Metal-Oxide Decorated Multilayered Three-Dimensional (3D) Porous

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Metal-oxide Decorated Multilayered 3D Porous Carbon Thin Films for Supercapacitor Electrodes Kunal Mondal, Rudra Kumar, and Ashutosh Sharma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03396 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 19, 2016

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Industrial & Engineering Chemistry Research

Metal-oxide Decorated Multilayered 3D Porous Carbon Thin Films for Supercapacitor Electrodes Kunal Mondal‡,#, Rudra Kumar# and Ashutosh Sharma* Department of Chemical Engineering, Indian Institute of Technology Kanpur Kanpur-208016, Uttar Pradesh, India ABSTRACT We demonstrate an easy, scalable, and two step synthesis of macroporous carbon, carbon/TiO2 (cTiO2), carbon/MnO2 (cMnO2), and carbon/TiO2/MnO2 (cTiO2/MnO2) composite thin films for energy storage applications. The direct synthesis of the hybrid films was achieved by spin coating, followed by carbonization. The unique multilayered 3D pore structure of the film permits the synthesis of carbon/TiO2/MnO2 nanocomposites with enhanced metal-oxide nanoparticle loading up to 50 wt.%. The as-synthesized porous carbon thin films were tested for their supercapacitor activity and a maximum specific capacitance ~44 Fg-1 was achieved with a film thickness of 350 nm. The as-prepared cTiO2, cMnO2, and cTiO2/MnO2 electrodes exhibit high specific capacitances of 178, 237, and 297 Fg-1, respectively at 5 mVs-1 because of their unique properties with impregnated nanoparticles, and direct fabrication on conductive substrates. This simple scalable coating technique is compatible with the high speed roll-to-roll manufacturing processes and easily generalized for other carbon/metal oxide composites. Keywords: Porous carbon film; thin film supercapacitor, Carbon/TiO2, Carbon/MnO2, multilayered porous *Corresponding author E-mail: [email protected], #Both the authors contributed equally to this work ‡

Present Address: Department of Chemical and Biomolecular Engineering, North Carolina State University, 911 Partners Way, Raleigh, North Carolina 27695, United States 1 ACS Paragon Plus Environment


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1. INTRODUCTION In recent years, the supercapacitor, which is also called the electrochemical double layer capacitor or ultracapacitor, has been widely studied because of the growing demand for energy storage for portable electronic devices.1-3 Supercapacitors are a type of energy storage unit and can provide an extremely large capacitance and a high power density in a short period of time, as well as a long cycle life that is essential for power delivery.4 Here, electrical energy is generally stored by rapid and reversible redox reactions or phase changes on the subsurface and/or surface of electrode materials.5 Supercapacitors are most studied as energy storage devices over conventional dielectric capacitors and batteries because they have limitations such as unstretchablilty, comparatively low power, and long charging time.6 Supercapacitors provide promising features such as faster charge-discharge rates, and safe operation.7 However, supercapacitors frequently suffer significantly from low energy performance, which is commonly assessed by specific capacitance and energy density. Therefore, it is necessary to increase their energy performance to achieve the larger necessities of future energy applications, extending from small portable electronics, flexible devices, such as roll-up displays, artificial electronic skin, and distributed sensors, to hybrid vehicles and massive industrial equipment.8-1213 For the improvement of these energy devices, nanostructured materials for supercapacitor electrodes are the key elements that need to be studied, as they promise not only higher electrical capacities but also superior response rates, compared to other traditional materials.14, 15 There has been substantial interest in using carbon nanomaterials as electrodes for supercapacitors because of their advantages such as light weight, unique nanostructure, good electrical conductivity, low mass density, remarkable chemical stability, and high electrochemical surface area.4,

