Facile Synthesis of Three Dimensional Sandwiched MnO2@GCs

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Facile Synthesis of Three Dimensional Sandwiched MnO2@GCs@MnO2 Hybrid Nanostructured Electrode for Electrochemical Capacitors Xian Jian, Shiyu Liu, Yuqi Gao, Wanli Zhang, Weidong He, Asif Mahmood, Chandrasekar M Subramaniyam, Xiaolin Wang, Nasir Mahmood, and Shixue Dou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 19, 2017

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ACS Applied Materials & Interfaces

Facile Synthesis of Three Dimensional Sandwiched MnO2@GCs@MnO2 Hybrid Nanostructured Electrode for Electrochemical Capacitors Xian Jiana,b,ξ, Shiyu Liua,ξ, Yuqi Gaoa,ξ, Wanli Zhang a,*, Weidong Hea,*, Asif Mahmoodc,d, Chandrasekar M Subramaniyamb, Xiaolin Wangb, Nasir Mahmoodb,e* and Shi Xue Doub a

Center of Micro-Nano Functional Materials and Devices, School of Energy Science and

Engineering, State key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 611731, China b

Institute for Superconducting and Electronic Materials, Australian Institute for Innovative

Materials, University of Wollongong, North Wollongong, 2500, Australia c

Department of Materials Science and Engineering, College of Engineering, Peking University,

Beijing, 178001, China d

e

Department of Physics, South University of Sciences and Technology, Shenzhen, P.R. China

Key Laboratory for Green Chemical Technology of the Ministry of Education, School of

Chemical Engineering and Technology, Collaborative Innovative Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China

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KEYWORDS: graphite-like capsules; MnO2; catalytic chemical vapour deposition; microwave synthesis; electrochemical capacitors

ABSTRACT: Designable control over morphology and structure of active materials is highlydesirable for achieving high-performance devices. Here, we develop a facile microwave assisted synthesis to decorate MnO2 nanocrystals on three dimensional (3D) graphite-like capsules (GCs) for obtaining sandwich nanostructures (3D MnO2@GCs@MnO2) as electrode materials for electrochemical capacitors (ECs). A templated growth of 3D GCs is carried out via catalytic chemical vapour deposition (CCVD) and MnO2 is decorated on exterior and interior surfaces of GC walls through microwave irradiation to build engineered architecture with robust structural and morphological stability. The unique sandwiched architecture owns large interfacial surface area, and allows for rapid electrolyte diffusion through hollow/open framework and fast electronic motion via carbon backbone. Furthermore, tough and rigid nature of GCs provide necessary structural stability, while strong synergy among MnO2 and GCs leads to high electrochemical activity in both neutral (265.1 F/g at 0.5 A/g) and alkaline (390 F/g at 0.5 A/g) electrolytes. The developed hybrid exhibits stable capacitance up to 6000 cycles in 1 M Na2SO4. The hybrid is a potential candidate for future ECs and the present study opens up an effective avenue to design hybrid materials for various applications.

1. INTRODUCTION The energy storage devices are of great importance in developing alternative, low cost and environmentally benign clean energy to address the environmental issues and depletion of fossil fuels. Electrochemical capacitors (ECs) are alternative attractive power source differing from traditional capacitors and batteries. The great advantages of ECs include maintenance-free, high 2 ACS Paragon Plus Environment

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power, long cycle-life, no memory effect, generally safer, etc., favour their promising applications in the field of wind power generation, heavy machinery, rail transit, etc. For instance, ECs can be coupled with other energy storage devices such as fuel cells or batteries to deliver high power needed during acceleration and to recover the energy during braking.1 The performance of ECs is mainly based on their electrodes efficiency and activity, therefore, electrode materials with well-engineered structure allowing efficient utilization of materials and compositions for effective redox process are highly demanded.2, 3 To date, various materials with different dimensions and morphologies have been explored to improve the efficiencies of the ECs.4-6 Recently, carbon-based materials (e.g. graphene, amorphous carbon, porous carbon, carbon nanotubes (CNTs) etc.) and non-precious metal-based materials e.g. metal oxides, hydroxides and sulphides got tremendous attention.7-11 Among all these materials, MnO2 stands out as an ECs electrode due to its multivalence states and high theoretical specific capacitance (~1370 F/ g), environment benignity and low cost.12-15 However, large scale applications of MnO2 based electrode materials are greatly hindered by its inherent lower conductivity (105–106 S/cm) and limited activity.16 Most of the reported MnO2-based electrode materials are dense/bulk

and lack of effective charge transmission channels, easy to aggregate which limit the potential high performance and their applications at large scale thereafter. Thus, it is urgent to develop new hybrid nanostructures to address these challenges. At present, most of the nanomaterials do not meet necessary requirements for improved electrochemical response due to inherent limitations e.g. limited redox sites, slower kinetics and poor stability.17 Therefore, it is assumed that the hybrid/heterogeneous structures can overcome the limitation of individual materials by tailoring each component of hybrid structure to address the associated issues and attain higher materials characteristics for improved electrochemical 3 ACS Paragon Plus Environment


