CVD-grown polypyrrole nanofilms on highly mesoporous structure MnO2 for high performance asymmetric supercapacitors

Accepted Manuscript CVD-grown polypyrrole nanofilms on highly mesoporous structure MnO2 for high performance asymmetric supercapacitors Ni Wang, Peng Zhao, Kun Liang, Mengqi Yao, Yang Yang, Wencheng Hu PII: DOI: Reference:

S1385-8947(16)31148-2 http://dx.doi.org/10.1016/j.cej.2016.08.074 CEJ 15640

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

18 May 2016 14 August 2016 16 August 2016

Please cite this article as: N. Wang, P. Zhao, K. Liang, M. Yao, Y. Yang, W. Hu, CVD-grown polypyrrole nanofilms on highly mesoporous structure MnO2 for high performance asymmetric supercapacitors, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.08.074

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CVD-grown polypyrrole nanofilms on highly mesoporous structure MnO2 for high performance asymmetric supercapacitors Ni Wanga, Peng Zhaoa, Kun Liangb, Mengqi Yaoa, Yang Yangb*, and Wencheng Hua* a

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science

and Technology of China, Chengdu, 610054, China b

NanoScience Technology Center, Department of Materials and Engineering, University of Central

Florida. Orlando FL, 32826 *Corresponding Authors: Wencheng Hu ([email protected]), Yang Yang ([email protected]) Abstract: High-porosity MnO2 with a mesoporous structure is synthesized through a facile redox reaction, and the polypyrrole (PPy) nanofilms are grown on the synthesized mesoporous MnO2 by chemical vapor deposition to form a 3D nanocomposite structure. The as-prepared high-porosity MnO2/PPy with a nanocomposite structure presents a high specific capacitance of 320 F g−1 and a 91.4% capacitance retention after 5000 charge–discharge cycles. MnO2/PPy and N-doped active carbon are employed as a positive electrode and a negative electrode, respectively, to assemble an asymmetric supercapacitor (ASC), which exhibits a high energy density of 38.6 Wh kg−1 at 0.5 A g−1 and long-term cyclic stability (90.6% capacity retention after 5000 cycles). Such high-performance capacitive behavior is attributed to the high porosity, mesoporous structure, and improved electrical conductivity of the fabricated ASC, which is promising for various supercapacitor applications. Keywords: High-porosity MnO2; PPy nanofilms; Chemical vapor deposition; Asymmetric supercapacitor; High energy density

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1. Introduction Pseudocapacitors, with an energy storage mechanism based on reversible surface Faradaic reactions, have been widely demonstrated to provide much higher energy densities than double-layer energy-storage devices [1-4]. As promising alternative electrode materials of RuO2, transition metal oxides have been widely studied because of their good capacitive behaviours, low toxicity, and low cost [5-8]. Among these transition metal oxides, manganese oxide (MnO2) is widely considered an attractive material for nextgeneration supercapacitors because of its high specific capacitance, which is approximately 1110 F g−1 based on the calculation of Faradaic law [9], and well-defined electrochemical redox activity [10,11]. In recent years, the design and fabrication of nanomaterials with mesoporous structures have captured growing interest in their practical application in electrochemical supercapacitors. Apparently, the high surface area and fast electrical pathways of these nanomaterials are beneficial to faradic redox reactions [12-14]. Many forms have been suggested for the development of nanostructured manganese oxides with high surface area, such as nanorod arrays [15], nanoflowers [16], two-dimensional ordered nanopore arrays [17], nanosheets [18], and well-ordered whisker-like arrays [19]. Although manganese oxides are considered the most attractive electrode material, a key challenge in their practical application to energy storage systems is their low electrical conductivity (10−3-10−2 S cm−1) [20], which limits fast electron transport and deteriorates rate capabilities for high power energy storage devices. To enhance the electroconductibility of MnOx-based electrode materials, many effective approaches have been proposed using conductive grapheme [21,22], carbon spheres [23] and carbon nanotubes [24] as supporting materials. However, the high cost possibly restricts their practical applications. Manganese oxides modified by conductive polymers, such as polyaniline [25], polypyrrole (PPy) [26], and poly(3,4-ethylenedioxythiophene) [27], have been widely investigated. Conductive polymers offer high electrical conductivity and possess pseudocapacitance properties themselves [28], thereby significantly improving the capacitance of manganese oxide-based materials. Various methods have been employed to fabricate PPy/ manganese oxide composites, such as the redox process including pyrrole monomer and KMnO4 in an acid solution [29], MnO2 coating by PPy derived

