Electrostatic-Interaction-Assisted Construction of 3D Networks of

Electrostatic-Interaction-Assisted Construction of 3D Networks of Manganese Dioxide Nanosheets for Flexible High-Performance Solid-State Asymmetric Supercapacitors Na Liu,† Yanli Su,*,† Zhiqiang Wang,† Zhen Wang,‡ Jinsong Xia,§ Yong Chen,§ Zhigang Zhao,*,‡ Qingwen Li,‡ and Fengxia Geng*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Renai Road, Suzhou 215123, China Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou Industry Park, Suzhou 215123, China § State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, 58 Renmin Road, Haikou 570228, China ‡

S Supporting Information *

ABSTRACT: A three-dimensional (3D) macroscopic network of manganese oxide (MnO2) sheets was synthesized by an easily scalable solution approach, grafting the negatively charged surfaces of the MnO2 sheets with an aniline monomer by electrostatic interactions followed by a quick chemical oxidizing polymerization reaction. The obtained structure possessed MnO2 sheets interconnected with polyaniline chains, producing a 3D monolith rich in mesopores. The MnO2 sheets had almost all their reactive centers exposed on the electrode surface, and combined with the electron transport highways provided by polyaniline and the shortened diffusion paths provided by the porous structure, the deliberately designed electrode achieved an excellent capacitance of 762 F g−1 at a current of 1 A g−1 and cycling performance with a capacity retention of 90% over 8000 cycles. Furthermore, a flexible asymmetric supercapacitor based on the constructed electrode and activated carbon serving as the positive and negative electrodes, respectively, was successfully fabricated, delivering a maximum energy density of 40.2 Wh kg−1 (0.113 Wh cm−2) and power density of 6227.0 W kg−1 (17.44 W cm−2) in a potential window of 0−1.7 V in a PVA/Na2SO4 gel electrolyte. KEYWORDS: two-dimensional sheets, manganese oxide, 3D network, electrostatic interaction, flexible devices

T

pseudocapacitive supercapacitor applications owing to its numerous advantages such as environmental compatibility, cost effectiveness, and natural abundance.10,11 Notably, charge storage in MnO2 mainly arises from a redox reaction occurring only in the first few nanometers of the electrode surface between Mn3+ and Mn4+ with almost no capacitance contribution from bulk intercalation.12−14 Therefore, it is highly necessary to downsize the electrode material to achieve a high specific surface area to enhance pseudocapacitive properties. The sheet form of redox-active MnO2, derived by delaminating a birnessite-type layered manganese oxide, is a unilamellar crystallite with a thickness on the nanometer scale

he rapidly growing commercial markets of portable electronics and electric vehicles have stimulated an imperative requirement for high-performance energy storage devices.1−4 Advances in supercapacitors, merited by their higher energy density than traditional capacitors, superior power delivery than recyclable batteries, and ultralong cycle life, are playing unparalleled roles in markets to meet the continuously increasing demand for energy consumption and to alleviate the growing concern about the energy crisis.5−8 To have supercapacitors with the capability to store sufficient energy and meet the higher performance criteria of future systems, pseudocapacitive transition metal oxides are very competent electrode material candidates, which possess variable oxidation states and thus facilitate fast and successive redox reactions.9 Among the rich pool of various transition metal oxides, manganese oxide (MnO2) has sparked particularly great attention as a very attractive electrode material platform toward © 2017 American Chemical Society