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activities.18-20 The hybrid materials display unique features e.g. faster recombination rate, longer stability and higher carrier mobility.21-23 However, the synergism among two components of hybrid materials mainly depend on (i) selection of appropriate components, (ii) designing and engineering of structures to utilize maximum material with better stability and (iii) the low cost and scalable synthesis method that can successfully tailor the structure, morphology and composition as per requirement of the system.24 Thus, the hybrid materials that can overcome the limitation of their counterpart and structure that allows faster ionic and electronic movements as well as provide high electrode/electrolyte interface for improved electrochemical performance are highly desirable. These concerns associated with MnO2 can be addressed by its hybrid structures with highly conductive substrates that can provide the quick access to the redox sites through open framework by improving the electrolyte penetration and ionic movements while the electronic conductivity can be boosted up via carbon backbone.25-27 Here, we have developed a facile method to design a unique 3D hybrid nanostructure of MnO2@GCs@MnO2 as an electrode for ECs by employing catalytic chemical vapor deposition (CCVD) to grow graphite-like capsules (GCs) in conjunction with microwave assisted growth of MnO2. The 3D nanostructures of MnO2@GCs@MnO2 hybrid possess high effective surface for enhanced electrode/electrolyte interface. In addition, MnO2 nanocrystals are decorated both inner- and outer sides of GCs walls which bring extra hybridized area to facilitate faster ionic and electronic transport during charging/discharging processes, prevent the aggregation of MnO2 nanocrystals and tailor the distance between each MnO2 nanocrystal on densely stacked graphene, which opens up more effective way for ions and electrons to access the redox sites. Hence, selectively tailored 3D design of the hybrid allows full use of available active surface to enhance the electrochemical performance and results in high capacitance (265.1 F/g at 0.5 A/g) and 4 ACS Paragon Plus Environment

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outstanding cycling stability up to 6000 cycles at 2 A/g in 1 M Na2SO4. We believe that 3D MnO2@GCs@MnO2 is an attractive candidate for future ECs and our developed strategy is an effective way to introduce unique electrode materials with better electrochemical performances. 2. EXPERIMENTAL SECTION Synthesis of GCs: The zinc oxide (ZnO) nanocrystals were used as template to develop GCs through CCVD. Initially, ZnO nanocrystals were placed in a tubular furnace at room temperature and heated up to 700 °C at the heating rate of 5 °C/min under N2. Afterwards, ethylene (C2H2) was introduced with the flow of 80 mL/min while the temperature was kept at 700 °C for 30 min. Then again N2 was introduced to cool down the product under neutral atmosphere. At last, the as-prepared core-shell of ZnO@GCs were dispersed in adequate amount of HNO3 to obtain the hollow GC structures by leaching out the ZnO nanocrystals. The final product was washed three times with distilled water to neutralize it and dried at 60 °C for 5 h. Synthesis of 3D MnO2@GCs@MnO2 Hybrid Nanostructures: To prepare the hybrid nanostructures, primarily 2.1 g of KMnO4 powder was dissolved in 133 mL distilled water and 100 mL suspension of as-synthesized GCs (120 mg) were added into the KMnO4 solution. Subsequently, the suspension was processed by ultrasonic vibration for 1 h. Then, the purple suspension was put into microwave oven to mount the structure for 5 min at the power energy of 750 W. Afterwards, the reaction system was cooled down to room temperature. The black sediment was filtered out from the parent solution through centrifugation and repeatedly washed 5 times with distilled water and alcohol, respectively. The final product was dried at 100 °C for 12 h to collect the hybrid nanostructure. Furthermore, to achieve the better synergism and