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from the reaction between pyrrole monomer and ammonium persulfate [30], and electrodeposition from MnSO4 and pyrrole monomer systems [31]. However, MnO2 molecules are prone to react with pyrrole monomers; as a result, the nanostructured morphology of MnO2 is destroyed. In this work, we report a novel and facile synthesis route for PPy chemical vapor deposition (CVD)-grown on manganese oxide with a high porosity and mesoporous structure. The synthesized MnO2/PPy is used as a supercapacitor electrode material. High-porosity manganese oxide is first synthesized through a redox reaction between KMnO4 and maleic acid/hexadecyltrimethyl-ammonium bromide, followed by the CVD method to uniformly deposit PPy nanofilms on mesoporous manganese oxides to form unique hierarchical mesoporous architectures. In a supercapacitor, the relationship of the stored energy (E) and the potential window (V) can be described as: E=1/2CV2, and many previous references [32-35] have confirmed that the potential window of asymmetric supercapacitors (ASCs) in an aqueous solution can be extended beyond the thermodynamic limit of water. ASCs usually employ metal oxides/ hydroxides as the positive electrode and carbon-based materials as the negative electrode [36-38]. A novel N-doped mesoporous carbon derived from PPy with an excellent electroconductibility and a high specific surface is an ideal negative material [39,40]. In the present work, the similar route employed in a previous study [39] is employed to prepare N-doped mesoporous carbon, which is used as the negative electrode of an ASC to evaluate the electrochemical properties of PPy-modified mesoporous manganese oxide (PMMO). 2. Experimental 2.1 Materials synthesis In the present procedure, a part of maleic acid and hexadecyltrimethylammonium bromide (CTAB) were used as reducing agents for reduction of potassium permanganate to form MnO2 which was in situ coated on the gelatinized organic materials. The excessive maleic acid and CTAB were further employed as sacrificial templates with different molecular weights for various mesopore sizes. A schematic for the synthesis of PMMO is shown in Fig. 1. In a typical synthesis of mesoporous manganese oxide, 1.0 g of maleic acid and 0.75 g of CTAB were added in 0.1 L of pure water to form a transparent solution at 40

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°C, and 10 mL of 0.2 M potassium permanganate solution was mixed with this solution under efficient stirring to form a gel. The gel was sequentially aged for 24 h at room temperature and rigorously rinsed with ethyl alcohol and pure water to remove unreacted reagents and impurity ions. The sample was dried in a vacuum oven for 10 h at 80 °C to obtain the manganese oxide with a mesoporous structure, abbreviated as MMO. The formation mechanism of MMO was shown in Supplementary Materials. The mesoporous powders were immersed into an ethyl alcohol solution containing 5 wt% ferrous chloride at room temperature for 20 min under tenth atmospheric pressure and then directly dried at 80 °C in atmospheric environment; as a result, ferrous ions were transformed into ferric ions. The sample was spread out in a culture dish, which was heated to 80 °C in a vacuum chamber. After the pressure of the vacuum chamber reached one-tenth of the atmospheric pressure, the vapor of the pyrrole monomer was continuously introduced to the vacuum chamber, which was maintained at 0.15 atm. The deposition of PPy thin films lasted for 20 min to finish the synthesis of PMMO. 2.2 Materials characterization FT-IR analysis was carried out in a FTIR8400S FT-IR spectrometer (Shimadzu) with an infrared region of 4000–400 cm−1. Raman spectra were performed on an IDRaman micro-532 Raman spectrometer (Ocean Optics) using 532 nm laser lines. The samples were analyzed in an X-ray diffraction (XRD, DMax-γ type A, Rigaku Co., Japan) to detect the crystal structures. The morphologies of the samples were observed by FESEM (Inspect F, FEI Co., USA) and HRTEM (Libra 200FE, Germany). The valence state of the manganese element was identified by X-ray photoelectron spectroscopy (XPS, XSAM800, KRATOS, UK). A JW-BK112 Surface Characterization Analyzer (Beijing JWGB Sci & Tech Co., China) was employed to obtain the N2 adsorption and desorption data. The BET equation and the BJH method were used to calculate the specific surface and pore size distributions. 2.3 Electrochemical measurements The prepared sample, acetylene black, and polyvinylidene fluoride were employed as the electrochemical active material, conductive agent and binder with a mass ratio of 80:10:10, and Nmethyl-2-pyrrolidone as solvent to prepare the working electrode. The carbon felt was used as the current