Received: April 4, 2017 Accepted: July 18, 2017 Published: July 18, 2017 7879

DOI: 10.1021/acsnano.7b02344 ACS Nano 2017, 11, 7879−7888

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ACS Nano and lateral dimensions on the micrometer scale.15−17 Very recently, a macroporous assembly of MnO2 sheets obtained by flocculating a MnO2 colloid with HCl was reported with a controlled Mn3+/Mn4+ ratio or cation vacancies, which showed improved electrochemical capacitance at a higher concentration of vacancies.11 This material is envisioned to show incomparable advantages in electrochemical behavior with all electroactive metallic centers exposed on the surface, allowing a high degree of electrode material utilization and intimate contact at the electrode to electrolyte interface, thus promising enormous possibilities to yield impressive performance as an electrode material for electrochemical supercapacitor applications. Moreover, it is well recognized that deliberately designed three-dimensional (3D) structures rich in pores are favorable for ameliorating electrochemical performances, owing to their advantages of fast electrolyte penetration, more exposed active sites, etc.18−20 For example, an interconnected graphene foam loaded with lithium titanate and lithium iron phosphate as a battery anode and cathode showed excellent high-rate battery performance owing to the rapid electron and ion transport in the porous structure.21 Alternatively, a hierarchical nanostructured polyaniline showed significantly improved energy storage properties, especially in terms of its high rate of electrochemical process and cyclic stability.22 However, the construction of MnO2 sheets into a complex, interconnected porous structure faces serious obstacles. First, although various MnO2 nanostructures have been used as decorated components for graphene-based or other 3D nanostructures,23,24 birnessitetype MnO2 sheets are of relatively high rigidity compared with sheets of graphene, posing difficulties for hierarchical assembly. Second, Mn in MnO2 sheets is heterovalent, i.e., Mn3+ and Mn4+, giving the sheets a high density of negative charges, for which the sheets have a high tendency to agglomerate into restacked structures in solution, and inevitably dramatic loss in electrochemically reactive surface would occur. Therefore, although it is greatly desired to assemble MnO2 sheets into a 3D porous structure to achieve full utilization of the electrode materials, it remains a critical challenge due to the restrictions of the synthetic method and limitations of the templates. Herein, we demonstrate a facile electrostatic-interactionassisted approach to synthesize 3D mesoporous MnO 2 networks backboned with polyaniline as an efficient supercapacitor electrode. The negatively charged surfaces of the MnO2 sheets were first grafted with protonated aniline monomers, after which the in situ polymerization of aniline connected MnO2 sheets into a 3D interpenetrated porous structure, as schematically illustrated in Figure 1. Fortunately, owing to the high conductivity of polyaniline, it additionally ameliorated electron transfer in the redox reactions of MnO2 while structurally connecting the MnO2 sheets into a macroscopic monolith. Thus, the “wiring” of the MnO2 sheets by the conductive polyaniline chains along with the mesoporous features provides highways for both electron and ion transport, and additionally the reduced dimensions of the constituent active materials would contribute to shorten path lengths for transport and diffusion of electrolyte ions, resulting in improved electrochemical performances, characterized by a large specific capacity (762 F g−1 at a current density of 1 A g−1), decent rate performance (capacity retention of 77% on increasing the current density from 1 A g−1 to 10 A g−1), and long-cycle stability at high current density (retention of 90% over 8000 cycles at a moderate density of 5 A g−1).

Figure 1. Synthesis procedure for the 3D network of MnO2 sheets backboned with polyaniline chains. Inset: Photograph of the 3D hybrid hydrogel. From the tube-inversion experiment, the solution was confirmed to be completely gelled with full fluidity loss.

Furthermore, an asymmetric supercapacitor device in a gel electrolyte of PVA/Na2SO4 was assembled with the deliberately designed 3D electrode and activated carbon as positive and negative electrodes, respectively, which delivered an energy density maximum of 40.2 Wh kg−1/0.113 Wh cm−2 (at a power density of 340.0 W kg−1/0.95 W cm−2) and power density of 6227.0 W kg−1/17.44 W cm−2 (at an energy density of 19.0 Wh kg−1/0.053 Wh cm−2) in a wide operating voltage window of 1.7 V, suggesting that the hybrid electrode could be a very promising electrode candidate for potential uses in flexible supercapacitors.

RESULTS AND DISCUSSION The 3D network of MnO2 sheets interconnected with polyaniline chains was obtained via a solution-based reaction, as illustrated in Figure 1. Unilamellar MnO2 sheets possessing edge-sharing MnO6 octahedra were synthesized through a typical top-down delamination approach15 (detailed characterizations for the precursors are in the Supporting Information, S1−2). It needs to be mentioned that the obtained MnO2 sheets were negative in net charge, which resulted from the inclusion of heterovalent Mn3+ along with Mn4+. Thereafter, the MnO2 nanosheets were grafted with protonated aniline through electrostatic interactions, which was achieved by homogeneously dispersing the sheets in aqueous aniline in the presence of a cyclic acid saturated with six phosphoric acid groups, i.e., phytic acid (C6H6(H2PO4)6). Upon addition of an oxidizing initiator, for example, ammonium peroxydisulfate ((NH4)2S2O 8), polymerization of aniline was initiated, connecting the MnO2 sheets into a 3D interconnected structure. Typically, the mixed solution gelled within no more than 5 min at 0 °C, transforming the mixture solution into a hydrogel with little fluidity, as shown by a tube-inversion experiment (inset in Figure 1). The gel was dark green in color, characteristic of the emeraldine salt form of polyaniline, which confirmed the formation of polyaniline chains. As the polymerization was initiated from the surfaces of MnO2 nanosheets, strong interaction between the two components was ensured, combining the nanostructured features of MnO2 sheets and the 3D porous structure, which would endow the designed material with efficient mass transport and excellent electrochemical activity. 7880


