Î²-MnO2

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Low Charge-carriers Scattering 3-Dimensional Alpha-MnO2/BetaMnO2 Networks for Ultrahigh-rate Asymmetrical Supercapacitors Shijin Zhu, Tian Wang, Xiaoying Liu, Yuxin Zhang, Fei Li, Fan Dong, Han Zhang, and Lili Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01592 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 17, 2018

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ACS Applied Energy Materials

Low Charge-carriers Scattering 3-Dimensional Alpha-MnO2/Beta-MnO2 Networks for Ultrahigh-rate Asymmetrical Supercapacitors Shijin Zhu†, Tian Wang†, Xiaoying Liu‡, Yuxin Zhang†*, Fei Li†, Fan Dong⊥, Han Zhang§, Lili Zhang∥* † State Key Laboratory of Mechanical Transmissions, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P.R. China ‡ Engineering Research Center for Waste Oil Recovery Technology and Equipment of Ministry of Education, College of Environment and Resources, Chongqing Technology and Business University, Chongqing, 400067, China ⊥Research Center for Environmental Science & Technology, Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 611731, China. § Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of OptoelectronicEngineering, Shenzhen University, Shenzhen 518060, P.R. China ∥ Institute of Chemical and Engineering Sciences (ICES), Agency for Science, Technology and Research (A*STAR), 1 Pesek Road, Jurong Island, Singapore 62783 *Corresponding author, E-mail addresses: [email protected] (Prof. Dr. Y.X. Zhang); [email protected] (Prof. Dr. L.L. Zhang)

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ABSTRACT The excessive concern of energy density of supercapacitor is changing its applied direction while the power density is always overlooked. Supercapacitor should be considered as a high power energy device rather than a neither fish nor fowl energy device. Herein, ultrafine α-MnO2 needle was formed on β-MnO2 networks not distributing randomly but standing on the surface of β-MnO2 vertically forming an array structure with low charge carriers scattering. These novel structures possess a rational arrangement of the needles resulting in high capacitance (278.2 F g-1 for α-MnO2/βMnO2 networks) and excellent rate capability (41.0% remained with the specific current increased form 0.25 A g-1 to 64.0 A g-1). Asymmetrical supercapacitors fabricated by reduced graphene oxide (RGO) as anode and as-prepared structures as cathode deliver excellent electrochemical performance. Specifically, the devices give a favorable specific energy (29.8 Wh kg-1) considering the weight of α-MnO2/β-MnO2 and reduced graphene oxide (RGO) as well as ultrahigh specific power (64.0 kW kg-1) and excellent cyclic stability (>95% of initial capacitance was remained after 10000 cycles). This work opens new avenues for promoting the high-power asymmetrical supercapacitors.

KEYWORDS: Manganese dioxide; Nanocomposite; Charge-carriers scattering; Asymmetrical supercapacitors; Power density.


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INTRODUCTION Research on high-performance energy-storing material materials has been a hot area with the increasing power and energy demands for modern lifestyle.1-5 Supercapacitors, also referred as electrochemical capacitors (ECs), offer high specific energy, good stability and low level of maintenance.1, 5-7 Additionally, ECs play an important role in electric vehicles and other types electric devices based on clean and renewable energy resources.8 Power density, as one of the most important performance index, should be concerned during the research and development of supercapacitor. Without high power density, supercapacitor will be a middlebrow energy-storage device because of the ultrahigh energy density of batteries, which means the supercapacitor will lose its superiority.9,10 Unfortunately, researchers are keeping a watchful eye on the specific energy and neglecting the specific power and even sacrifice the specific power of supercapacitors to enhancing the energy density, leading to a neither fish nor fowl energy device. Thus, one thing which should be recognized clearly is that the power density is of equal importance with the energy density in developing high performance supercapacitors.11,12 Two kinds of supercapacitors, symmetrical supercapacitor and asymmetrical supercapacitor, have been widely explored by scientific researchers.13,14 For symmetrical supercapacitors, same active materials are applied in cathode and anode such as carbonaceous materials with high electronic conductivity. Carbonaceous materials store energy from reversible ions adsorption onto their surfaces, leading to high specific power because of their high electronic conductivity but low specific energy ascribing to the low specific capacitance of carbon and narrow working potential windows of symmetrical supercapacitor (< 1.0 V in aqueous electrolyte).15,16 By contrast, transition-metal oxides are often used as anode materials in asymmetrical supercapacitor and exhibit high electrochemical performance because they exhibit alterable oxidation states during effective pseudo redox reactions. However, the specific power is a wide gap for asymmetrical supercapacitor because of their low electronic conductivity of the electrodes.17 Additionally, it is hard to match the electrochemical property of cathode and anode not only because of their compatibility of