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collector (1.0 × 1.0 cm2). Electrochemical studies were conducted by AC impedance spectroscopy and cyclic voltammetry (CV) using an electrochemical workstation (CHI660D, Chenhua, China) using a calomel electrode and Pt plate as the reference electrode and counter electrode in a beaker-type threeelectrode cell. The mass of MnO2/PPy composite loaded on each working electrode was ca. 5.0 mg cm-2. An ASC device was used to further evaluate the capacitive performance of PMMO electrode for potential application. The ASC device was assembled using mesoporous carbon derived from PPy as the negative electrode and the as-synthesized porous PMMO as the positive and electrode in 1 M Na2SO4, and the separator is a piece of cellulose paper. The N-doped mesoporous carbon negative electrode and PMMO positive electrode were both overspread with the electrolyte solution for a few minutes and then placed in a vacuum environment for an additional few minutes such that the surface of the mesopores of both electrodes were completely wet by the electrolyte solution. Subsequently, a standard 2032 coin cell was assembled for the electrochemical characterization. 3. Results and discussion 3.1 Structural characteristics of PMMO Fig. 2a shows the FT-IR spectrum of the prepared PPy/MnO2 composite. The peaks near 1635 cm–1 are assigned to the vibration of C=C/C–C, and the pyrrole ring vibration appeared at 1394 and 1352cm–1 [41]. The peaks at 1027, 1598 and 3443 cm–1 are attributed to the N–H in–plane deformation, bending and stretching vibrations, respectively. The bands located at 1135 and 960 cm–1 ascribed to the C–N stretching vibrations and the C–H out-of plane vibration. The Raman spectra of PPy / MnO2 are shown in Fig. 2b. Raman peaks in the spectral region 500–700 cm−1 are caused by the Mn−O lattice vibrations, and the double peaks at 1050 and 1080 cm−1 are assigned to the characteristic peaks of the ring stretching mode and C=C backbone stretching of PPy [42]. The results of FT-IR and Raman spectra suggest the synthesis of PPy/MnO2 composite. The PPy/MnO2 composite was also investigated using XPS, and the XPS surface scan spectra are shown in Fig. 3. The XPS full spectrum exhibits Mn 2p, O 1s, N 1s and C 1s peaks located at 642.0, 535.1 , 400.1 and 284.8 eV, respectively, implying that the composite mainly consists of these four

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elements. The magnified Mn 2p, C 1s and N 1s spectrum are presented in Fig. 3b, 3c and 3d respectively. The intense peak at 642.0 eV with a satellite at 653.6 eV is typical characteristic of manganese dioxide, suggesting the presence of MnO2 in our sample. The C 1s spectrum exhibits C=C, C−C, C−O and C−N, bonds corresponding to the peaks at 284.4, 285.2, 286.5 and 288.5 eV, respectively, and de-convoluted N 1s peaks at 398.9, 400.1 and 401.8 eV are assigned to −C=N−, −NH− and −NH+− bonds [43], indicating the incorporation of PPy into the composite. Fig. 4a shows the representative XRD patterns of MMO and PMMO. Both samples are similar, suggesting that almost no change occurs in crystallinity after the PPy deposition process. All peaks can be indexed to the diffraction data of β-MnO2 (JCPDS No. 42-0735). The XRD diffraction peaks are relatively broad and have low intensity, revealing the imperfect crystallization of the sample and existence of only tiny crystals in the samples. Fig. 4b shows the FESEM micrographs of PMMO. The sample consists of large micron-scale agglomerates containing numerous pores with different diameters to form a 3D continuous network. Those agglomerates consist of many fine particles, and a rough is evident. The morphological properties were evaluated by HRTEM, as displayed in Fig. 4c and 4d. The micrograph of MMO shows that its structure is a network, whose skeleton is constituted of nano particles with very rough suface, revealing that the skeleton is a mesoporous structure. The higher magnification HRTEM image (the inset of Fig. 4c) suggests that the well-defined lattice fringes are present, and the mesopores are less than 5 nm. After the deposition of PPy on mesoporous MnO2 (Fig. 4d), the skeleton of the network seems to slightly smoothen; this result is attributed to the surface modification of MMO by the CVD-deposited PPy. The inset image suggests the MnO2 particles are enwrapped by a PPy nanofilm with uniform thickness. The PPy nanofilm with uniform thickness is derived from the unique method used in this study. When the gas pressure decreases, most of the air is exhausted. The evaporative pyrrole monomers rapidly occupy the spaces of the mesopores. The high concentration of the reactant and proper temperature lead to the rapid formation of the uniform PPy nanofilm. The adsorption and desorption isotherms of MMO and PMMO are shown in Fig. 4e. The two samples exhibit type IV adsorption