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delaminated MnO2 sheets.15,16 Upon mixing the MnO2 colloid with the aniline monomer solution, while no obvious precipitation was observed (Supporting Information, S4), the characteristic absorption shifted to higher energies, which is likely related to the grafting of the MnO2 sheet surfaces with aniline monomers. The close interaction of the aniline monomers to the MnO2 sheets facilitated the interconnection of the MnO2 sheets with polyaniline into a uniform 3D structure. The solvent, H2O, was entrapped in the network and occupied the majority of the volume, and therefore, a highly porous structure could be readily obtained simply by a freeze-drying treatment, which has been a simple and efficient template-free approach to the scalable preparation of 3D highly porous monoliths. In this process, the contraction of the total volume was negligible, suggesting little contraction of the network in the freeze-drying treatment. The porous architecture was templated from the spaces occupied by H2O by sublimating the ice crystals. The density of the thus-obtained sample was approximately 0.167 g cm−3, which is comparable to some other reported 3D porous structures, for example, 0.55 g cm−3 for SnO2 impregnated in a carbon porous structure,25 0.18 g cm−3 for Ru nanoparticles attached on a carbon network surface,26 and 0.674 g cm−3 for Si nanoparticles composited in a polyaniline network.27 Owing to the ultralow density of the obtained aerogel, it could even be balanced on a dandelion, as displayed in Figure 3a. The nitrogen adsorption and desorption isotherms of the obtained porous structure were also characterized, as presented in Figure 3b, which displayed typical type IV mesoporous characteristics with obvious hysteresis. The sharp increase in nitrogen uptake at P/P0 > 0.9 might be ascribed to the presence of pores forming much larger, secondary mesoporosity.28 The inset of Figure 3b shows the corresponding pore size distribution plot from a nitrogen adsorption branch analyzed with the Barrett− Joyner−Halenda (BJH) method, revealing the presence of mesopores with a distribution peak at ∼4.3 nm, which is attributable to the interconnected polyaniline bones. Further-

The stable colloidal suspension of exfoliated MnO2 sheets was characterized with atomic force microscopy (AFM), with typical images provided in Figure 2a, which observed two-

Figure 2. (a) Tapping-mode AFM image of the MnO2 sheets deposited onto a Si wafer substrate precoated with polyethylenimine. Inset: Picture of the aqueous colloid for the exfoliated MnO2 sheets displaying an obvious Tyndall scattering effect. The phenomenon resulted from the scattering of an irradiated beam by the exfoliated sheets in solution. (b) UV−vis absorption spectrum for the MnO2 sheets grafted with protonated aniline in comparison with that for the neat MnO2 sheets.

dimensional sheets with lateral dimensions in the range of 1−2 μm, although a few smaller fragmented pieces were also detected. The corresponding height diagram showed that the sheets had a uniform thickness of approximately 1.0 nm, in close agreement with a sum value of crystallographic thickness for a monolayer MnO 2 sheet (0.52 nm, Supporting Information, S3) and the surface covering moieties of H2O molecules along with some counterbalancing ions, TMA+,15,16 which confirmed the unilamellar nature of the delaminated sheets. The obtained aqueous colloid was a transparent brownish color, which exhibited an apparent Tyndall light scattering when being irradiated through with a red laser beam (inset in Figure 2a), also confirming the sheets were welldispersed in water. The optical absorption of the colloid exhibited a broad peak centered at 374 nm, characteristic of

Figure 3. (a) Photograph of the obtained 3D monolith supported on a dandelion, implying its low density. (b) N2 adsorption and desorption isotherms. Inset: Derived pore size distribution with BJH analysis. (c) XRD patterns for the hybrid and neat polyaniline;. (d) SEM image and (e) corresponding EDS mapping demonstrating the mesoporous structure and homogeneous distribution of all elements. The arrows in (d) indicate the sheets of a few micrometers in lateral size supporting polyaniline backbones. (f) XPS spectra of N1s for the hybrid structure and the neat polyaniline counterpart. 7881


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Figure 4. Electrochemical behavior of the hybrid electrode measured in a three-electrode system. (a) Comparing the CV curve of the hybrid with those for neat MnO2 and polyaniline at scan rates of 10 mV s−1, demonstrating the obvious synergistic effect. (b) CV curves at variable scan rates and (c) GCD profiles at various current densities of the hybrid electrode. (d) Specific capacitances of the hybrid electrode compared with the neat polyaniline electrode and MnO2 electrode depending on current density. At all current densities, the hybrid electrode exhibited capacity enhancement, which was much more obvious at higher current densities. (e) Cycling performance over 8000 cycles at a current density of 5 A g−1. Inset: GCD profiles for the last 10 cycles.