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electrochemical results the mole ratios of MnO2 and GCs are controlled by adjusting the initial reactant concentrations (KMnO4: GCs =3:4, 4:4 and 6:4 by molar ratio). Physical Characterizations: The morphological and structural features of pure GCs and hybrid were characterized by using JSM-7600F field emission scanning electron microscopy (FESEM; JEOL, Ltd, Tokyo, Japan) and a JEM-2100F transmission electron microscopy (TEM; JEOL, Ltd, Tokyo, Japan) equipped with operating voltage of 200 kV. The x-ray diffraction (XRD) patterns were recorded in the 2Ѳ range of 15-85° using an x-ray diffractometer (XRD-7000; Shimadzu, equipped with Cu Kα, λ=0.154178 nm). Elemental composition of hybrids was analyzed by xray photoelectron spectroscopy (XPS), Kartos Axis Ultra with monochromatized Al Kα radiation (1486.6 eV) and scanning transmission electron microscopy (STEM) analysis was done using JEM-2100F. The BET analysis was done using ASAP 2010. The thermogravimetric analysis (TGA) was done by a SDT Q600 (USA) in air at a heating rate of 5 °C/min from 25 to 800 °C. Raman spectra were recorded using Renishaw 1000 Raman imaging microscope system with an excitation wavelength of 632.8 nm. Electrochemical Measurements: Electrochemical measurements of GCs, MnO2 and hybrid were performed on a LAND CT 2001A analyzer. The working electrode was composed of active material, conductive agent (acetylene black) and polymer binder (polytetrafluoroethylene, PTFE) in a weight ratio of 80:10:10, mixed well and loaded on nickel foam (1×1 cm) with mass loadings of active materials (2-3 mg) to assure the repeatability of the electrochemical results and avoid uncertainties due to small mass loadings. To remove the moisture or solvent contents, the electrode was dried at 60 °C for 12 h under vacuum. The Ag/AgCl and Hg/HgO were used as the reference electrode for 1 M Na2SO4 and 6 M KOH electrolytes, respectively and Pt foil was used 6 ACS Paragon Plus Environment

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as counter electrode. The cyclic voltammetry (CV) analysis were carried out at different scanning rates (10-1000 mV/s) between -0.1 to 0.5 V vs. Ag/AgCl and between -0.2 to 0.3 V vs. Hg/HgO on CHI 760C (Shanghai Chenhua). The electrochemical impedance spectroscopy (EIS) was also done using CHI 760C (Shanghai Chenhua) in the range of 100 kHz to 10 mHz. All the electrochemical characterizations were performed at room temperature. 3. RESULTS AND DISCUSSION The 3D MnO2@GCs@MnO2 hybrid was synthesized by a self-limiting deposition of MnO2 on the exterior and interior surfaces of GCs walls as presented in Figure 1a. The acetylene gas was catalyzed on the surface of ZnO using CCVD process at 700 °C, while high-purity hollow GCs were obtained using acid leaching of ZnO. The growth mechanism of the GCs is based on our newly defined vapor–dissociation–solid (VDS) growth mode, where acetylene molecule first adsorbed on ZnO surface and then dissociate into active carbon atoms, that rearranged themselves into GCs on ZnO surfaces.28 The ZnO particles play key role not only as catalyst to catalyze the carbon precursor but also generate necessary open space by acting as sacrificial template for the growth of MnO2 on the interior and exterior surface of GCs. The as-prepared GCs have highly unique structural features with hollow interior cavity and open framework that makes

them

a

promising

carrier

for

active

materials

(Figure

1a).

Finally

3D

MnO2@GCs@MnO2 hybrid sandwich nanostructures are obtained by microwave assisted reaction between KMnO4 and GCs. The redox reaction was mediated by GCs that serve as a reductant by transferring electron to the oxidant and convert MnO4¯ ions to insoluble MnO2 nanocrystals. By adjusting the initial reactant mass ratios, the mass loading capacity, structure and morphology of MnO2 decorated on GCs is adjusted to obtain the final product such as the sample with mass ratio of KMnO4 : GCs= 15:4 shown in Figure S1. After the redox reaction, 7 ACS Paragon Plus Environment