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capacitance but also their compatibility of rate capability, cyclic stability, etc. In general, energy density should be obtained without sacrificing the power density. Thus, the key point of high performance supercapacitors lies on: 1) designing asymmetrical supercapacitors; 2) enhancing the electroconductibility of active materials to achieve ultrahigh specific power; 3) boosting the capacitance of cathode and anode to increase the energy density; 4) matching the electrochemical property of cathode and anode to elevate the whole properties of asymmetrical supercapacitors. For cathode materials, carbonaceous materials are tremendously popular for asymmetrical supercapacitors because of their high capacitance and wonderful rate capability compared to other cathode materials, which can be ascribed to the ultrahigh specific surface area, favorable electroconductibility and their energy storage mechanism.18,19 However, the electrochemical properties of asymmetrical supercapacitors are often following the cask effect, which means anode materials and cathode materials should be brought to the forefront at the same time. Manganese dioxide (MnO2), as the most promising anode materials used in asymmetrical supercapacitors because of its ultrahigh theoretical capacitance, wide working potential windows (-0.2 V– 1.2 V) and low cost, has attracted a lot of attention in these years. However, only 20%-30% of its theoretical capacitance can be obtained in almost all of the previous works as well as the poor rate capability ascribing to the poor electroconductibility (1*10-6 s m-1) and severe agglomeration of MnO2 nanoparticles. For this case, a system, which is the using of pseudocapacitive material– conductive matrix hybrid nanostructures, was designed.2,20 The electrochemical performance of MnO2 electrode was improved a lot by this way but still far away from its theoretical capacitance. Meanwhile, some researchers have also tried to fabricating hybrid materials, for example, mixing MnO2 with other metal oxides.21 However, some properties of this kind of electrode materials have been largely unsatisfactory (e.g. poor rate capability, narrowed potential window or poor cycling stability) due to the defective experimental design.22 Specifically, this design are often stalled by the higher charge carriers scattering ascribed to the defects between this two metal oxides or plenty of crystal boundary. Thus, eyes should be focused on the basic properties of MnO2 including structure and the energy storage


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mechanism. Smart MnO2-based electrode should be designed as conductive materials act as substrate to support active ultrathin or ultrafine MnO2 with high degree of crystallinity and low defects in the whole materials. Additionally, the active MnO2 should be well arranged from their size to shape rather than distributed on the surface of substrate randomly as the arrangement of them will influence the pore structure, transmission path of electrolyte ions and charge transfer route significantly.23,24 Here, we present a simple route to fabricate a unique morphology metal oxide structure for electrochemical capacitors. The as-prepared composites were consisted of β-MnO2 as substrate and α MnO2 needles stand on the surface of β-MnO2 networks. These structures process large specific surface area, high electronic conductivity and affluent active sites for electrochemical reactions due to the unique morphology with low crystal defects and low charge carriers scattering. An asymmetric supercapacitor assembled by using this novel structure as anode and reduced graphene oxides (RGO) as cathode achieves a high specific energy of 29.8 Wh kg-1 and a maximum specific power of 64.0 kW kg-1 due to the rational matching of anode and cathode. Remarkably, this as-prepared supercapacitors also exhibit excellent rate abilities and outstanding cycling abilities, which will meet the high power and perdurable requirement.

RESULTS AND DISCUSSION Characterization of Structure and Composition The growth mechanism of α-MnO2/β-MnO2 network involves two steps (Figure 1a): The formation of MnO6 octahedron nuclei and the growth of highly-ordered and ultrafine α-MnO2 needle during the hydrothermal. After the MnOOH networks were dispersed into KMnO4 solution, a spontaneous reaction between MnOOH and KMnO4 took place because of the unstable chemical property of MnOOH and high oxidability of KMnO4 in acidic solution forming MnO2 nuclei on the surfaces of MnOOH networks and the surface area of MnOOH was oxidized to β-MnO2 at the same time. It should emphasize that the distribution of MnO2 nuclei influenced by the component of the solution is