structure gave pores with sizes of tens of nanometers. In addition, secondary macropores were also observed, consistent with the nitrogen adsorption−desorption characterizations. The energy-dispersive X-ray spectroscopy (EDS) spectrum of the sample showed elements of Mn and O along with C, N, and P, in addition to the combined features of polyaniline and MnO2 observed in the infrared spectroscopy and Raman characterizations (Supporting Information, S7−9), all of which undoubtedly and consistently evidenced the successful hybridization of polyaniline and MnO2. The elemental mapping displayed in Figure 3e shows even color distributions for all elements (Mn, O, C, and P), suggesting the homogeneous hybridization of the two constituents. Due to the close grafting of polyaniline on the surface of the MnO2 sheets, the incorporation of the MnO2 sheets did not impair the whole conductivity. The conductivity of polyaniline is heavily dependent on the doping level of nitrogen, and therefore X-ray photoelectron spectroscopy (XPS) features of N1s for the obtained hybrid structure were compared with those for neat polyaniline, as displayed in Figure 3f. The fitted XPS peaks of neat polyaniline were centered at 399.5 and 400.7 eV, which were ascribable to benzenoid amines and cationic nitrogen radicals, respectively. In the meantime, the presence of the cationic nitrogen peak was suggestive of the protonation doping of polyaniline with phytic acid.29 In contrast, for the hybrid structure the proportion of cationic nitrogen was

more, the addition of MnO2 sheets into the polyaniline network brought a slight increase in the corresponding surface area (Supporting Information, S5), which should be ascribed to the ultrathin thickness of the MnO2 sheets. The product phase was identified by X-ray diffraction (XRD) analysis and is depicted in Figure 3c, showing diffraction features characteristic of polyaniline chains, three broad peaks with maxima at 15.3°, 20.5°, and 25.6°,22,28 and an additional halo at the low-angle region centered at 6.8°, which may originate from the possible occurrence of some restacking of sheets during the mixing and polymerization process. The average restacking degree or stacked thickness could be estimated according to the Scherrer equation, which gave a value of 1.9 nm, corresponding to restack limited to 2 or 3 layers, additionally evidencing that the MnO2 incorporated in the hybrid was in an ultrathin sheet form. To further confirm the 3D porous network in the obtained hybrid aerogel, the sample was characterized using scanning electron microscopy (SEM). MnO2 sheets of a few micrometers in lateral size could be apparently found uniformly dispersing in the network and closely connecting the fibers (Figure 3d and Supporting Information, S6), which should result from the well-dispersed sheets in the aqueous aniline system and the fact that the polymerization of aniline was initiated from the surface of MnO2 sheets. The polyaniline bones interpenetrating among the MnO2 sheets manifested diameters of 60−100 nm, and this 7882


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because of the heavily porous structure and efficient charge transfer at the component interfaces. The enhancement of the specific capacitance was also evidenced by the GCD curves (Supporting Information, S12), in which the hybrid electrode exhibited the longest discharge time. The GCD curves for the hybrid hydrogel electrode with increasing current density from 1 to 10 A g−1 are presented in Figure 4c, in which all the curves well preserved symmetrical or mirror-like shapes, implying good reversibility of the electrode. Impressively, the specific capacitance of the hybrid electrode estimated from the galvanostatic discharge curves was 762, 680, 643, and 587 F g−1 at increasing current densities of 1, 2, 5, and 10 A g−1, respectively. In other words, 77% of the initial capacity at 1 A g−1 was retained on increasing the current density up to 10 A g−1. This behavior is very competitive when compared with other previously reported MnO2-based hybrid materials, for example, MnO2/polypyrrole co-deposited on single-wall carbon nanotube bundles (353 F g−1 at 1 A g−1),31 graphene oxide sheets supporting needle-like MnO2 (216 F g−1 at 0.15 A g−1),32 and polyaniline nanofibers sandwiched between graphene layers (210 F g−1 at 0.3 A g−1).33 A detailed comparison with more example materials and their electrochemical performances is listed in the Supporting Information (Table S1). The obtained high capacitance should originate from the synergy working of the polyaniline backbone and ultrathin MnO2 sheets in this deliberately designed structure. The specific capacitances of the hybrid electrodes derived from the GCD curves were compared with those for neat polyaniline and MnO2 materials, at all current densities, as shown in Figure 4d, from which it was obvious that the hybrid electrode showed a significant enhancement, especially at high current densities. The capacitances were 464, 383, 264, and 163 F g−1 and 144, 123, 100, and 64 F g−1 at current densities of 1, 2, 5, and 10 A g−1 for neat polyaniline and MnO2 materials, respectively. Although the values were respectively comparable to those for previously documented polyaniline22,33 and MnO2 materials,10,11,34,35 it is noteworthy that the hybrid manifested a much enlarged capacitance compared with the calculated specific capacitance assuming a proportional contribution from each component, even with the MnO2 fraction contributing the theoretical capacitance of 1370 F g−1 (Supporting Information, Table S2). The great performance enhancement implied a significant synergistic effect between the two components. The MnO2 sheets with such ultrathin feature and in a 3D conductive network probably achieved full utilization and showed an obvious profit in contribution to the overall capacitance compared with other MnO2-containing composites, for example, chemically synthesized MnO2 needles contributing ∼200 F g−1 at 0.2 A g−1 composited on graphene oxide,32 a maximum 800 F g−1 at 5 mV s−1 for electrodeposited MnO2 on carbon nanoparticles,36 and 1145 F g−1 at 5 mV s−1 for MnO2 thin film electroless-plated on porous metal.6 As the hybrid capacitance even outperforms the one supposing MnO2 possessing a theoretical value, particularly at high densities, the polyaniline performance should have been also elevated, which could be due to the efficient charge exchange at the interface and the resultant modification to the doping level of polyaniline and/or the suppression of swelling, shrinkage, or cracking of the polyaniline chains by the penetrated MnO2 sheets, which however still needs further in-depth study before complete elucidation. To further verify the positive function of the 3D structure and chemical interactions at polyaniline fiber/

