafluoroethylene binder onto the nickel foam current collector (1 cm* 1 cm). The mass loading of the electrode materials was ~3 mg cm−2. The electrochemical measurements of the individual electrode were conducted by a conventional three-electrode system in 0.5 mol L−1 Na2SO4 aqueous solution. The platinum foil and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The symmetric supercapacitor was assembled as shown as Scheme 1, and it tested in 0.5 mol L−1 Na2SO4 electrolyte and the voltage range was 0–1.8 V. The loading mass ratio of positive electrode material and negative electrode material were estimated according to the equation [37]: mþ C −  ΔE− ¼ m− C þ  ΔEþ

ð1Þ

where m is the mass of each active material (g), C is the specific capacitance (F g−1), ΔE is the voltage range for the charge/discharge process (V).

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The gravimetric specific capacitance (C) was calculated based on the following equation [38]: Cm ¼

i  Δt ΔV  m

ð2Þ

where Δt is the discharge time, i is the discharge current, ΔV is the voltage potential, and m is the mass of the active materials. The power density P (W kg−1) and energy density E (Wh kg−1) of supercapacitor were calculated using the equation [39,40]: P¼

E  3600 Δt

1 1 E ¼ C m  ΔV 2  2 3:6

ð3Þ ð4Þ

where Δt (s) is the discharge time, ΔV (V) is the discharge voltage range (excluding the IR drop) and Cm (F g−1) is the specific capacitance of asymmetrical capacitor. 3. Results and discussion Fig. 1 shows the SEM and TEM images of the obtained PAC and MnO2@PAC samples. Comparing with AC exhibit a bulk rock morphology without obvious porous structure (Fig. S1), the PAC (Fig. 1a and b) displays an obvious honeycomb-like structure, and the diameters of macropores are varying from 0.6 to 3.5 μm. The low resolution TEM (Fig. 1e) confirms the PAC sample consisting with an interconnected hierarchical porous structure of mesopores, macropores and rich micropores could be obviously observed in the high-resolution TEM image (Fig. 1f). Apparently, it proves that the PAC has a hierarchical porous structure, which is beneficial to shorten ions diffusion path and further leading to excellent rate performance. As seen in Fig. 1c and d, MnO2 nanosheets have been successfully deposited on the PAC after hydrothermal reaction. It also could be observed the porous structure between manganese dioxide and PAC, which is beneficial for the ion and electron transport. Fig. 1g shows the morphology of MnO2, which exhibits the nanosheet structure with ~5 nm thickness. The HR-TEM image (Fig. 1h) further revealed the as-prepared MnO2 nanosheet had a crystal lattice structure with 0.69 and 0.35 nm, corresponding to the (110) and (002) plane of birnessite-type MnO2. The phase structure of the carbon products is analyzed by X-ray diffraction analysis (Fig. 2a). As seen, the PAC exhibits two broad peaks centered at 26° and 43.5° corresponding to the (002) and (100) planes of graphite, exhibiting a typical feature of amorphous nature. Moreover, the (002) peak of PAC is weaker and broader in comparison to the AC (Fig. S2). It is mainly because of the activation process lead to the existence of rich pores, thus to decrease the graphitization degree and crystallinity. After MnO2 nanosheets are successfully decorated on PAC, in addition to the (002) peak of carbon, three diffraction peaks located at 12.8°, 37.5° and 65.1°can be observed, which are attributed to the (110), (211) and (002) planes of MnO2 (JCPDS card No. 44-0141). Fig. 2b shows the Raman spectra of the as-prepared PAC and MnO2@ PAC samples. Two sharp peaks locate at 1350 cm−1 and 1580 cm−1 that corresponding to the typical vibration mode of D-band and Gband, respectively. It is known that the relative intensity ratio of D/G is widely applied in evaluating the degree of edge roughness, structural defects and the domain size of carbon-based materials [41,42]. Compared with PAC, the ID/IG value for MnO2@PAC sample composites ascends from 0.93 to 0.95, manifesting the formation of MnO2 nanosheet on PAC surfaces result in the increasing of disorder. For the MnO2@PAC sample, two other peaks located at 574 cm−1 and 645 cm−1, can be assigned to the characteristic stretching vibration bands of Mn\\O [43], indicating the formation of MnO2. Thus, XRD and Raman results can clearly demonstrate the successful introduction of MnO2 nanosheets decorated on the PAC. To further determine the

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Fig. 1. The SEM images of PAC (a and b) and MnO2@PAC samples (c and d). The TEM and HRTEM of PAC (e and f) and MnO2@PAC samples (g and h).

pore structure of resultant samples, nitrogen adsorption-desorption isotherms is performed and the results are presented in Fig. 2c. As shown, the curves of PAC are similar to the combined type I and IV isotherms and exhibit a steep rise at very low pressure area (b0.1) owing to the existence of micropore. Moreover, the isotherms have appeared hysteresis loops among the pressure area of 0.4–1.0, which is consistent with the characteristic of micropores. The pore size distribution calculated via density functional theory (DFT) model that show in Fig. 2d further confirm the above conclusion. Except for the micropores center at ~1.5 nm, PAC also exhibits a broad peak locates at 10–35 nm, confirms the coexisting of micropores and mesopores, agree well with the observation of SEM and TEM. The specific surface area (SSA) of MnO2@PAC (129 m2 g−1) is smaller than PAC (1080 m2 g−1), which is due to the MnO2 nanosheets homogenously anchor on the surface of PAC resulting in some nano-pore blockage of PAC.