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the key point for obtaining highly-ordered α-MnO2. The KMnO4 solution with high acidity lead to a fast redox reaction. Solitary MnO2 nuclei sprout from the surface of MnOOH networks and then α-MnO2 needles grow up in the acidic solution while the MnOOH turn to β-MnO2 in the final product. As the quality of matrix materials influenced the uniformity of final product directly, the MnOOH network was well-designed. The structural information and crystalline phase purity of MnOOH networks were examined by FIB/SEM and X-ray powder diffraction. As exhibited in Figure 1b, a uniform net makes up of nanorods with the diameter range from 300 nm to 400 nm. The nanorods are very smooth without any other component. Meanwhile, all the peaks are belong to the typical MnOOH (JCPDS 411379, Figure 1c) and no impure peaks can be found demonstrating the ultrahigh purity of MnOOH networks which means the MnOOH networks could provide a homogeneous nucleation site for other phase of manganese oxides.25,26 With these high quality MnOOH networks, uniform and high ordered α-MnO2 can be loaded on it via secondary hydrothermal reactions. After reacting with KMnO4 under the acidic condition, as displayed in Figure 1d, no diffraction can be found expect for α-MnO2 (JCPDS no. 44-0141) and β-MnO2 (81-2261),27,28 indicating the products are the physical mixture of β-MnO2 and α-MnO2 or α-MnO2/β-MnO2 nanostructures. Additionally, the crystalline degree of β-MnO2 and α-MnO2 are pretty high which will signally reduce the defects in the nanostructure because of their affinal constitutional units. As we known, the MnOOH is metastable and easily transform to β-MnO2. While, αMnO2 is often obtained when treated KMnO4 solution with heating under acidic condition. Thus, the final products can be confirmed to be β-MnO2 derived from MnOOH and α-MnO2 originated from KMnO4. Note that, the perks around 28.6o and 37.3o are aberrant strong, which can be ascribed to the simultaneous diffraction of α-MnO2 and βMnO2 at these two angles. To confirm the combination of α-MnO2 and β-MnO2, the structural information and morphologies of the asprepared networks are investigated by FIB/SEM. After treating with KMnO4 in high acidity condition, a large amount of ultrafine nanowires, which can be indexed to α-MnO2 nanowires, are standing on the surfaces of the networks,


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which are almost sprout out of the surfaces of β-MnO2 (Figure S1). The slim α-MnO2 nanowires with sharp tip-endings are well apart from each other and homogeneously aligned, forming a unique nanoarray with interlaced fashion forming a sweater structure (Figure 2a and 2b). The nanowires are abundant to conceal the surface of the substrate. Thus, the morphology of novel nanostructures is affirmed to be α-MnO2 nanowires supported by β- MnO2 networks instead of physical mixture of β-MnO2 and α-MnO2. As we can see from Figure 2c, the α-MnO2 nanowires are arranged in rows with similar degree of inclination, which can be attributed to the non-cylindrical section of β-MnO2 nanorods. Furthermore, the α-MnO2 nanowires range from several nanometer to dozens of nanometers with smooth surfaces. Thus, the independent nanowires standing on the surfaces of β-MnO2 nanorods are expected to enhance the electric conductivity of the composite structures due to 1D and 2D conductive channels. For comparison, pure αMnO2 (Figure S2a) and α-MnO2/β-MnO2 nanorods (Figure S2b) was synthesized at the same conditions. The details synthetic route was provide in Supplementary Information. As displayed in Figure S2, the α-MnO2 in these two sample keep its needle-like morphology and the length and diameter are almost the same. The pure α-MnO2 are aggregate together forming an urchin-like structure. While, the α-MnO2 are standing on the surfaces β-MnO2 in α-MnO2/β-MnO2 nanorods with the substrate distribute randomly. As shown in Figure 2d, the α-MnO2 nanowires with the average diameters of 22 nm are not standing up straight on β-MnO2 networks matching well with the SEM images shown in Figure 2c. The ultrathin nanowires provide large surfaces for the contact with electrolyte, which ensures outstanding electrochemical property. The lattice planes were marked for β-MnO2 in Figure 2e. The crystalline interplanar spacings of 0.31 and 0.29 nm were corresponded to the (110) and (001) planes of β-MnO2. However, the electron beam is not perpendicular to a single crystal face of α-MnO2 corresponding to selected area electron diffraction (SAED). The interplanar spacings of 0.70 and 0.247 nm were corresponded to the (110) and (400) planes of α-MnO2 (Figure 2f). Thus, a distinct growth mechanism has been put forward. In detail, at the initial stage, when the KMnO4 met with MnOOH nanorods, the MnOOH nanorods was