significantly increased, which should originate from the strong electronic interactions between the two constituents. The room-temperature electrical conductivity of the hybrid aerogel was measured to be 0.08 S cm−1 (details in the Supporting Information, S10), very close to that of the neat polyaniline aerogel (0.09 S cm−1). Therefore, there was almost no deterioration in electrical conductivity after hybridizing polyaniline with poorly conductive MnO2, owing to the strong chemical coupling between the two constituents. The polyaniline chain in this constructed 3D architecture served as an efficient electron highway for redox reactions of MnO2, combining the high percentage of reactive centers possessed by the MnO2 sheets and the porous structure, which would allow excellent contact between the electrode and liquid electrolyte and also rapid mass transportation. All the favorable characteristics suggest that the constructed porous electrode based on MnO2 sheets will display excellent electrochemical performance. The so-obtained 3D network of MnO 2 nanosheets backboned with the conductive polyaniline was expected to generate a high electrode/electrolyte interfacial contact area and also short paths for electron/ion diffusion when being used as an electrochemical electrode material. Electrochemical performance of the sample was first evaluated in a threeelectrode configuration with typical techniques of cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrochemical impedance spectroscopy (EIS). The electrochemical performances of the 3D network containing a range of amounts of MnO2 sheets were studied, and the optimal amount was applied for detailed electrochemical characterization. While the improvement in electrochemical capacity was quite limited when the MnO2 content was low, the sheets impeded the formation of a uniform 3D network upon increasing MnO2 to a high level, resulting in an irregular microstructure, which degraded the electrochemical behaviors (Supporting Information, S11). Figure 4a demonstrates a typical CV curve of the optimal hybrid electrode compared with those for the two individual components, all supported on carbon cloths, measured in a voltage window ranging from −0.2 to 0.7 V and at a scan rate of 10 mV s−1. Only negligible current was detected for the bare carbon cloth, and therefore under the conditions adopted in the present work the obtained capacitance should originate from the subject material with almost no contribution from the electrode support. Notably, the integrated area surrounded by the CV curve for the hybrid structure was significantly larger than that for both unitary constituents, either neat polyaniline or the MnO2 sheets, revealing that the hybrid electrode possessed a significantly higher specific capacitance than both neat constituents, which was attributable to the synergistic effect of the MnO2 sheets and polyaniline backbone. Two pairs of redox peaks, R1/R1′ and R2/R2′ marked in Figure 4a, were clearly observed, which should be related with the Faradaic redox reactions occurring for the polyaniline molecules between a fully reduced state (semiconducting leucoemeraldine) and a partially oxidized state (conducting polaronic emeraldine form) and the emeraldine state to a fully oxidized pernigraniline, respectively.30 No welldefined peaks characteristic of MnO2 were found because the reversible redox reactions of MnO2 typically occur in a fast and successive way. The CV profiles for the hybrid electrode with increasing scan rates from 5 to 100 mV s−1 are shown in Figure 4b, displaying no significant changes in the shapes of the curves at differing scan rates, which suggests a good rate performance 7883


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Figure 5. (a) Schematic for the fabricated asymmetric supercapacitor device. (b) CV curves for activated carbon (negative electrode) and the hybrid electrode (positive electrode) in a three-electrode configuration at a scan rate of 20 mV s−1 in a 1 M Na2SO4 aqueous solution. (c) CV curves of the asymmetric supercapacitor device measured at 20 mV s−1 with the potential window broadening from 0.8 V to 2.0 V. (d) Specific capacitance estimated from the CV curves as a function of differing potential windows. (e) CV curves at differing scan rates and (f) GCD curves of the assembled asymmetric supercapacitor device at various currents and at an operation voltage window of 1.7 V.