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GCs retains the original morphology with thinner walls beautifully decorated with MnO2 nanocrystals, which are firmly anchored both inside and outside of GCs walls, resulting in a fascinating sandwich structure of 3D MnO2@GCs@MnO2 (Figure 1a). The resulting product consist of large available surface owing to accessibility of both inner and outer surfaces of the MnO2@GCs@MnO2 hybrids enriched with highly active sites for charge storage; smaller diffusion distances for ions and their faster transport due to 3D framework with hollow structure. This unique 3D sandwich hollow structure will be effective to promote the efficient utilization of all the material in charge storage. The morphology of ZnO@GCs nanostructures is determined by the ZnO particles used as catalyst and template (Figure 1b). The ZnO template is removed through acid leaching that result in well-defined hollow GCs with smooth surface and noticeable large opening (Figure 1c). The opening results from the limited room between ZnO2 particle that hinders the graphene growth and provide channels for mass (aqueous permanganate (MnO4¯)) transport for redox reaction between MnO4¯ and GCs to yield MnO2 nanocrystals on interior side of GCs alongside the exterior surface of GCs (Figure 1d). The opening is further enlarged by depletion of carbon during redox reaction between GCs and MnO4¯ ions. This redox reaction results in densely packed decoration of MnO2 on inner and outer sides of GCs to produce highly dense 3D MnO2@GCs@MnO2 hybrid. The in-depth morphological and microstructural analyses of GC and hybrid products were carried out with the help of TEM and powder XRD as shown in Figure 1e-i). It is quite clear that pure GCs have a smooth surface and uniform walls with a thickness of ~10 nm and an average diameter of ~100 nm (Figure 1e). Compared with the pure GCs, 3D MnO2@GCs@MnO2 hybrid possesses uneven surface because of β-MnO2 nanocrystals dispersing on both inside and outside of GCs (Figure 1f). It is worth noting that GCs retain their 8 ACS Paragon Plus Environment

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morphology during the growth of β-MnO2 nanocrystals which promotes the formation of a sandwich structure of 3D MnO2@GCs@MnO2 hybrid.



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Figure 1. (a) Schematic illustration for the synthesis of sandwiched 3D MnO2@GCs@MnO2 hybrid nanostructures. SEM images of (b) ZnO particles, (c) pure GC and (d) 3D MnO2@GCs@MnO2 hybrid. The TEM images of (e) GCs and (f) 3D MnO2@GCs@MnO2 hybrid. (g) The high magnification TEM image of 3D MnO2@GCs@MnO2 hybrid. The HRTEM images and Fourier transform patterns (h) outside and (i) inside of 3D MnO2@GCs@MnO2 hybrid. (j) The XRD spectra of ZnO@GCs, GCs and 3D MnO2@GCs@MnO2 hybrid. The high magnification TEM image in Figure 1g clearly shows that MnO2 nanocrystals are attached both on the interior and exterior sides of GC walls with strong adhesion that will inhibit their aggregation during electrochemical testing. The high resolution TEM (HRTEM) observations further provide insights into 3D MnO2@GCs@MnO2 hybrid while analysis at edges confirm that MnO2 nanocrystals are attached both on the outside (Figure 1h) and inside (Figure 1i) of GC walls. In addition to the parallel growth with walls, some anisotropic MnO2 growth on vertical mode was also observed as shown in Figure 1h. The size of MnO2 nanocrystals was found in the range of 5-10 nm, while the fast Fourier transform (FFT) analyses demonstrate that these nanocrystals grew in multi-direction (the inset of Figure 1h&i). The XRD patterns of ZnO@GCs, GCs and MnO2@GCs@MnO2 are presented in Figure 1j. The ZnO@GCs bears the intense diffraction peaks, which are well-consistent with standard ZnO (ICDD card PDF# 00-036-1451). However, after the removal of ZnO template two broad peaks at 2θ of 25° and 44° are observed for hollow GCs, attributing to the (0 0 15) and (107) planes of graphitic carbon (ICDD card PDF# 01-074-2329), respectively. Absence of clear diffraction peak for GCs in ZnO@GCs sample could be attributed to their thin layer growth and highly intense diffraction by ZnO. After the redox reaction between GCs and KMnO4, two relatively strong and broad XRD peaks appear at about 37° and 67°can be index to delta-MnO2 (JCPDS 10 ACS Paragon Plus Environment

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42-1317), respectively.29, 30 These structural results confirm the successful loading of MnO2 on GC walls, further existence of broad peaks in the XRD spectra of 3D MnO2@GCs@MnO2 hybrid assures the strong interaction of MnO2 nanocrystals with GCs. The core levels of C, O and Mn in the XPS full spectrum (Figure S2) of the 3D MnO2@GCs@MnO2 hybrid confirm the desired chemical composition. The small amount of Zn (~1.18%) is also detected that might be the ZnC produced at catalyst and GCs interface during acetylene catalysis. The Figure 2a presents the de-convoluted C1s spectrum, where it is obvious that carbon has multiple bonding states with C, O and carboxyl groups as confirmed from the XPS binding energies of 284.8, 285.9 and 288.7 eV, respectively. The existence of bonding between carbon and oxygen is an indication of strong interaction among MnO2 and GCs walls, which brings strong synergy between these two components. Further, the presence of carboxyl group is favorable for better wettability of electrode with electrolyte and facilitates the redox process. Meanwhile, the Mn 2p spectrum shows two peaks centered at 642.3 and 654.2 eV which can be designated to the binding energy of Mn 2p3/2 and Mn 2p1/2, respectively as presented in Figure 2b, indicating Mn4+ ions are dominant in the hybrid product. As the difference of binding energy among satellite peak and 2p1/2 is ~11.28 eV that is larger than the 6 eV for Mn (II) and assures the higher oxidation state of Mn (IV) in hybrid.31 It further confirms the Mn (IV) oxide coating on GCs without any detectable residuals of MnO4¯. Moreover, In addition, the spin energy separation difference of 4.92 in the doublet peaks of Mn 3s also suggest the oxidation state of 4 for Mn in the hybrid (Figure S3).32, 33 the elemental mapping is recorded by energy dispersive x-ray (EDX) analysis of single nanocapsule of 3D MnO2@GCs@MnO2 hybrid using TEM which further confirm the existence of Mn, C and O elements, and homogeneous distribution of nanoscale MnO2 throughout the 3D hollow nanocapsule (Figure 2c-f). Thus, the 11 ACS Paragon Plus Environment