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oxidized to β-MnO2 with several MnO2 nucleus restored from KMnO4 adhere to the surface. After transferred into hydrothermal reactor, the MnO2 nucleus began to grow and formed α-MnO2 nanowires finally. After the completion of the reaction, the α-MnO2 nanowires are stand on the surfaces of the β-MnO2 network. Thus, this novel structure are deemed to give a satisfactory specific area and rational pore structure (Figure 2g and 2h). In detail, the isotherms exhibits a type III according to the IUPAC-classification.29 An favourable surface area of 46 m2 g-1 was obtained for the structure, which can be ascribed to the unique morphology and independent distribution of α-MnO2. These result demonstrate this structure will provide an abundant active site for the electrochemical reactions. Additionally, the Mn 2p spectrum of the sample was displayed in Figure 2i. The binding energy of Mn 2p3/2 are located at 642.4 eV and Mn 2p1/2 located at 654.1 eV, with an energy difference of 11.7 eV. These values dovetail well with the literature report.30 These results indicate the successful growing of α-MnO2 with high electrochemical active area, high crystallinity and well arrangement on the β-MnO2, which ensure an excellent electrochemical performance. Electrochemical properties and performances Compare to pure α-MnO2 and α-MnO2/β-MnO2 nanorods, the advantages of the networks structures is distinct. As illuminated in Figure 3a, as the electrochemical active materials, α-MnO2 react with the Na+ ions during the electrochemical testing process, which can be classify as the pseudoactive actions. Specifically, the MnO6 octahedron in the surface and 2×2 channel adsorb Na+ ions during the discharging process and negative charges originated from this electrochemical reaction. The charges migrate form the surfaces to the bulk of α-MnO2 and then collected by the current collector with the assistance of conductive agent (super P). For pure urchin-like α-MnO2, the only transmission route is the needles which will be influenced by the inherent electronic conductivity, length and diameter of α-MnO2 needle. However, for the other two composites, the charges can be transferred via the β-MnO2, which will enhance the electroconductivity of the sample as the β-MnO2 possess the highest electronic conductivity among manganese oxides. The β-MnO2 networks perform better than single β-MnO2 in charge transferring as they gives more


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transmission route. Additionally, the α-MnO2/β-MnO2 networks can reduce the charge carriers scattering dramatically compare to pure MnO2, α-MnO2/β-MnO2 nanorods and other metal oxide-metal oxide structure. As we known, the charge transfer route in electrode is pretty complex including the plenty contact interfaces between active materials, super P, electrolyte, current collector. These contact interfaces will give a very high resistance which be in direct proportion to the contact interfaces for the charges. However, the main resistance comes from the active materials which is inherent and unbridgeable. For the directional movement of charges in crystal, the electron mobility (μe) can be unscrambled as the follow Equation: μe =τe/m*

(1)

where m* is the electronic mass, and τ is relaxation time. The m* is always determined by the crystal lattice, while τ is always influence by the charge scattering. As we known, the mobility of electrons in crystal will be affected by phonon, impurity and defects' scattered. A higher electron mobility requires a low temperature and a low crystal defect as these two factor is the key points influencing the value of τ. In our work, the advantage in electron mobility of the nanocomposites can be elucidated as: 1) both the α-MnO2 and β-MnO2 is high crystalline, which ensures the structure constituent of them is uniform and ordered ensuring a low crystal boundary content inside of them. The high crystalline will obviously decrease the charge carriers scatting. 2) These two components (alpha-MnO2 and beta-MnO2) is sib as the basic element of these two structures is MnO6 octahedron. A lots of MnO6 octahedron will be shared in their structure which means the electron can step over the interface easily which will also increasing the electron mobility. 3) The α-MnO2/β-MnO2 structure is widespread which will provide a shortcut for charges to travel inside of the structure instead of stepping over plenty of phase interface. Obviously, these advantages ensures the high electronic conductivity of the novel structure. Thus, the α-MnO2/β-MnO2 networks will obviously give an ideal electrochemical properties. The capacitive properties of the novel structures was evaluated in 1 M Na2SO4 solution shown in Figure 3b. The α-MnO2/β-MnO2