16, 17

There are innumerable exciting opportunities to develop

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innovative strategies for such energy storage devices if the carbon electrodes can be engineered and modified to acquire new functionalities. Nanoporous carbon thin films are the most promising materials for supercapacitor electrodes, because of their unique underlying nanostructures, high-surface-area porous templates, good electron transfer characteristics, environmentally friendly nature, and availability at low cost.18 These nonporous carbon films can be engineered further by making composites with carbon nanotubes, graphene, metals, metaloxide nanoparticles etc., all of which have been extensively explored for high power electrode materials because of their reasonable electrical conductivity and readily available surface areas. Moreover, their porous morphology and multilayered interconnected pore networks facilitate rapid electron transfer and the transport of reactants and products during electrochemical reactions, which make them an ideal support for functional nanomaterials.19 Several nanostructured materials such as graphene, carbon nanotubes, carbon nanofibers, conducting polymers, and transition metal oxides/hydroxides like MnO2, RuO2, and NiO have been studied as potentially useful materials in the field of supercapacitors.20 Among the metaloxide materials, MnO2 has been intensively explored because of its electrochemical behavior, low cost, structural flexibility, and plentiful availability.21 Also, MnO2 can function in neutral aqueous electrolytes, unlike NiOOH and RuO2.xH2O that function only in strongly alkaline or acidic electrolytes, hence initiating environmental problems.22 A redox reaction, Mn4+/Mn3+ system comprising of single-electron transfer accounts for the MnO2 pseudocapacitance.23 Unfortunately, even the best MnO2 nanostructured materials still suffer from poor energy density like other metal-oxides, because of drastic structural alterations throughout the course of the charge-discharge cycles. Thus, there is an immediate requirement to design and fabricate a MnO2 or MnO2 composite electrode with a porous nanostructure that affords both high energy density 3 ACS Paragon Plus Environment


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and outstanding power capability. To address those problems, nanocomposites combined with other metal oxides (e.g. RuO2, NiO, TiO2) carbon film, carbon nanotube, and conducting polymer, graphene etc., which may offer unique and critical function to reach optimized electrochemical properties, have been proposed and studied.24 As one type of transition metal oxide, MnO2 is believed to be the most promising pseudo-capacitive material owing to its high energy density; unfortunately, however, its low electrical conductivity (∼10−5 S·cm−1) is a deterrent to achieving high electrochemical performance.25, 26 To resolve this problem, intensive efforts have been dedicated to incorporating additional materials such as metal oxides, graphene, carbon nanotubes, porous carbon, and activated carbons.26-29 Combination of titania with manganese dioxide and carbon provides an interesting solution where carbon/TiO2 offers excellent electrochemical stability and improved conductivity to the composite whereas carbon/MnO2 enhances the pseudo capacitance.30 This enhancement in the capacitance is mainly due to the interplay of the high electronic conductivity of carbon/TiO2 phase (formed due to composite structure) and the high electrochemical activity of MnO2. Furthermore, TiO2 also contributes excellent cycle stability and MnO2 delivers high capacity.30, 31 In this paper, we report an easy but rapid, two-step, economical, powerful method to prepare porous carbon films, impregnated with titanium dioxide (cTiO2), manganese dioxide (cMnO2), and a binary mixture of these two metal-oxides (cTiO2/MnO2) on stainless steel (SS) substrates by self-organization in a solution-cast polymer film followed by carbonization. This proposed technique is also applicable for solution-cast high speed roll-to-roll thin film synthesis processes and is specifically suitable in applications necessitating thin polymer/metal-oxide and/or carbon/metal-oxide hybrid films; for example, thin film electrochemical supercapacitors and lithium-ion batteries where a strong interaction between carbon and metal-oxides is 4 ACS Paragon Plus Environment

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indispensable.32,

33

The fabricated porous composite carbon architecture can also serve as a

support for functional nanoparticles for instance MnO2 and TiO2 nanoparticles without hindering their activity. The synthesized TiO2 and MnO2 were in the form of the pure rutile and Mn3O4 phases, respectively, which are chemically and thermally the most stable. Because of the unique properties of these composite films, such as porous nanostructures, huge surface-to-volume ratios, high crystallinity, and direct fabrication on conductive substrates, the as-prepared cTiO2, cMnO2, and cTiO2/MnO2 electrodes exhibit a high specific capacitance of 178, 237 and, 297 F/g, respectively at 5 mVs-1. The enhanced loading of TiO2 nanoparticles has also been proven to increase the specific capacitance of the cTiO2/MnO2 electrodes. To the best our knowledge, in spite of the increasing interest in porous carbon architectures for electrochemical energy storage applications such as lithium-ion batteries and fuel cells, the use of these materials and the proposed fabrication method of thin films for supercapacitor applications have still not been reported. However, the deposition of thin films on conducting substrates with decorated functional nanostructured MnO2 and TiO2 is believed to be an ideal approach to enhance the capacitive behavior of MnO2 films and has an encouraging future as potential electrode material for high-performance electrochemical supercapacitors. 2. EXPERIMENTAL 2.1 Materials. Polyacrylonitrile (PAN; MW=150,000), Na2SO4, titanium dioxide and manganese dioxide were obtained from Sigma–Aldrich Co., USA. N, N-dimethyl formamide (DMF) was obtained from Fischer Scientific, India. All reagents were of analytical grade. 2.2 Preparation of porous carbon, carbon TiO2 and MnO2 composite films. The porous carbon films and the metal-oxide impregnated porous carbon films were synthesized using spin