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MnO2 nanocrystals are evenly dispersed in the 3D MnO2@GCs@MnO2 hybrid without any detectable agglomerations.

Figure 2. The high resolution XPS spectra of (a) C1s and (b) Mn2p of 3D MnO2@GCs@MnO2 hybrid. (c) The TEM image of 3D MnO2@GCs@MnO2 hybrid (the inset shows the EDS line scanning spectra where the blue, red and green lines represent the elements of Mn, C and O, respectively). The elemental mapping images of 3D MnO2@GCs@MnO2 hybrid: (d) Mn, (e) C and (f) O. The presence of MnO2 nanocrystals in 3D MnO2@GCs@MnO2 hybrid is also supported by the Raman spectroscopic analysis as shown in Figure 3a. Two diagnostic peaks of GCs appear at 1337 and 1590 cm¯1, corresponding to the breathing mode of k-point phonons of A1g symmetry and first-order scattering of E2g phonons, respectively.34 In comparison with pure GCs, there are 12 ACS Paragon Plus Environment

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several different peaks centered at 302, 428, 656 attributing to MnO2 crystal35, 36 and the peak at 1119 cm¯1 attributing to carbon-metal connection in hybrid.37 Besides, the intensity of the two diagnostic Raman peaks for GCs is decreased because of the coating of MnO2 nanocrystals over GCs and the lower amount of GCs due to the redox reaction with MnO4¯. These results further verified that the MnO2 and GCs have strong chemical interaction that builds a powerful synergy among the components of 3D MnO2@GCs@MnO2 hybrid to enhance its electrochemical performance. Furthermore, TGA analysis under air environment was employed to determine actual content of each component in 3D MnO2@GCs@MnO2 hybrid (Figure 3b). A total weight loss of 16.87% is observed in the product from which 5-7% loss was observed up to 100 °C due to loss of moisture and water contents while the remaining 9.87-12.87% loss is observed up to 500 °C that corresponds to the removal of GCs by thermal oxidation which is well-stand with the experimental value of 9.4%. The surface area and porosity of material largely affect its electrochemical performance, thus Brunauer–Emmett–Teller (BET) and Barrett-Joyner-Halenda (BJH) studies were carried out as shown in Figure 3c and d, respectively. The 3D MnO2@GCs@MnO2 hybrid bears high surface area of 190 m²/g with fine pore size distribution from micro to macro size. The high surface area will bring the larger number of active sites and space for charge storage. While the large pore size distribution will favor the easy wettability of electrode by electrolyte through macropores, access to redox sites through mesopores and micropores will provide the additional space for charge storage.



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Figure 3. (a) Raman spectra of pure GCs and 3D MnO2@GCs@MnO2 hybrid. (b) TGA pattern, (c) N2 adsorption and desorption curves and (d) BJH pore size distribution of 3D MnO2@GCs@MnO2 hybrid. Considering the unique features of as-synthesized 3D MnO2@GCs@MnO2 hybrid, the hybrid was investigated as electrode of ECs. The suitability of the 3D MnO2@GCs@MnO2 hybrid for electrochemical applications is supported by (i) its 3D hollow framework which provide enough space for electrolyte and avoid the aggregation of MnO2 nanocrystals during ongoing charge/discharge process (ii) favorable size of MnO2 nanocrystals which is less than 10 nm, since 14 ACS Paragon Plus Environment