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networks exhibit a more rectangular shape compare to other two samples, indicating its higher specific capacitance.31 The area of these three samples reflecting the capacitance at the same scan rate are roughly the same which can be ascribed to the different α-MnO2 content (100% for pure α-MnO2, 77.4% for nanorods and 78.6% for networks). The CV curves of as-prepared nanocomposites were shown in Figure 3c. The α-MnO2/β-MnO2 networks exhibit rectangular and symmetric CV even if at a high scan rate (200 mV s-1), indicating the good pseudo-capacitive nature of the nanocomposites ascribing to the ultrafine electrochemical active nanowire and the novel morphology providing an independent surroundings for each α-MnO2 nanowire and improved electroconductivity resulting in the enhanced charge transplant, for which charges originated from ultrafast farad electrochemical reaction between active materials and electrolyte31,32. To further evaluate the electrochemical properties, the galvanostatic charge-discharge (GCD) tests of the αMnO2/β-MnO2 networks at a series of specific currents were displayed in Figure 3d. The one-to-one correspondence between the charge curves and their discharge counterparts indicate ultrafast and highly reversible Faradic reaction between MnO2 and electrolyte.33 The symmetrical triangular shapes with linear line structure suggested a faster charge-discharge process, being agree with the cyclic voltammetry tests. This can be cause for the sufficient Faradic reaction between α-MnO2 and alkali cations as the novel structures providing ultrafine nanowires with rational pore structures. As we all know it is hard for bulky MnO2 to devote all its capacitance for the rejected access of alkali cations to the interior of the bulk, because of the poor electroconductivity of semiconductor. For a given scan rate or charge-discharge current, the penetrating thickness of alkali cations should be fixed. Thus, it is unfavorable for brawny nanowires to react with electrolyte adequately, resulting in a poor capacitance.34 Therefore, the α-MnO2 should be fine for the fully access of alkali cations but with sufficient pseudo capacitance supporter, which call for a reasonable pore structures and rational arrangement of electrochemical active unit. A high specific capacitance of 278.2 F g-1 for the networks composites electrode was obtained (current density: 0.25 A g-1), with a columbic efficiency of almost 100%


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(Figure 3e). The favorable capacitance can certainly be ascribed to the novel morphology with benign electronic conductivity of the structures. High-current charge−discharge curves are displayed in Figure S3, and the low IR drops revealed low internal resistance even applied high current. This also implies the preeminent conductivity of the structures which can be ascribed to the high crystalline degree and low charge carriers scatting of the nanostructure. Note that, about 41.1% of the initial capacitance was remained for α-MnO2/β-MnO2 networks indicating the excellent rate capability of α-MnO2/β-MnO2 networks. The satisfactory rate performance can be attributed to the high electroconductivity and ultrafast electrochemical reaction between α-MnO2 and the electrolyte owning to the unimpeded surroundings. The electrochemical properties of pure α-MnO2 and α-MnO2/β-MnO2 composites is displayed in Figure S4. The α-MnO2/β-MnO2 nanorods perform better than pure MnO2 both in capacitance and rate capability but worse than α-MnO2/β-MnO2 networks highlighting the electrochemical reaction and charge transfer mechanism of these three structures displayed in Figure 3a. To estimate the fundamentally electrochemical behavior of these three electrode, The EIS plot is exhibited in Figure 3f. All the plots reveals a near-semicircle part and a linear part. To analyze these impedance spectra, an equivalent circuit diagram was presented (inset of Figure 3f). Note that the large semicircle corresponds to high interfacial charge-transfer resistance, ascribing to low electroconductivity of active materials; while the better verticality reflected an ideal capacitive behavior.35 The electrode of α-MnO2/β-MnO2 networks gave a relatively small ohmic resistance (1.06 Ω) and interfacial charge-transfer resistance (2.8 Ω). These findings demonstrate the high electronic conductivity of the α-MnO2/β-MnO2 networks, which lead to the outstanding electrochemical properties and promising their use in high performance asymmetrical supercapacitors. Asymmetric supercapacitor assembled by as-prepared nanocomposite and reduced graphene oxides (RGO) are displayed in Figure 4a. These two electrodes were settled in a steel cell with metallic conductor and steel shell as the current transferring medium. The electrochemical performance of negative electrode can be found in Figure S5. As we can see, both the capacitance and rate capability of negative electrode (RGO) are parallel to that of positive