promotes better electrolyte accessibility within the electrode materials along with more efficient exposure of the active sites to the electrolyte; the 3D interconnected porous structure allowed fast ion diffusion and reduced the transport distances of the electrolyte ions during the charge and discharge processes; the strong chemical coupling at the MnO2 and polyaniline interface ensured efficient charge transfer for fast redox reactions of the MnO2 sheets (Supporting Information, S14). All the factors worked concertedly and accordingly maximized the synergistic function of the constituent components. To further examine the potential uses of the deliberately designed hybrid electrode, an asymmetric supercapacitor device was measured in a two-electrode system with the 3D hybrid electrode as the positive electrode in combination with activated carbon as the negative one while having a PVA/ Na2SO4 gel electrolyte sandwiched between the electrodes, as schematically illustrated in Figure 5a, by which a high energy density and power density could promisingly be simultaneously achieved. The CV curves at various scan rates and GCD profiles at different current densities for activated carbon showed good electrochemical response and reversibility (Supporting Information, S15). From the CV tests on the three-electrode system with the positive hybrid electrode and negative activated carbon electrode in 1 M aqueous Na2SO4 at a scan rate of 20 mV s−1, presented in Figure 5b, the calculated electrode specific capacitance was 562 and 201 F g−1 operating stably in the potential windows of −1 to 0 V and −0.2 to 0.7 V, respectively. For achieving charge balance between the two electrodes in this assembled asymmetric cell, the mass ratio of m(positive)/

MnO2 sheet interfaces, control experiments studying electrochemical performances of their simple physical mixture were carried out, which showed much inferior electrochemical capacity (Supporting Information, S13). For example, the hybrid exhibited a specific capacitance of 650 F g−1 at a current of 10 A g−1, while comparatively only 376 F g−1 was obtained for the physical mixture of these two components. The results unambiguously underscored the advantages of such a constructed pore-rich skeleton structure and the nanoscale hybridization between the electrostatically coupled polyaniline and MnO2 sheets, which would offer a synergistic effect and thus yield improved electrochemical performances. In addition to the significantly enhanced electrochemical capacitance and rate performance, the hybrid electrode possessed a superior long-cycle stability. The hybrid electrode showed little capacitance loss and kept a high capacitive retention of 90% even after 8000 charge/discharge cycles, demonstrating the very high stability of the hybrid electrode. In sharp contrast, the neat polyaniline electrode showed a capacity retention of only ∼70%, and capacity loss was particularly severe at extended cycles beyond 6000 cycles. The inset shows the GCD profiles of the last 10 cycles (7990th to 8000th), displaying that symmetrical triangles were still well maintained. All the results unambiguously evidenced that the hybrid electrode exhibited excellent long-cycle electrochemical stability. The superior electrochemical performance of the obtained 3D networks of MnO2 sheets backboned by polyaniline can be ascribed to three factors, described as follows: the high percentage of reactive centers possessed by the MnO2 sheets 7884


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Figure 6. (a) Ragone plot (energy density vs power density) for the assembled asymmetric supercapacitor device. (b) CV curves collected at a scan rate of 20 mV s−1 for the device under various bending conditions, suggesting excellent mechanical flexibility.

areal capacitance of 132.7 mF cm−2, which should originate from the high electrochemically active surface area and efficient mass transportation of the constructed 3D nanosheet porous structure, as proved by the Nyquist plot depicted in the Supporting Information, S17. The Ragone diagram plotting the energy and power densities of the fabricated device, which were attained from calculation based on the galvanostatic discharge plots, is depicted in Figure 6a. Impressively, with an operating voltage of 1.7 V, the fabricated asymmetric supercapacitor device exhibited a maximum energy density of 40.2 Wh kg−1 at a power density of 340.0 W kg−1 and still maintained 19.0 Wh kg−1 at a high power density of 6227.0 W kg−1. The corresponding maximum areal energy and power density were 0.113 Wh cm−2 and 17.44 W cm−2, respectively. These numbers are superior to previously documented asymmetric supercapacitor devices (details in the Supporting Information, Table S3), listing a few examples, MnO2−rGO//carbon nanotube−rGO (31.8 Wh kg−1 at 453.6 W kg−1),40 layered NaMnO2//activated carbon (19.5 Wh kg−1 at 130.0 W kg−1),42 MnO2 nanofiber−graphitic hollow carbon spheres (GHCS)//GHCS (22.1 Wh kg−1 at 100.0 W kg−1),43 and bacterial-cellulose-derived carbon nanofiber network coated MnO2//nitrogen-doped bacterial cellulose (32.91 Wh kg−1 at ∼250 W kg−1),44 and some others.45−48 Importantly, due to the high mechanical stability of the 3D porous structure and the strong chemical interactions between the constituent components, the as-fabricated asymmetric supercapacitor device exhibited excellent flexibility with no significant degradation in electrochemical performances. CV measurements of the asymmetric supercapacitor device were conducted at 20 mV s−1 under differing bending angles of 45°, 90°, and 180° (Figure 6b), and it was found that the obtained CV curves showed no significant change, manifesting excellent mechanical flexibility and stability possessed by the device. As such, this flexible and cost-effective asymmetric supercapacitor based on the designed electrode showing high electrochemical performance could serve as a promising candidate for a host of future flexible and gadget electronic applications.