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it is well-known that electrolyte cannot penetrate more than 20 nm in case of crystalline material38 (iii) presence of carbon backbone in sandwiched 3D MnO2@GCs@MnO2 hybrid having good conductivity will provide the highway to electrons and its tough nature will stabilize the structure of overall electrode material resulting in high performance and stable longevity. Given the excellent material features, the developed hybrid was tested in both neutral and alkaline electrolytes. It is generally assumed that electrolyte plays an important role in the electrochemical performance of ECs, thus 1 M Na2SO4 and 6 M KOH were employed as neutral and alkaline electrolytes, respectively, for the electrochemical analysis. Figure 4 shows morphological and electrochemical comparison of various 3D MnO2@GCs@MnO2 hybrids having variable amount of GC and MnO2 as well as pure GCs in 1 M Na2SO4 in the range of -0.1 to 0.5 V vs. Ag/AgCl. It is found that the pure GC exhibiting poor specific capacitance of about 1.41~5.53 F/g at the scan rate of 10~1000 mV/s (Figure 4a), confirming its role as only a conducive buffer and effective substrate to prevent the aggregation of MnO2 nanocrystals. While, the 3D MnO2@GCs@MnO2 hybrid shows a quasi-rectangle shape of CV curves revealing a good capacitive feature even at higher scan rates of 10-100 mV/s (Figure 4c, 4e and 4g), which is consistent with the electrochemical behavior of the Mn based electrode materials in neutral electrolytes. To achieve the better synergism and appropriate mass loadings, a series of experiments were carried out by adjusting the mass ratios of initial reactants (KMnO4 vs. GCs ranged from 5.27 to 17.5) for better electrochemical performance. The microstructure and regularity of as-synthesized 3D MnO2@GCs@MnO2 hybrid is greatly affected by the initial reactant concentrations. At the mass ratio of 17.5, larger MnO2 nanocrystals are formed as represented by the TEM image (Figure 4h) by almost complete utilization of all the GCs. Importantly, at lower mass ratios of 13.13 (Figure 4d), 8.75 (Figure 4f) and 5.27 (Figure S1), 15 ACS Paragon Plus Environment


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GCs are found clearly within the hybrid structure as observed from TEM images. At appropriate mass ratios 13.13 and 8.75, the 3D MnO2@GCs@MnO2 hybrids contain uniformly dispersed MnO2 nanocrystals at inside and outside of GCs walls. But only a few MnO2 nanocrystals can be found when the mass ratio was reduced to 5.27. Comparative analysis of morphologies of various products confirmed that the 3D MnO2@GCs@MnO2 hybrid prepared at mass ratio of 13.13 delineates better dispersion of MnO2 nanocrystals. Similarly, it is also found the ratio of GCs to KMnO4 largely affect the charge storage capability of resulted hybrid as presented in Table S1 and among all these 3D MnO2@GCs@MnO2 hybrid with mass ratio of 13.13 (molar ratio of GC:Mn=4:4) delivers high specific capacitance of 265.1 F/g at 0.5 A/g in 1 M Na2SO4. Besides CV, the galvanostatic charge-discharge analysis was carried out to further analyze the charge storage behavior of MnO2@GCs@MnO2 hybrid in neutral media as shown in Figure S5. Linear charge-discharge profiles were observed using 1 M Na2SO4 which confirms nearly an ideal capacitive behavior and a typical feature of reversible redox reaction of hybrid. The specific capacitances of 265, 254 and 153 at the current densities of 0.5, 1 and 2 A/g, respectively (Figure S5c, which are much higher than the pure GC (Figure S5a), hybrid with other molar ratio (Figure S5b, d) and commercial MnO2 (Figure S6), the values are given in Table S1.


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Figure 4. CV curves of pure GC (a) and 3D MnO2@GCs@MnO2 hybrid with different loading ratio of GC:Mn 6:4 (c), 4:4 (e) and 4:3 (g) at the scan rate of 0.01, 0.05 and 0.1 V/s in the 17 ACS Paragon Plus Environment


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voltage range of -0.1 to 0.5 V vs. Ag/AgCl in 1 M Na2SO4. TEM images of pure GC (b) and 3D MnO2@GCs@MnO2 hybrid with different loading ratio of GC:Mn 6:4 (d), 4:4 (f) and 4:3 (h). In contrast, when tested in 6 M KOH, the 3D MnO2@GCs@MnO2 hybrid shows clear oxidation and reduction peaks (Figure 5). The existence of clear redox peaks in strong alkaline electrolyte could be ascribed to reversible insertion/desertion of anions and variation of Mn valence between Mn(IV) and Mn(III),