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electrode, indicating a good match between α-MnO2/β-MnO2 networks and the reduced graphene oxides (RGO). The mass ratio of the cathode (RGO) to the anode (α-MnO2/β-MnO2) was adjusted based on the charge balance (Figure S6).10,36 Figure 4b exhibited cyclic voltammetry curves of the device with different working voltages. The box-like CV curves indicated the favorable electrochemical behavior of the cell. Moreover, the rectangular CV shape was well maintained even at high scan rate (Figure 4c), demonstrating a desirable rate capability of this supercapacitors attributing to the excellent rate capability of anode and cathode.37,38 The galvanostatic charge–discharge curves of the cell are displayed in Figure 4d. The well symmetrical properties of charge and discharge curves indicate the satisfactory configuration between the anode and cathode. In detail, the anode store energy by faradaic pseudocapacitive reactions, while cathode store energy by electric double layer adsorption/desorption. From the time span of the discharge curve, the gravimetric capacitance (Ct) of the device is measured to be 53.6 F g-1 for α-MnO2/β-MnO2//RGO supercapacitor (specific current: 0.25 A g-1) based on the whole mass of active materials (α-MnO2/β- MnO2+RGO). Meanwhile, these capacitors also possess good rate capability because of the rational mass matching of active materials in both electrode, the ultrafast electrochemical reactions in two electrode and high ionic conductivity of the applied electrolyte (Figure 4e). The excellent rate capability implies a high specific power for the supercapacitor. Simultaneously, the wide potential working window (2.0 V) and high capacitance in aqueous electrolyte (1 M Na2SO4) afforded the supercapacitor high specific energy. The Ragone plot of the device at various specific current was exhibited in Figure 4f. The α-MnO2/β-MnO2//RGO delivered high specific energy of 29.8 Wh kg-1 at a specific power of 0.25 kW kg-1. When the specific power was at 64.0 kW kg-1, a satisfactory specific energy of 1.81 Wh kg-1 was obtained. The observed electrochemical properties are attributed to the employ of aqueous electrolyte which provide ultrahigh ionic conductivities and high-efficiency electrochemical reactions occurred in these two asymmetric electrodes. The cathode of RGO collected charges via EDLC which offered very fast electronic transfer, while the novel structure of the nanocomposite shortens the electrolyte diffusion length and


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affords abundant electrochemical active sites for Faradic reactions. Thus, a red LED light was lighted by this assembled devices shown in insert of Figure 4f. The cycling stability of this capacitor was also measured against the specific current. The device expresses outstanding cycling stability with about 95.3% of its capacitance retained (Figure 4g). These can be ascribed to the strong combination between α-MnO2 and β-MnO2 in positive electrode as well as the highly stable electrochemical properties of RGO. Specifically, RGO stores energy by ultrafast surface adsorption and the positive electrode stores energy by Faradic reactions between alpha-MnO2 and electrolyte which is also an ultrafast process. And no transition involved in these two electrode during the testing process.39 The high power density of the supercapacitors will meet the demands of practical applications.

CONCLUSIONS Summarily, novel nanocomposite was fabricated by using β-MnO2 networks as the substrate to support ultrafine active MnO2 needles with lower charge carriers scattering ensuring its high electronic conductivity. The ultrafine α-MnO2 guarantee an ultrafast and high reversible Faradic reaction with electrolyte. While, αMnO2/β-MnO2 exhibits a capacitance of 278.2 F g-1 with an favorite rate capability due to the 1D and 2D conductive channels. Asymmetrical supercapacitors assembled by using RGO as the anode and the fabricated nanocomposite structures as the cathode deliver promising electrochemical properties. An specific energy of 29.8 Wh kg-1 with a maximum specific power of 64.0 kW kg-1 was obtained for α-MnO2/β-MnO2 network//RGO capacitor. Remarkably, the as-prepared supercapacitor also exhibits excellent rate abilities and outstanding cycling abilities (>95% of initial capacitance was remained after 10000 cycles). Importantly, due to the novel structures, these nanocomposites are supposed to find other potential applications like battery, fuel cell, catalysis, etc.