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coating technique reported earlier.34 Briefly, A 8 wt% PAN:DMF blend

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was prepared by

dissolving a known amount of PAN polymer in DMF and heated at 65 C with constant stirring for 25 min. A transparent solution was obtained and spin coated on cleaned stainless steel wafer and then stabilized in air at 250 C for 2 h followed by the carbonization under 150 mL/min argon flow at 900 C for 2 h, with a 5 oC/minute heating rate, to gel a macroporous carbon film. Three samples like, TiO2/carbon (cTiO2), MnO2/carbon (cMnO2) and TiO2/MnO2/carbon (cTiO2/MnO2) hybrid films with different TiO2 and MnO2 loadings (cTiO2/MnO2-1 and cTiO2/MnO2 - 2) were prepared by following the similar strategy as described above, however, during the preparation of PAN:DMF solution 25 wt/wt % of TiO2 and MnO2 nanoparticles (with respect to the PAN) was also added. The same procedure was followed for carbonization. After preparing PAN:DMF blend solution as discussed above, known quantity of pre-purchased titanium dioxide and manganese dioxide powder were mixed and kept on a magnet stirrer for around 24 h to obtain a good dispersion of TiO2 and MnO2 particles in the polymer blend. The combined viscous solution was used for spin coating. The parameter like viscosity of polymer blend and rpm of spin coating and TiO2, MnO2 nanoparticles loadings were optimized to get different thin film samples. 2.3 Characterizations. The Field Emission Scanning Electron Microscopy (FESEM, Quanta 200, Zeiss, Germany) was utilized to characterize the surface morphologies of porous carbon film and its Ag hybrid. The Powder X-ray diffraction (XRD) data were collected on a PANanalytical, Netherlands X-ray diffractometer with Cu Kα radiation (λ=0.15418 nm) at 50 kV and 200 mA at room temperature over an angular range of 7°-80°, with a step scan rate of 0.02° S-1 to get structural information of the MnO2 and TiO2 impregnated hybrid carbon films. High resolution transmission electron microscopy (HRTEM) was carried out on a Tecnai G2, USA 6 ACS Paragon Plus Environment

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microscope to gain further insights into the synthesized material. For this purpose, small quantities of carbon films were removed from silicon wafer and sandwiched between carbon coated copper grids and subjected for HRTEM imaging. X-ray photoelectron spectroscopy (XPS; PHI 5000 Versa Prob II, FEI Inc.) was performed to determine the oxidation states and surface properties. Thermo gravimetric analysis (TGA) was carried out in air atmosphere from room temperature to 900 °C at a heating rate of 10 C min-1 (TGA,TA Instruments 2960). Raman spectral analysis was performed on WiTec, Germany using laser light of 543 nm wavelength to characterize the graphitic nature of the carbon material. The surface topography of the carbon film was further established by the Atomic Force Microscopy (AFM) using an Agilent Technologies atomic force microscope (Model 5500) operating in noncontact/ACAFM mode. Micro-fabricated silicon nitride cantilevers with a spring constant (C) of 50 N/m and resonant frequency (f) of 175 kHz were used. The average thickness (T), width (W), and length (L) of the cantilever were approximately 700, 38, and 225 μm, respectively. Data acquisition and analysis was carried out using PicoView 1.4 and Pico Image Basic software, respectively. Surface profiling and the thicknesses of the films structure are estimated using an optical profiling system (NanoMap-D, AepTechnlogy, USA). The pore size distribution and total surface area were calculated using the BET method and Autosorb1 software (Quantachrome Instruments, USA). The pore size distribution was deliberated by the Barrett–Joyner–Halenda (BJH) method. 2.4 Sample preparation for electrochemical characterizations. All the catalytic reactions were performed in a controlled atmosphere (room temperature 23 oC and humidity level of 32%). The electrochemical characterization of the as-prepared samples were investigated with a Potentiostat/galvanostat 302N (Autolab, Metroohm) using cyclic voltammetry (CV) and galvanostatic charge/discharge in three-electrode cell. Electrochemical impedance spectroscopy