39, 40

surface adsorption of the electrolyte cations as well

as the electrochemical conversion of residual ZnC to Zn(OH)2. Minute quantities of Zn species in the form of insoluble ZnC (ZnC is generally formed on the grain boundaries of ZnO catalyst and growing GCs) could be retained in the product during acid leaching. The ZnC is assumed to convert into electrochemically active Zn(OH)2 upon exposure to strong alkaline solution, which is confirmed by the XRD studies of KOH treated GCs as shown in Figure S4. From XRD results it is obvious that Zn species changed to Zn(OH)2 on exposure to aqueous KOH as all the peaks are well-matched with the standard card No. 00-020-1437. Besides, the existence of clear redox peaks in strong alkaline electrolyte could also own to mutual interaction among OH¯ ions and electrode material.41, 42 It is hypothesized that the electrochemical reactions of MnO2 in alkaline medium occur in a complex process as follows: 3Mn(OH)2

Mn3O4•2H2O + 2H+ + 2e-

Mn3O4•2H2O + OH4MnOOH+ 2Mn(OH)3+OH-

MnOOH + Mn(OH)3+2e(6MnO2) •5H2O + 3H+ + 6e-

Further, a linear enhancement in the peak current with high scan rates significantly confirm that electronic and mass transfer is fast enough to maintain the redox reaction (Figure 5a-c). Further only a slight shift in the peak position with increasing scan rate verifies that the hybrid still maintain its structure and conductivity signifying that the hybrid retain its redox active ability 18 ACS Paragon Plus Environment

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even at high scan rates, where most materials show huge shift in peak position due to larger barrier for ionic diffusion. Similarly, charge/discharge curves of hybrid in 6 M KOH solution also exhibited clear Faradaic reaction when analyzed in the voltage range of -0.2 to 0.3 V vs. Hg/HgO. Given the increasingly exposed surface of MnO2 nanocrystals present on both inside and outside of GC walls, the 3D MnO2@GCs@MnO2 hybrid shows excellent electrochemical performance in alkaline electrolyte with a high capacitance of 390 F/g at 0.5 A/g. Similar trend of comparative performance is also observed in 6M KOH for different kind of hybrids prepared with different mole ratios of GCs and KMnO4 as delineated by their CV (Figure 5a-c) and charge-discharge profile (Figure 5d-f), the resulted capacitance values are listed in Table S1. Besides, Table S2 compares the type of working electrode, electrolyte, and specific capacitance of MnO2 of this work with other literature.



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Figure 5. The (a-c) CV curves at the scan rate of 0.01, 0.05, 0.1, 0.5 and 1 V/s and (d-f) chargedischarge curves of 3D MnO2@GCs@MnO2 hybrid in 6 M KOH over the voltage range of -0.2 to 0.3 V vs. Hg/HgO. The 3D MnO2@GCs@MnO2 hybrid performs better in KOH than in Na2SO4 solution at the relative higher current density as discussed above. The enhanced capacitance in the highly concentrated KOH electrolyte arises due to abundant availability of OH¯ ions, higher ionic conductivity as well as electrochemically active nature of Zn species in KOH solution. The higher concentration of KOH leads to excessive and highly mobile K+ moieties in the electrolyte which not only improve the ionic conductivity but also leads to faster intercalation/deintercalation of K+ ions in the 3D MnO2@GCs@MnO2 hybrid providing higher capacitive performance at fast charge-discharge rates. It is well understood that hydration shell on the dissociated electrolyte ions determines the actual capacitive capabilities of the electrolyte on a particular electrode material. The hydration shell not only affects the size of the electrolyte ion but also determines the ionic mobility and diffusion. Although, ionic size of K+ (1.33 Å) is larger than Na+ (0.95 Å), however, size of fully solvated K+ (3.31 Å) ions is considerably smaller than fully solvated Na+ (3.58 Å) which ultimately lead to better performance of the developed hybrid in the KOH electrolyte. Similarly, the smaller ionic solvation radius of OH¯ (1.33 Å) in comparison to SO42¯ (2.58 Å) and the less number of atoms in OH¯ ion result in the fast desorption of these alkaline ions from the electrode and move back into the electrolytes reversibly.43 Hence, the hybrid exhibit excellent electrochemical activity in KOH solution where the smaller ionic size help in faster diffusion in short time to maintain high capacitance.