m(negative) was carefully adjusted and the optimized one was set to be 0.4 (calculation details in the Experimental Methods). For attaining the optimized working potential, the comparative CV curves of the assembled asymmetric supercapacitor prototype device in a range of voltage windows were taken at a scan rate of 20 mV s−1 and displayed in Figure 5c. The asymmetric supercapacitor device exhibited stable behavior in the operating voltage window of 0−1.7 V, and oxygen evolution occurred when the potential reached 2.0 V or higher.37 The specific capacitances under a current of 2 mA in different potential windows were calculated and are depicted in Figure 5d, which showed that the capacitance values increased almost linearly from 48 to 72 F g−1 with the operating voltage extending from 0.8 to 1.7 V. Meanwhile, even at 1.7 V, GCD curve of the asymmetric cell still maintained a nearly symmetric shape, indicating the fast current response at this voltage (Supporting Information, S16). Therefore, a potential window of 1.7 V, almost twice that for the conventional activated carbon-based symmetric capacitors in aqueous electrolyte (∼1.0 V),38 was chosen for further detailed investigations of the electrochemical performance for the assembled asymmetric supercapacitor. The CV curves for the as-prepared asymmetric supercapacitor are shown in Figure 5e. Unlike the three-electrode electrochemical performance featuring obvious redox signals, the asymmetric supercapacitor exhibited quasi-rectangular curves, which resulted from the combination of the capacitive behaviors of both pseudocapacitance and electric double-layer capacitance in the as-assembled asymmetric configuration. The current increased with increasing scan rates, and the shape was well maintained even at a scan rate increasing up to 100 mV s−1. In addition, the GCD plots at different current densities, displayed in Figure 5f, presented symmetrical charge/discharge curves, indicating ideal capacitive characteristics and a high Coulombic efficiency. The specific capacitance was evaluated by the total mass including both the positive and negative electrodes (2.8 mg), and the calculated gravimetric capacitance for our asymmetric supercapacitor cell was 100.2 F g−1 at a current of 1 mA (a density of approximately 0.36 A g−1). The area for the assembled device was 1 cm2, and thus the corresponding normalized areal capacitance was 280.6 mF cm−2. The capacitance value is superior to most other recently reported asymmetric supercapacitors based on MnO2 composite materials, for example, CuO@MnO2 core−shell architecture//microwave exfoliated graphene oxide (49.2 F g−1 at 0.25 A g−1),39 MnO2−reduced graphene oxide (rGO)//carbon nanotube−rGO (69.4 F g−1 at 0.5 A g−1),40 and MnO2−rGO// rGO (22.7 F g−1 at 0.01 A g−1).41 Even with the current increasing 20 times to 20 mA, the asymmetric supercapacitor device still showed a high specific capacitance of 47.4 F g−1 and

CONCLUSION A 3D network of MnO2 sheets “wired” with polyaniline chains was developed, which was assisted by the strong electrostatic interactions between the MnO2 sheets bearing negative charges and the protonated aniline monomers. The structure possessed three favorable characteristics: (1) the MnO2 sheets had all electrochemically reactive metal centers exposed on the surface, maximizing the utilization of the electrode materials; (2) polyaniline has a high conductivity and is strongly coupled with the MnO2 sheets, which provided a highway for electron transfer in the redox reactions of the electrode materials; (3) 7885


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put in ice bath at 0−4 °C. After reaction for 24 h, the sample supported on carbon cloth was immersed in H2O for 24 h to remove any excess reactants. The control electrode of neat polyaniline was made using a similar procedure except without adding MnO2 sheets. To prepare the control electrode of neat MnO2 sheets or the negative electrode for the asymmetric assembly, sheets separated from the colloid by high-speed centrifugation or activated carbon, polyvinylidene fluoride binder, and Super P current collector were dispersed in a weight ratio of 8:1:1 in N-methylpyrrolidone (NMP) and mixed thoroughly, after which the obtained slurry was uniformly supported on a carbon cloth by blade coating. To avoid possible sheet restacking or aggregation in the electrode-making process, the separated unilamellar MnO2 sheets in a wet glue state were immediately redispersed in some NMP before mixing with others. Finally, the electrodes were dried at 60 °C in an oven for 24 h to ensure complete evaporation of the NMP solvent. The loading mass of active materials was controlled at approximately 2 mg cm−2. The specific capacitance (Cs, F g−1) of one electrode in a three-

the porous structure enabled the close contact of electrolyte ions with the electrode materials and thus shortened the diffusion paths of the ions. Therefore, the deliberately designed electrode manifested superior electrochemical behavior in terms of high capacity (762 F g−1 at a current density of 1 A g−1), superior rate performance (587 F g−1 at a current density of 10 A g−1), and good cyclic stability (capacity retention of 90% even after 8000 cycles). In addition, the assembled asymmetric supercapacitor based on the designed electrode and activated carbon exhibited a high energy density (40.2 Wh kg−1 at 340.0 W kg−1/0.113 Wh cm−2 at 0.95 W cm−2) and power density (6227.0 W kg−1 at 19.0 Wh kg−1/17.44 W cm−2 at 0.053 Wh cm−2) within the operating voltage window of 0−1.7 V in PVA/Na2SO4 gel electrolyte. The special structure showed competitive electrochemical performance, making it a very attractive electrode candidate for high-performance supercapacitors in future flexible and portable energy storage device applications.