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(EIS) was performed in a frequency range from 100 kHz to 10 mHz at an open circuit potential of 0.01 V with an ac amplitude of 10 mV. The three-electrode cell configuration consisted of Pt rod as the counter electrode, Ag/AgCl (3M KCl) as the reference electrode, and the porous carbon samples as the working electrodes (WEs) with 1 M Na2SO4 as an electrolyte. 3. RESULTS AND DISCUSSION 3.1 Surface morphology of porous carbon and carbon/metal-oxide composite thin films. The porous carbon films were fabricated using dip coating of polyacrylonitrile (PAN) polymer under an optimized protocol which is schematically shown in Figure 1a. 34 The as-synthesized spin-coated PAN polymer derived and porous carbon films deposited on stainless steel wafers (3×1.5 cm) were imaged by a field-effect scanning electron microscope (FESEM), as shown in Figures 1b, c, and d. The well-ordered pore formation in the polymer film was extensively studied in our earlier work (the detailed study was published elsewhere).34, 35 The formation of the macropores depends on parameters like the viscosity of the polymer, film thickness, nature of the solvent, annealing conditions, etc. In particular, factors encouraging more destabilization forces on the spin-coated films such as thinner films, faster evaporation, and lower viscosity give better porosity.


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Figure 1: (a) Illustration shows a schematic for the fabrication of spin coated porous carbon and carbon/metal-oxide hybrid films. FESEM micrographs show polymer (b) thick film without/few pores, (b) thinner film with optimal macrospores, and (c) thinnest film with larger pore diameter. Inset of ‘c’ and ‘d’ show the magnified images of porous polymer films before carbonization.

It was found earlier that the pore size and the number density of pores in the polymer films increase with a decrease in polymer concentration. In addition, the thinner film produces bigger pores and higher pore density. The thickness of the pore wall also depends on polymer concentration and film thickness. Thus, optimizing these vital pore-forming parameters, we fabricated three different films including a thick film with few or no pores (sample CF1) (Figure 1b), a thinner film with optimal macropores (sample CF2) (diameter ~500–800 nm shown in Figure 1c), and the thinnest film having large-diameter pores (sample CF3) (~5–10 µm, shown in Figure 1d). Furthermore, we incorporated metal-oxide nanoparticles into the porous carbon films (specifically, into the CF2 films) and tested those samples for their electrochemical 9 ACS Paragon Plus Environment


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supercapacitor performance. The porous polymer film prepared by spin coating exhibited interconnected macropores and also a few mesopores embedded onto the macropore walls; these can be easily seen in the FESEM images shown in the inset of Figure 1c and d. The porous polymer film was then carbonized in an inert argon atmosphere to obtain the porous carbon films. It is important to note that the interconnected macropores developed in the PAN film by the asset of spin-coating technique, and were consequently transformed to porous carbon structures by the carbonization process.

Figure 2. FESEM image of (a) free-standing porous carbon films collected from the substrate and (b) higher magnification image shows the well-defined porous morphology of the film. (c) AFM image of spun coated carbon film confirms the presence of macropores (i) and three dimensional (3D) topographic image of the porous carbon film (ii). Figure 2a shows a FESEM image of a porous carbon film (sample CF2), while ordered macropores along with mesopores can be seen in Figure 2b. The porous carbon films were also studied under an atomic force microscope (AFM) as shown in Figure 2c; this confirmed the presence of macropores in the carbon films. Figure 2c(i) shows the interconnected macropores all over the carbon films while the 3D topography in Figure 2c(ii) confirms that the size of


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macropores is 3.5 µm. An optical profilometry study revealed further information about the thickness of all the films. Figure 3a (sample CF1) shows an optical image of a thick film with no pores, which after height profile analysis (Figure 3b) with 3D topography (Figure 3c) by the optical profiler shows that the average film thickness is 1.5 µm. Consequently, it was discovered that the thickness of the polymer film decreases by about 20–40% after carbonization at high temperature; this was investigated by measuring the thickness of the porous polymer and carbon films before and after carbonization. Figures 4a, b and c, respectively, show the thick carbon films (~1.5 µm) with few or no pores (CF1), the thinner carbon film (~350 nm) with optimal macrospores (CF2), and the thinnest film (~100 nm) having pores with larger pore diameters (CF3). Interestingly, one can compare this CF3 sample with 100 nm thin possessing pores 5 -10 µm as a web material. We have also efficaciously impregnated metal-oxide nanoparticles into the porous polymer as well as into carbon films by addition of metal-oxides at the time of solution preparation for the spin-coating process. This is an easy and scalable route for fabrication of carbon/metal-oxide composite films, which is beneficial for various applications, like porous carbon/TiO2 films for photocatalysis of organic pollutants.35 Here, we have demonstrated the fabrication of several hybrid films, including porous carbon/TiO2 (cTiO2 shown in Figure 4d), carbon/MnO2 (cMnO2 shown in Figure 4e) and carbon/TiO2/MnO2 (cTiO2/MnO2 shown in Figure 4f), to verify their electrochemical supercapacitor activity.