X-ray photoelectron spectroscopy (XPS) analysis is used to examine the chemical bonding and surface elemental composition of the MnO2@ PAC and PAC samples. The characteristic peaks of C, Mn, and O elements can be observed from the XPS survey scan spectrum presented in Fig. 3a, suggesting the successfully depositing of MnO2 on PAC surface. The high-resolution C1s spectra displays four signals (Fig. S3) located at 284.7, 285.1, 286.37 and 288.39 eV, which are corresponding to C\\C (aromatic rings), C\\N, C\\O (epoxy and alkoxy), C_O (carbonyl), respectively. The O1s spectra of PACs-50 is divided into three peaks of C_O quinone groups (~531.6 eV), C-OH phenol groups (~532.1 eV), and C-O-C ether groups (~533.1 eV) [44]. The N 1s spectra shown in Fig. 3b proves that the nitrogen atoms existed in the PAC sample and further deconvolved into three-type peaks at 398.3 eV, 399.8 eV and 400.6 eV, which are assigned to the N functional groups of pyridinic (N-6), pyrrolic (N-5), and graphitic (N-Q) nitrogen, respectively

Fig. 2. The XRD (a), Raman (b), the nitrogen adsorption–desorption isotherms (c) and pore-size distributions (d) of PAC and MnO2@PAC samples.

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Fig. 3. (a) The wide-scan XPS survey spectra of PAC and MnO2@PAC. XPS spectra of N1s (b), O1s (c), and Mn 2p (d).

[45,46]. The O 1s spectra of MnO2@PAC presented in Fig. 3c, which is composed of Mn-O-Mn (530 eV) and Mn-O-H (531.4 eV) functional groups [13]. The high-resolution spectrum of Mn 2p spectra is shown in Fig. 3d, the main peaks at 654 eV and 642.4 eV are ascribed to the Mn 2p1/2 and Mn 2p3/2, respectively. In addition, the energy separation of Mn 2p is 11.6 eV, revealing that manganese element in the form of tetravalent manganese is presented in the composite [47,48]. The electrochemical performances of PAC and MnO2@PAC electrodes are separately measured in 0.5 mol L−1 Na2SO4 electrolyte with a typical three-electrode configuration. The CV and GCD curves of PAC are shown in Fig. 4a and b, respectively. Obviously, the CV curves remain the rectangular shape in neutral solution, suggesting an ideal capacitive behavior. Moreover, the shapes of CV curves maintain well with the increasing of the scan rate, representing that lower contact resistance of PAC electrode. The GCD curves exhibit a nearly symmetric shape, revealing that the PAC electrode has excellent reversibility. After depositing MnO2 nanosheets on the PAC, the CV curves of MnO2@PAC also show the quasi-rectangular (Fig. 4c), and it has not especially obvious deformation for scan rates up to 100 mV s−1. In addition, contrasting with Fig. 4a, the CV surround area of MnO2@PAC composite is much larger than PAC at the same scan rate, which give prominence to the effect of MnO2 in increasing the capacitance of the composite electrode. Agrees with the CV results, the GCD curves of MnO2@PAC electrode exhibit much longer discharge time than the PAC at the same current density. The specific capacitances corresponding to different current densities are presented in Fig. 4e. As seen, the specific capacitance of MnO2@PAC is superior to PAC, and calculated as 258, 250, 244, 240, 238 and 235 F g−1 corresponding to the current densities of 1, 2, 4, 6, 10 and 20 A g−1, respectively. However, the capacitance retention of PAC with the current density ranging from 1 A g−1 to 20 A g−1 is 91.1%, which is a little higher than MnO2@PAC (82.8%). Besides, the electrochemical performance of the MnO2@PAC composites with different mass percentages of MnO2 (38.7%, 56.3% and 76.9%) was investigated in a 0.5 M Na2SO4 solution. From the Fig. S4, the CV curve of 56.3% MnO2@PAC shows the biggest area compared with other two