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ASSOCIATED CONTENT The Supporting Information is available free of charge on the: Experimental sections, material characterization, electrochemical testing, SEM image (Figure S1 and S2), Galvanostatic charge-discharge curves (Figure S3), CV Curves, Galvanostatic charge-discharge curves and rate capability (Figure S4 and S5), CV curves (Figure S6)

CONFLICTS OF INTEREST All authors have no conflict of interest to declare.

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No.21576034), the Innovative Research Team of Chongqing (CXTDG201602014), State Education Ministry and Fundamental Research Funds for the Central Universities (106112017CDJQJ138802, 106112017CDJSK04XK11 and 106112016CDJZR135506), Chongqing Special Postdoctoral Science Foundation (XmT2018043) and Technological projects of Chongqing Municipal Education Commission (KJZD-K201800801). The authors would like to thank Electron Microscopy Center of Chongqing University for material characterizations.

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27 Yu, M.; Zhai, T.; Lu, X.; Chen, X.; Xie, S.; Li, W.; Liang, C.; Zhao, W.; Zhang, L.; Tong, Y. Manganese Dioxide Nanorod Arrays on Carbon Fabric for Flexible Solid-State Supercapacitors. J. Power Sources 2013, 239, 64–71. 28 Wu, Q.; Tao, H.; Xie, K.; Liu, N.; Yu, L.; Hu, Z. Modified Redox Synthesis and Electrochemical Properties of Potassium Manganese Oxide Nanowires. J. Mater. Chem. 2011, 21, 17904–17908. 29 Choma, J.; Burakiewicz-Mortka, W.; Jaroniec, M.; Li, Z. Monitoring Changes in Surface and Structural Properties of Porous Carbons Modified by Different Oxidizing Agents. J. Colloid Inter. Sci. 1999, 214, 438–446. 30 Chen, Y. C.; Hsu, Y. K.; Lin, Y. G.; Lin, Y. K.; Horng, Y. Y.; Chen, L. C.; Chen, K. H. Highly Flexible Supercapacitors with Manganese Oxide Nanosheet/Carbon Cloth Electrode. Electrochim. Acta 2011, 56, 7124–7130. 31 Lv, P.; Feng, Y. Y.; Li, Y.; Feng, W. Carbon Fabric-aligned Carbon Nanotube/MnO2/Conducting Polymers Ternary Composite Electrodes with High Utilization and Mass Loading of MnO2 for SuperCapacitors. J. Power Sources 2012, 220, 160–163. 32 Zhi, M.; Xiang, C.; Li, J.; Li, M.; Wu, N. Nanostructured Carbon–metal Oxide Composite Electrodes for Supercapacitors: A Review. Nanoscale 2013, 5, 72–88. 33 Lei, Z.; Zhang, J.; Zhao, X. S. Ultrathin MnO2 Nanofibers Grown on Graphitic Carbon Spheres as HighPerformance Asymmetric Supercapacitor Electrodes. J. Mater. Chem. 2012, 22, 153–160. 34 Zhu, S. J.; Zhang, J.; Ma, J. J.; Zhang, Y. X.; Yao, K. X. Rational Design of Coaxial Mesoporous Birnessite Manganese Dioxide/Amorphous–Carbon Nanotubes Arrays for Advanced Asymmetric Supercapacitors. J. Power Sources, 2015, 278, 555–561. 35 Toupin, M.; Brousse, T.; Bélanger, D. Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor. Chem. Mater. 2004, 16, 3184–3190. 36 Jiang, H.; Li, C.; Sun, T.; Ma, J. A Green and High Energy Density Asymmetric Supercapacitor Based on Ultrathin MnO2 Nanostructures and Functional Mesoporous Carbon Nanotube Electrodes. Nanoscale 2012, 4, 807–812. 37 Wang, F.; Lv, X.; Zhang, L.; Zhang, H.; Zhu, Y.; Hu, Z.; Zhang, Y.; Ji, J.; Jiang W. Construction of Vertically Aligned PPy Nanosheets Networks Anchored on MnCo2O4 Nanobelts for High-Performance Asymmetric Supercapacitor. J. Power Sources, 2018, 393, 169–176. 38 Wang F.; Zhu; Y., Tian W.; Lv X.; Zhang H.; Hu Z.; Zhang Y.; Ji J.; Jiang W. Co-Doped Ni3S2@CNT Arrays Anchored on Graphite Foam with A Hierarchical Conductive Network for High–Performance Supercapacitors and Hydrogen Evolution Electrodes. J. Mater. Chem. A, 2018, 6, 10490–10496 39 Ji J.; Li, Y.; Peng, W.; Zhang, G.; Zhang, F.; Fan, X. Advanced Graphene‐Based Binder–Free Electrodes for High‐Performance Energy Storage. Adv. Mater. 2015, 27, 5264–5279.