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Figure 3. Optical profiling images carbon film (a) and height profile analysis of the film on a SS substrate (b), and (c) 3D topography of the optical image.

Figure 4. FESEM micrograph of carbon film with (a) no pore/few pores (CF1), (b) thinner carbon film with optimal pores (CF2), and (c) thinnest film with larger pores (CF3). Micrographs for the thinner macropores carbon film with impregnated metal-oxides: (d) TiO2 loaded cTiO2, (e) MnO2 loaded cMnO2 and (f) TiO2/MnO2 loaded cTiO2/MnO2 film. Inset showing the magnified images with nanoparticles distributed throughout the composite films.


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The scanning electron micrographs clearly show the presence of bead-like titanium dioxide and manganese dioxide MnO2 nanoparticles that are well distributed throughout the surface of the films and are also entrenched underneath the film layers (as shown in Figure 4f) with average diameters of around 60–70 nm for TiO2 and 35–50 nm for the MnO2. The insets of Figures 4d, e, and f clearly show that the TiO2, MnO2, and TiO2/MnO2 were distributed uniformly all over the porous films and underneath the layer of the pore walls. It is also observed that, if the particle loading in the polymer is increased, after a certain limit the distribution of particles become poor, and the particles start to accumulate on the film surface, which leads to agglomeration and nonuniform particle distribution.

Figure 5. EDX spectra and corresponding histogram plots for quantitative analysis of various elements present in cTiO2 (a, b), cMnO2 (c, d) and cTiO2/MnO2 (e, f) composite films, respectively.

The TiO2 and MnO2 nanoparticles embedded into the porous carbon films were analyzed by selected area energy dispersive x-ray (EDX) analysis (Figure 5), which was then confirmed by the Raman spectra and X-ray diffraction (XRD) analysis of the composite carbon composite films. Figures 5 a, c, and e show the EDX spectra of the TiO2-, MnO2-, and TiO2/MnO213 ACS Paragon Plus Environment


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impregnated carbon films while the corresponding wt.% of the elements present in the samples are summarized in the histograms of Figure 5b, d, and f, respectively. The insets of Figures 5a, c, and e show the FESEM micrographs of the composite films on which the analysis was performed. The stability of the composite film and the exact amount of metal oxide embedded in the carbon film was analysed by thermo gravimetric analysis. Figure S1 shows that the composite film is thermally stable at high temperature and a mass loss about 50 % was recorded at 700 oC. The TGA curve (Figure S2) displays that about 13% of metal oxides (TiO2 and MnO2) were present in cTiO2/MnO2 composite film.

Figure 6. (a) TEM micrograph of (a) porous carbon film, (b) porous cTiO2 film, (c) porous cMnO2 film, and (d) porous cTiO2/MnO2 film. SAED pattern shows (e) cTiO2 film, and (f) cTiO2/MnO2 film. The porous carbon film impregnated with bead-shaped metal-oxide nanoparticles was examined under the transmission electron microscope (TEM) for further insight about the porous layers


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and impregnated nanoparticles. The TEM image in Figure 6a shows a virgin porous carbon film, whereas Figures 6b–d demonstrate the films with impregnated metal-oxide nanoparticles. The virgin carbon film shows macropores (indicated by circles in Figures 6a and b); interestingly, few mesopores are also present on the pore wall as indicated by the arrow in Figure 6a. The sizes and shapes of the TiO2 and MnO2 nanoparticles in the porous carbon films are apparent from the TEM images. Figure 6b and c confirm that the diameters of the TiO2 and MnO2 nanoparticles are around 60–70 and 35–50 nm, respectively (the circles indicate the bead-like particles), which is also supported by the previous FESEM study. The TiO2 and MnO2 particles are also seen in Figure 6d where both are present together (marked by circles in the micrograph) within the carbon matrix. It is interesting that MnO2 nanoparticles can differentiated from titanium dioxide particles on the basis of their size differences. The virgin carbon film is amorphous, which is confirmed by the SAED image where no ring-like pattern was observed (data not shown). On the other hand, the metal-oxide nanoparticles are highly crystalline as can be seen by the sharp rings of the SAED patterns for TiO2 and MnO2 in Figures 6e and f. Each ring corresponds to the lattice plane of the crystals, which are indexed accordingly; this also supports the XRD results. Table 1. Pore size distribution from BET data. Sample details