electrodes, indicating the largest specific capacitance. The Fig. S5 exhibits the GCD curves with a current density of 1 A g−1, and the 56.3% MnO2@PAC obtains the longest discharge time which shows a consistent conclusion with the CV result. The specific capacitances calculated from the GCD measured with different current densities are exhibited in Fig. S6. Obviously, the capacitance increases with the mass loading of the MnO2 increased from 38.7% to 56.3% but declines for 76.9% MnO2@PAC. It may be excessive amount of MnO2 will block the pores of carbon material, which is not conducive to the transmission of ions and electrons, and also. hinders the mass transfer. Fig. 4f demonstrates the Nyquist plots of PAC and MnO2@PAC electrodes. It can be seen that PAC shows a steeper linear curve than that of MnO2@PAC in the low frequency region, suggesting that the PAC has ideal electric double layer capacitance behavior, and the MnO2@PAC exhibits the characteristics of pseudocapacitance. For the high frequency region, the charge-transfer resistance (Rct) of PAC is 0.74 Ω, which is a bit smaller than MnO2@PAC (1.25 Ω). This result suggests a good contact between the PAC and MnO2. The increase of the charge-transfer resistance of the MnO2@ PAC composite is mainly attributed to the low conductivity of MnO2 resulting in an increase in ESR. The PAC//MnO2@PAC asymmetric supercapacitor has been assembled in a 0.5 mol L−1 Na2SO4 aqueous electrolyte by using PAC as the negative electrode and MnO2@PACs composite as the positive electrode. Electrochemical characterizations of the PAC//MnO2@PAC asymmetric supercapacitor are presented in Fig. 5. Fig. 5a displays the CV curves of PAC//MnO2@PAC asymmetric supercapacitor at a scan rate of 50 mV s−1 in various voltage windows. It can be seen that the CV curves still retain a quasi-rectangular shape without obvious distortion when the voltage is up to 1.8 V, suggesting that the as-prepared symmetrical supercapacitor can be reversibly cycled at the potential voltage of 0–1.8 V. The CV plots at different sweeping rates are exhibited in Fig. 5b. What calls for attention is that the shape of CV curve at a higher scan rate of 200 mV s−1 still remains rectangular-like, indicating outstanding rate performance and electrochemical reversibility. Fig. 5c

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Fig. 4. Electrochemical characterizations of the PAC and MnO2@PAC electrodes in 0.5 mol L−1 Na2SO4 electrolyte. The CV curves of PAC (a) and MnO2@PAC composite (c). The GCD curves of PAC (b) and MnO2@PAC composite (d). The specific capacitance corresponding to various current densities (e). The Nyquist plots (f).

shows the GCD curves of PAC//MnO2@PAC asymmetric supercapacitors versus various current densities. As seen, the IR drop of the asymmetric supercapacitors is very small, indicating a lower internal resistance of the cell. The galvanostatic charge discharge test is based on the overall quality of positive and negative electrode, and the weight of each electrode is set according to the Eq. (1). It should note that the Coulomb efficiency (CE) of asymmetric supercapacitor at 0.5 A g−1 is 88%, which is caused by the electrode occurred side reactions in the charge process at a small current density. The CE increased to 100% at a higher current density, suggesting an ideal capacitance behavior. The Ragone plots of

PAC//MnO2@PAC symmetric supercapacitor is shown in Fig. 5d. According to the Eqs. (3) and (4), the asymmetric supercapacitor has a maximum energy density of 32.6 Wh kg−1 at a power density of 450 W kg−1, the excellent results are represented significantly higher and/or comparable than that of previous reports about MnO2-based asymmetric supercapacitor [47–54]. Furthermore, the capacitance retention of PAC//MnO2@PAC symmetric supercapacitor can retain 93.6% and the coulombic efficiency remains 99.7% after testing of 10,000 charge/discharge cycles (Fig. 5e). In order to check the structural evolution of MnO2@PAC after long cycle process, the XRD and Raman

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Fig. 5. Electrochemical characterizations of PAC//MnO2@PAC asymmetric supercapacitor. (a) The CV curves of the asymmetric two-electrode cell in various voltage windows at the scan rate of 50 mV s−1. (b) CV plots at different scan rates. (c) The GCD versus various current densities. (d) Ragone plots. (e) The cycling life and columbic efficiency test at a current density of 4 A g−1.

spectrum of MnO2/PAC electrode after cyclic test were performed and the results are presented in Figs. S7 and S8. Obviously, the peak intensity of MnO2@PAC has a bit weaker after charge/discharge cycling but the peak location remains unchanged. The ID/IG value of MnO2@PAC sample ascends from 0.95 to 1.01 after cyclic test, suggesting the structure has a little change result in the increasing of disorder. Therefore, the superior

electrochemical performances of MnO2@PAC composite is attributed to the followed reasons: (1) The hierarchical porous structure of PAC could effectively reduce the electron transportation resistance and the high specific surface area of PAC could provide abundant sites for the deposition of MnO2; (2) The ultra-thin MnO2 sheets remarkable increased the contact area with electrolyte and shortened the diffusion path of ion and