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isotherms according to the recommendation of the IUPAC [44], indicating that both samples are typical mesoporous materials. In addition, the adsorbed quantity of N2 molecules by PMMO is slightly less than that of MMO, resulting from the decrease of the specific surface because of the deposition of PPy. The pore size distributions of MMO and PMMO are shown in the inset of Fig. 4e. PMMO possesses a large number of mesopores, which meets the requirement of supercapacitors for both charge storage and rate capability. The surface area, pore volume, mean pore diameter, and mesopore ratio calculated from the isotherms are 523 m2/g, 1.68 cm3/g, 10.1 nm, and 92.5% for MMO, respectively, and 467 m2/g, 1.55cm3/g, 5.4 nm, and 93.7%, for PMMO, respectively. Apparently, the as-prepared PMMO composite mainly maintains the micrographs and pore/ channel structure of MMO after the deposition of the PPy nanofilm. 3.2 Electrochemical performance of PMMO electrode Fig. 5a shows the CV curves of the MMO and the PMMO electrodes with the same mass of electroactive materials in 1M Na2SO4 aqueous electrolyte at a scan rate of 10 mV s−1. The two CV curves are essentially nearly rectangular and display symmetrical current–potential characteristics of a capacitor. A comparison of the areas calculated from the depicted CV curves for MMO and PMMO demonstrates that the specific capacitance of the PMMO electrode is considerably higher than that of the MMO electrode. The improvement in specific capacitance is believed to have resulted from the deposited PPy with high electrical conductivity inducing more MnO2 molecules to participate in a faradic electrochemical reaction. Fig. 5b presents the CV curves of a PMMO electrode recorded at different potential scan rates. The rectangular feature of CV curves, which is maintained even at a high scan rate of 20 mV s−1, implies the kinetic reversibility of the intercalation–deintercalation process in the present study. Fig. 5c illustrates the galvanostatic charge–discharge curves of PMMO at different current densities of 0.5, 1, 2, 5, and 10 A g−1. As a comparison, MMO is also measured at 0.5 A g−1. The capacitor voltage varies linearly with time during both charging and discharging processes, indicating a rapid current–voltage response and good Coulombic efficiency. The specific capacitance of the electrochemical active material is dependent on the potential drop during discharge (∆V), constant discharge current (I), discharge time (∆t) and the mass of the active materials (m), and can be calculated by equation (1).

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୍×∆୲

C = ୫×∆୚

(1)

The specific capacitance value is 320 F g−1 for the PMMO electrode and 231 F g−1 for the MMO electrode at an applied current density of 0.5 A g−1 in the potential range of 0–1 V. The specific capacitance of PMMO measured in this study is much higher than those previously reported in related studies [29, 45]. An obvious IR drop can be observed in the discharge curve of the MMO electrode, suggesting that a relatively large internal resistance exists in this electrode and causes low specific capacitance. The calculated specific capacitances of the PMMO electrode at different current densities are given in Fig. 5d. The specific capacitance is still 248 F g−1 at a high charge–discharge rate of 10 A g−1, exhibiting good high-rate discharge ability. The electrochemical stability of electrodes is also a crucial parameter for their practical application. The long cycle life tests of the PMMO and MMO electrodes depend on the GCD measurements within the potential window of 0–1 V at a current density of 5 A g−1; the results of these tests are shown in Fig. 5e. The specific capacitance of the PMMO electrode declines slightly in the cycle experiment; after 5000 charge/discharge cycles, the specific capacitance is 234.9 F g−1, exhibiting 91.4% retention. The results demonstrate that the PMMO electrode has excellent electrochemical properties, including high specific capacitance, good charge–discharge stability, and long-term cycling life. By contrast, the MMO electrode shows a normal loss of discharge capacity in the first 500 cycles, after which the specific capacitance declines rapidly with an increase in cycle number. The value decreases to 38.4 F g-1, which is only 22% of the initial specific capacitance. From this result, the effect of PPy CVD-deposited on mesoporous MnO2 in improving electrochemical properties is prominent. The PPy nanofilm coated on mesoporous MnO2 facilitates electron transfer and, more importantly, prevents MnO2 from dissolving and maintains the stability of the mesoporous structure. 3.3 Characteristics of the asymmetric supercapacitor The assembled ASC should balance the charges stored in both electrodes based on the specific capacitance determined by the CV curves of the three-electrode system. The mass ratio (m+/m-) for