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Figure 6. (a) The plot of capacitance vs. current density for 3D MnO2@GCs@MnO2 in 6 M KOH and (b) Nyquist curves for GCs, MnO2 and MnO2@GCs@MnO2 over the frequency range of 100 kHz to 10 mHz (the inset shows the magnified Nyquist plots for clear comparison). The hybrid also bears high rate capability as delivers excellent specific capacitance values of 390, 385, 355, 326 and 291 F/g at the current densities of 0.5, 1, 2, 4 and 8 A/g, respectively (Figure 6a). In comparison to hybrid, the pure GCs delivered specific capacitance of 86, 80, 76, 69 and 59 F/g at scan rate of 0.5, 1, 2, 4 and 8 A/g in 6 M KOH (Figure S7). To verify the positive effect of GCs in the 3D MnO2@GCs@MnO2 hybrid on charge transportation, electrochemical impedance spectroscopy (EIS) was measured in the frequency of 10 mHz to 100 kHz (Figure 6b). The EIS spectrum of MnO2 electrode presents partially semicircles and a straight line bears lower slope that confirms its high resistance and larger barrier for ionic diffusion. But the 3D MnO2@GCs@MnO2 hybrid EIS spectrum contains more clearly overlapped semicircles with smaller diameter and a straight line with larger slope, which assures high electronic and ionic transportation in the hybrid. The impedance spectrum confirms that the hybrid can utilize the advantages of active site of MnO2 and good conductivity of GCs to reduce their resistance and provide faster highway for ion and electron movements. Thus, it is confirmed 21 ACS Paragon Plus Environment


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that designing of MnO2 on the interior and exterior sides of conductive backbone enhance its capacitive performance by improving its ionic and electronic conductivities. In addition to excellent charge storage capabilities, the 3D MnO2@GCs@MnO2 hybrid exhibited good cycling stability with 80.6% capacitance retention after 6000 cycles at a current density of 2 A/g (Figure 7), which further confirm the structural sustainability of this novel hybrid. The excellent capacitive performance of the 3D MnO2@GCs@MnO2 hybrid in both electrolytes proves its versatility to be used in various conditions for large number of applications.

Figure 7. Cyclic performance and capacitance retention of 3D MnO2@GCs@MnO2 hybrid at current density of 2 A/g in the voltage range of -0.1 to 0.5 V vs. Ag/AgCl in 1 M Na2SO4. CONCLUSIONS In summary, we have developed a facile method to fabricate the sandwich structure of 3D MnO2@GCs@MnO2 hybrid as an electrode for supercapacitor. MnO2 is coated on the interior and exterior surfaces of GCs using microwave assisted redox reaction between MnO2 and GCs. The ultrafine and evenly dispersed MnO2 on the exterior and interior GCs walls resulting in the hybrid with large active area and redox site, while hollow GCs play the role of 3D conducting 22 ACS Paragon Plus Environment

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framework leading to the shortened distance for ion diffusion and electron transport. The resulted 3D MnO2@GCs@MnO2 hybrid exhibited excellent electrochemical performance in 1 M Na2SO4 (265.1 F/g at 0.5 A/g) as well as a high capacitance of 390 F/g at current density of 0.5 A/g in 6 M KOH solution. Furthermore, the hybrid bears good rate capability and cycling stability by retaining the capacitance up to 80.6% after 6,000 cycles at 2 A/g in 1 M Na2SO4. We believe that our strategy to decorate active materials on conductive backbone with 3D open framework will open a new avenue for material designing for large number of application and present hybrid is a potential candidate for future ECs. ASSOCIATED CONTENT Supporting Information. The TEM images, XPS, XRD, Charge-discharge, CV and performance comparison of various 3D MnO2@GCs@MnO2 hybrids, pure GC and MnO2. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Wanli Zhang ([email protected]) * Weidong He ([email protected]) * Nasir Mahmood ([email protected] ) Author Contributions Xian Jian, Shiyu Liu, Yuqi Gao, Weidong He and Nasir Mahmood designed all the experiment and carried out synthesis of materials and performed all the characteristic and properties, they also wrote the manuscript. Asif Mahmood and Chandrasekar M Subramaniyam asisit with 23 ACS Paragon Plus Environment


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analysis of electrochemcial results. Prof. Xiaolin Wang, Wanli Zhang and Prof. Shi Xue Dou provided guidance in experiment and in writing manuscript. All the authors discussed the results. ζThese authors contributed equally. Acknowledgements This work was financially supported by the Fundamental Research Funds for the Chinese Central Universities (Grant No. ZYGX2016J139), the Science & Technology Support Funds of Sichuan Province (Grant No. 2016GZ0151), China Postdoctoral Science Foundation (2015M582539), the National Natural Science Foundation of China (Grant No. 51402040), the National Hi-Tech Research and Development Program of China (No. 2015AA034202) and the Automotive CRC 2020, Project 1-108. References 1.

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Table of Content (TOC)


Facile Synthesis of Three Dimensional Sandwiched MnO2@GCs

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