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Figures

Figure 1. (a) Schematic representation of fabricating α-MnO2/β-MnO2 network; (b) SEM image of MnOOH networks; (c) XRD pattern of MnOOH networks; (d) XRD pattern of α-MnO2/β-MnO2.



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Figure 2. (a-c) SEM images of α-MnO2/β-MnO2 networks (d-f) TEM images of α-MnO2/β-MnO2 networks; (g) Nitrogen adsorption and desorption isotherms of α-MnO2/β-MnO2 networks; (h) pore-size distribution curves of α-MnO2/β-MnO2 networks; (i) XPS spectra (Mn 2p) of α-MnO2/β-MnO2 networks.


alpha-MnO2/beta-MnO2 alpha-MnO2/beta-MnO2 network

2 0 -2 -4 Scan rate: 20 mV s-1

-8

0.0

0.2

0.4

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10 0 -10

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(e)

350

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99.5%

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alpha-MnO2/beta-MnO2 network

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alpha-MnO2/beta-MnO2

alpha-MnO2 needle

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150 120 90

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30

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0.6

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-20

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(d)

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20

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(c)

50 mV s-1 100 mV s-1 200 mV s-1

Z''(ohm)

-6

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Z''(ohm)

4

40

Potential (V)

(b)

alpha-MnO2 needle

6

Coulombic efficiency(%)

Current density (A g-1)

8

Capacitance (F g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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0

50


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10

15

Z'(ohm) 150

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250

Z'(ohm)

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450

Figure 3. The electrochemical performance of the three kinds of α-MnO2-based nanocomposite in 1 M Na2SO4 solution: (a) electrochemical reaction mechanism of pure alpha-MnO2, α-MnO2/β-MnO2 nanorods and α-MnO2/βMnO2 networks; (b) CV curves of pure alpha-MnO2, α-MnO2/β-MnO2 nanorods and α-MnO2/β-MnO2 networks; (c) CV curves of α-MnO2/β-MnO2 networks at different scan rate; (d) Galvanostatic charge–discharge curves of α-MnO2/β-MnO2 networks; (e) Capacitance retention ratio as a function of the potential scan rate the current densities. (f) Nyquist plots with insets showing the high-frequency parts and equivalent circuit.


3

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(b)

Alpha-MnO2@beta-MnO2 network//AGO

2 1 0 -1 -2 -3

0-1.2 V 0-1.4 V

0-1.0 V

0.0

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1.0

0-1.6 V

0-1.8 V

0-2. V

1.5

2.0

Potential (V) 8

5 mV s-1 10 mV s-1 20 mV s-1

6

2.0

(c)

50 mV s-1 100 mV s-1 200 mV s-1

Potential (V)


10

4 2 0 -2 -4 -6 -8

(d)

1.5

0.25 A g-1 0.5 A g-1 1.0 A g-1 2.0 A g-1 4.0 A g-1

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Capacitance (F g-1)

(e)

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(g)

Alpha-MnO2@beta-MnO2 network//AGO

Capacitance (F g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



60

0.25 A g

50

-1

0.25 A g 0.5 A g-1

95.3%

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2.0 A g-1

1.0 A g-1

2.0 A g-1

30

-1

4.0 A g-1

20 0

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Cycle number

Figure 4. Asymmetrical supercapacitors made with the smart structures and reduced graphene oxide (RGO). (a) Schematic illustration of the fabrications of α-MnO2/β-MnO2//RGO capacitor; (b) CV curves recorded at different cell voltages (scan rate: 50 mV s-1); (c) CV curves recorded at different scan rates with a maximum cell voltage of 2.0 V; (d) galvanostatic charge-discharge curves at different current densities; (e) variations of the capacitance and Columbic efficiency with the current densities; (f) Ragone plots of the supercapacitors; (g) variations of the capacitance with cycle number at different current densities of the asymmetric supercapacitors.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Energy density (Wh kg-1)

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1000 Alpha-MnO2@beta-MnO2 network//AGO

100 10 1 0.1 0.01 0.1

1

10

Power density (kW kg-1)


100

Î²-MnO2

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