electrode system was calculated based on the equations Cs =

∫ I dV ΔVmv

and

I Δt mΔV

when estimating from the CV and GCD curves, Cs = respectively, where I, ΔV, m, v, and Δt respectively represent the applied specific discharge current (A), the voltage window for one scanning segment (V), the mass of the electrode (g), the voltage scan rate (V s−1), and the period for one discharge cycle (s). For the constructed asymmetric capacitors in a two-electrode system, specific capacitances (Casy, F g−1) derived from GCD curves were obtained via I Δt the equation Casy = M ΔV , where M (M = m+ + m−) means the sum active material masses on both positive and negative electrodes (g). To obtain charge balance, the optimized mass ratio between positive (m+) and negative electrodes (m−) was obtained using the m C V equation m+ = C−V− . The energy density E (Wh kg−1) and the power

EXPERIMENTAL METHODS Preparation of MnO2 Nanosheets. A colloidal suspension of MnO2 sheets was prepared according to previous reports with minor modifications. K-birnessite nanobelts were first obtained through a hydrothermal method at 175 °C for 2 days, which were then successively treated with an aqueous solution of 0.5 M ammonium persulfate (APS, (NH4)2S2O8) at 60 °C for 12 h and 1 M HCl at 25 °C for 2 h to give the protonated H-birnessite phase. Before collection, the solid was washed with Milli-Q water and air-dried. For delaminating H-birnessite to produce the colloidal suspension of MnO2 sheets, a weighed amount (0.4 g) of as-prepared H-birnessite was immersed in 100 mL of aqueous tetramethylammonium hydroxide (TMAOH) solution, and the system was mechanically shaken at 150 rpm for 2 days. Then the system underwent an ultrasonication treatment for 8 h. The colloidal suspension of MnO2 sheets was finally obtained after centrifugation at 3500 rpm to remove the nanoparticles that remained laminated. The concentration of the obtained MnO2 colloid was determined to be approximately 2.0 mg mL−1. Preparation of the 3D Network of MnO2 Sheets Backboned with Polyaniline. In a typical synthesis, an aliquot of MnO2 sheets separated by high-speed centrifugation (10 000 rpm) was immediately dispersed in a monomer solution of 2 mL containing 2.5 M aniline and 0.5 M phytic acid by a short ultrasonication to achieve a homogeneous mixture. To initiate the polymerization, 1 mL of an aqueous solution of 1.25 M APS was added into the above mixture and subjected to quick stirring for ∼30 s under an ice bath. In approximately 5 min, the mixed solution changed from a brown color to dark green and in the meantime became viscous and gel-like, implying completion of the polymerization. Finally, the as-prepared monolith was washed three times to remove any excess acid and byproducts from the polymerization. Electrochemical Measurements. All electrochemical studies were conducted on an electrochemical analyzer (CHI660E, Shanghai) at room temperature. For evaluating the electrochemical behavior of the individual electrode in a three-electrode configuration, 1 M aqueous Na2SO4 was used as the electrolyte with a saturated calomel electrode (SCE) and a Pt sheet employed as the reference and counter electrode, respectively. In the asymmetric two-electrode measurement, the hybrid electrode was paired with activated carbon separated with a commercial NKK separator. PVA/Na2SO4 gel was employed as the electrolyte, which was made by mixing aqueous Na2SO4 (0.05 g mL−1) and PVA (0.2 g mL−1) before heating at 95 °C under continuous vigorous stirring until the system became transparent. The specific capacitance for the fabricated asymmetric supercapacitor was evaluated considering the total active masses on both positive and negative electrodes. To make the electrode of the network of MnO2 sheets backboned with polyaniline, a solution mixture containing all reactants was quickly dropped onto a carbon cloth before it lost fluidity and subsequently

−

+ +

density P (W kg−1) were evaluated by means of GCD curves following the equations E =

2 1 1000Casy ΔV 2 3600

and P =

3600E . Δt

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02344. Additional information (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fengxia Geng: 0000-0001-5557-4165 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (51402204), Thousand Young Talents Program, Jiangsu Specially-Appointed Professor Program, and a project funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. Z.G.Z. thanks the National Natural Science Foundation of China (51372266, 51572286) and the Outstanding Youth Fund of Jiangsu Province (BK20160011) for financial support . 7886


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