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optimal electrochemical performance between PMMO and N-doped porous carbon is determined as 0.83, as calculated by equation (2): ୫శ ୫ష

େ ∆୚

= େష ∆୚ష శ

(2)

శ

As shown in Fig. 6a, the CV curves of a PMMO//N-doped mesoporous carbon ASC cell measured at different potential windows and 10 mV s−1 exhibit rectangular-like shapes even at 2.0 V, indicating good reversibility and ideal capacitive behavior. No decomposition reaction of the electrolyte solution occurs in the charge and discharge processes. Fig. 6b shows the typical CV curves at different scan rates from 2 to 50 mV s−1. This result suggests that the CV curves retain the rectangular shape even at a high scan rate of 50 mV s-1, further implying the rapid transportation of electrolyte ions and excellent rate capability. Fig. 6c shows the GCD curves of the ASC cell at different current densities ranging from 0.5 A g−1 to 10 A g−1 at a cell voltage of 2.0 V. Quasi linear and symmetrical GCD curves of the ASC cell are observed, suggesting a rapid voltage−current response and excellent electrochemical reversibility. The specific capacitance of the ASC cell is 69.5 F g −1 based on the total mass of the active materials on the two electrodes. This specific capacitance is higher than those of MnO2/MnCO3/rGO aerogels//rGO aerogel

[46],

MnO2/hierarchically

porous

carbon//hierarchically

porous

carbon

[47],

and

FeOOH/graphene nanosheets/carbon nanotubes//MnO2/graphene nanosheets [48] and comparable with that of NiCo2O4@MnO2 core–shell nanowire arrays on nickel foam//activated carbon [49]. Based on the discharge curves, the specific capacitances are 69.5, 60.2, 54.7, 45.2 and 40.2 F g−1 at the current densities of 0.5, 1, 2, 5, and 10 A g−1, respectively, as shown in Fig. 6d. Remarkably, up to 58% of the capacitance is retained when the discharge current density is increased from 0.5 A g−1 to 10 A g−1, indicating a relatively high utilization efficiency of the active materials even at high discharge current. The long-term cycle stability of the ASC examined using GCD measurements at 5 A g−1 up to 5000 cycles is shown in Fig. 6e. The specific capacitances slightly decrease in the long-term cycle process, and only 9.4% capacitance loss is observed after 5000 cycles; this result is superior to those of some reported

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MnO2-based asymmetric cells, such as MnO2 nanowire/ graphene//graphene [50], MnO2 nanoplates on nickel foam//3D graphene hydrogel [51], and MnO2/rGO hydrogel//rGO hydrogel [52]. Energy and power densities are usually used to characterize the electrochemical properties of a supercapacitor cell. The energy and average power densities are calculated from the GCD curves at ଵ