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offer abundant electro-active sites for charges accumulation; (3) The excellent interfacial contact force between MnO2 and carbon ensure to the structure stability during the cyclic process. 4. Conclusion In summary, we have fabricated a N-doped porous carbon material with high surface area (1058m2 g−1) by a facile and controllable molten salt method, using low-cost wheat bran as carbon source and the eutectic mixture NaCl/ZnCl2 as combined solvent-porogen. MnO2 nanosheets were deposited on the resultant porous carbon through a simple onepot hydrothermal reaction. The synergistic effect of the hierarchically porous structure and the crosslinked ultrathin MnO2 nanosheets are responsible for the enhanced electrochemical performance. As a result, the MnO2@PAC hydrous electrode displays high specific capacitance (258 F g−1 at 1 A g−1) outstanding rate performance (235 F g−1 at 20 A g−1). Furthermore, the assembled PAC//MnO2@PAC asymmetric supercapacitor delivers a high energy density of 32.6 Wh kg−1 and outstanding long-term cyclic stability (93.6% capacitance retention after 10,000 cycles). Therefore, such porous carbon materials derived from biomass waste combining with manganese dioxide have a lot of potential to open a green channel for energy conversion and storage. Acknowledgment We gratefully acknowledge the financial support of this research by Natural Science Foundation Project of Chongqing Science and Technology Commission (No. cstc2019jcyj-msxmX0738, No. cstc2017jcyjAX0100) and National Natural Science Foundation of China. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jelechem.2019.113412. References [1] F. Wang, X. Wu, X. Yuan, Z. Liu, Y. Zhang, L. Fu, Y. Zhu, Latest advances in supercapacitors: from new electrode materials to novel device designs, Chem. Soc. Rev. 46 (2017) 6816. [2] Z. Wu, L. Li, J. Yan, X. Zhang, Materials design and system construction for conventional and new-concept supercapacitors, Advanced Science 4 (2017). [3] R. Kumar, R.K. Singh, A.K. Singh, A.R. Vaz, C.S. Rout, S.A. Moshkalev, Facile and single step synthesis of three dimensional reduced graphene oxide-NiCoO2 composite using microwave for enhanced electron field emission properties, Appl. Surf. Sci. 416 (2017) 259–265. [4] R. Kumar, M.M. Abdel-Galeil, K. Zay Ya, K. Fujita, W.K. Tan, A. Matsuda, Facile and fast microwave-assisted formation of reduced graphene oxide-wrapped manganese cobaltite ternary hybrids as improved supercapacitor electrode material, Appl. Surf. Sci. 418 (2019) 296–306. [5] R. Kumar, R.K. Singh, A.R. Vaz, R.M. Yadav, C.S. Routd, S.A. Moshkalev, Synthesis of reduced graphene oxide nanosheets supported agglomerated cobalt oxide nanoparticles and their enhanced electron field emission properties, New J. Chem. 41 (2017) 8431–8436. [6] K. Singh, B. Kirubasankar, S. Angaiah, Synthesis and electrochemical performance of P2-Na0.67AlxCo1-xO2 (0.0≤X≤0.5) nanopowders for sodium-ion capacitors, Ionics 23 (2017) 731–736. [7] S. Arunachalam, B. Kirubasankar, E.R. Nagarajan, D. Vellasamy, S. Angaiah, A facile chemical precipitation method for the synthesis of Nd(OH)3 and La(OH)3 nanopowders and their supercapacitor performances, ChemistrySelect 3 (2018) 12719–12724. [8] A. Subasri, K. Balakrishnan, E.R. Nagarajan, V. Devadoss, A. Subramania, Development of 2D La(OH)3/graphene nanohybrid by a facile solvothermal reduction process for high-performance supercapacitors, Electrochim. Acta 218 (2018) 329–337. [9] B. Kirubasankar, V. Murugadoss, S. Angaiah, Hydrothermal assisted in situ growth of CoSe onto graphene nanosheets as a nanohybrid positive electrode for asymmetric supercapacitors, RSC Adv. 7 (2017) 5853–5862. [10] K. Balakrishnan, M. Kumar, A. Subramania, Synthesis of polythiophene and its carbonaceous nanofibers as electrode materials for asymmetric supercapacitors, Adv. Mater. Res. 938 (2014) 151–157. [11] B. Kirubasankar, V. Murugadoss, J. Lin, T. Ding, M.Y. Dong, H. Liu, J.X. Zhang, T.X. Li, N. Wang, Z.H. Guo, S. Angaiah, In situ grown nickel selenide on graphene nanohybrid electrodes for high energy density asymmetric supercapacitors, Nanoscale 10 (2018) 20414–20425.

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