different current densities according to E = ଶ CV ଶ and P = E/t, respectively. Fig. 6f shows the Ragone plot of the PMMO//N-doped mesoporous carbon ASC. The supercapacitor device delivers a high energy density of 38.6 Wh kg−1 at a power density of 900 W kg−1 and still retains an energy density of 25.1 Wh kg−1 at a power density of 9 kW kg−1. The energy density of the as-fabricated ASC system is highly comparable with previously reported MnO2-based ASC systems, such as activated carbon//NaMnO2 [53], MnO2 //grapheme [54] and MnO2@CCNs//CCNs [55]. An electrochemical impedance spectroscopy (EIS) experiment was conducted to further examine the detailed electrical properties of the ASC cell in the electrolytes. Fig. 6g presents the Nyquist plots obtained from the ASC before and after the long cycle measurements. The Nyquist plots include a semicircle in the high-frequency region followed by a straight line at the low-frequency region. These results reveal that the electrode process is controlled by an electrochemical reaction at high frequencies and by mass transfer at low frequencies [56]. The as-prepared ASC possesses a relatively large slope at the low-frequency region before the cycle measurement, proving a nearly ideal capacitive response [57]. The solution/electrolyte resistance (Rs) and the charge–transfer resistance (Rct) calculated from the intercept of the plot in the EIS spectrum and the diameter of the semicircle are 1.21 and 1.69 Ω, respectively. The low values are attributed to the porous structure and excellent electrical conductivity of the PMMO-based ASC. Obviously, polypyrrole doped with chloride ions introduced by ferrous chloride in our investigation shows an excellent conductivity [58]. During the charge–discharge measurement after 5000 cycles, the values of Rs and Rct increase to 1.53 and 1.83 Ω, and the straight line at the lowfrequency region shows a significant reduction in the slope, which may be due to the partial electrochemical degradation of PPy during the long cycle measurement [59].

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4. Conclusions This paper successfully presents an effective strategy for fabricating a CVD-deposited PPy/MnO2 composite. The composite has a highly mesoporous structure and excellent electrical conductivity. Physicochemical investigations reveal that the PMMO electrode possesses a high specific capacitance, high energy density, and good cycling stability in neutral aqueous Na2SO4 electrolyte. An ASC with PMMO as positive electrode and N-doped mesoporous carbon as negative electrode has been successfully assembled. The ASC shows a high specific capacitance and high energy and power densities at an operation voltage window of 2.0 V in highly safe aqueous Na2SO4 electrolyte solution. This PMMO electrode material configured with mesoporous N-doped carbon is a promising research direction for next-generation supercapacitors that can meet the high energy density and power storage demands of new energy devices. Acknowledgment The authors gratefully acknowledge the funding support by Laboratory of Precision Manufacturing Technology, CAEP (Grant No. KF15003).

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Fig. 1. Schematic showing synthesis of PMMO by CVD method

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Fig. 2. (a) FT-IR spectrum of PPy/MnO2 composite; (b) Raman spectra of pure MnO2 and PPy/MnO2 composite.

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Fig. 3. XPS spectra of PPy/MnO2 composite (a) survey spectrum; (b) Mn 2p spectrum; (c) C 1s spectrum; (d) N 1s spectrum.

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Fig. 4. (a) XRD patterns of MMO and PMMO; (b) A typical SEM image of PMMO; (c) TEM image with high resolution TEM image inset of MMO; (d) TEM image with high resolution TEM image inset of PMMO; (e) N2 adsorption–desorption isotherms and pore size distributions (inset) of MMO and PMMO.

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Fig. 5. Electrochemical behaviours of MMO and PMMO electrode materials: (a) Cyclic voltammograms of MMO and PMMO recorded in 1M Na2SO4 at 5 mV s-1; (b) Cyclic voltammograms of the PMMO electrode at different scan rates; (c) Galvanostatic charge-discharge curves of the PMMO electrode at different current densities (the bold line represents the GCD curve of MMO measured at 0.5 A g-1; (d) Plot of specific capacitance a function of current density of PMMO; (e) Cycling stability of the MMO and PMMO electrodes.

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Fig. 6. Electrochemical performances of the ASC: (a) CV curves at different potential windows at 10 mV s-1; (b) CV curves at different scan rates; (c) Galvanostatic charge/discharge curves at different current densities ranging from 0.5 A g−1 to 10 A g-1; (d) Specific capacitances at different current densities; (e) Cycle life at a current density of 5 A g-1, the inset shows the GCD curve last 20 cycles; (f) Ragone plots of the PMMO//NMC ASC and some reported results; (g) Nyquist plots obtained from the ASC before and after the long cycle measurements.

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Highlights • Mesoporous MnO2 with a specific surface area of 523 m2 g−1 is prepared. • PPy nanofilm is CVD-deposited on the surface of mesoporous MnO2. • The as-prepared composite maintains the micrographs and pore/channel structure. • The sample shows a high specific capacitance of 320F g−1 and a long cyclic stability. • An assembled ASC exhibits a high energy density of 38.6 Wh kg−1 at 0.5 A g−1.

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