Total pore volume (cc/g)

BET area (m2/g)

CF1 CF2 CF3 cTiO2 cMnO2 cTiO2/MnO2

0.006 0.02 0.013 0.110 0.142 0.181

12.2 169 175 121.8 163.6 180.2

Macropore volume (cc/g) 0.0005 0.013 0.012 0.076 0.102 0.143

Mesopore volume (cc/g) 0.0053 0.009 0.007 0.020 0.028 0.0283

Micropore volume (cc/g) 0.0002 0.001 0.00075 0.014 0.0127 0.0089



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3.2 BET analysis. The N2 adsorption-desorption isotherms of the composite film are shown in Figure 7a; the existence of a well-developed porous structure is established by the well-defined hysteresis loop. The specific surface areas and pore size distribution are summarized in Table 1. Nanoparticle-impregnated composite carbon film has a slightly higher specific surface area (180.2 m2/g) and a larger pore volume (0.181 cc/g) than pristine carbon film (surface are of 175 m2/g, and pore volume of 0.013 cm3/g)34 because they have MnO2 and TiO2 nanoparticles impregnated within the matrix, which increase the pore volume. Even though the incorporation of nanoparticles increases the specific surface area of the composite film, the macroporosity (~80%) decreases compared to the virgin carbon film (~97%); moreover, the mesoporosity was found to be ~16%, and the microporosity, ~4%. The heat treatment of the nanoparticles during carbonization might open up a few meso- and micro-pores in the wall of the macroporous carbon film. This huge macroporosity and its connection to the mesopores seem advantageous in terms of allowing ionic transportation through the electrolyte during the supercapacitor chargingdischarging cycles.

Figure 7: (a) Nitrogen adsorption/desorption isotherm and pore size distribution plot (inset) of the cTiO2/MnO2 composite film, (b) Raman spectral analysis and (c) XRD plot of cTiO2, cMnO2 and c TiO2/mnO2 films.


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3.3 Raman analysis. Raman spectroscopy was performed on the porous carbon film, as well as the cTiO2, cMnO2, and cTiO2/MnO2 composite films after carbonization to investigate the phase composition and graphitic content; the results are shown in Figure 7b. The Raman spectrum of the porous carbon film (CF2) comprises two major sharp peaks at 1352 and 1595 cm−1 and a broad peak at 2800 cm−1 conforming to the major D and G and 2D bands for sp2 bonded carbon.35 The G-band originates from the C–C stretching (E2g) mode of graphite, and the D-band (or defect band) is ascribed to the defects due to the restricted crystallite size or edges of graphitic layers.36 This band is a result of out-of-plane vibrations of sp2 bonded carbon atoms and is attributed to the presence of structural defects. The G-band (or graphitic bond) signifies the inplane bond-stretching vibration of sp2 bonded carbon atoms and demonstrates the existence of crystalline graphene layers. The frequency of the 2D band is determined by a second-order process from two zone-boundary longitudinal optical phonon vibrations and occurred in the wavenumber region from 2500 to 2900 cm-1.35 This bond indicates sp2 bonded carbon atoms with surface defect modes. The porous carbon film with incorporated TiO2 shows peaks at 247, 429, and 602 cm-1 followed by other characteristic carbon Raman peaks (D, G, and 2D) (shown by the blue line in Figure 7b). The presence of TiO2 nanoparticles in the composite film leads to robust Raman characteristic peaks at 247, 429, and 602 cm−1 corresponding to the Eg and A1g modes for the rutile phase.35, 37 The rutile phase of the titanium dioxide is to be expected because TiO2 experiences heat treatment at 900 °C, and the anatase phase has thereby been converted entirely to the rutile phase. The Raman analysis of manganese dioxide (MnO2) shows that it is a Mn3O4 polymorph of manganese dioxide. Here, cMnO2 was prepared by heating the carbon and manganese dioxide at 900 °C; thus, there are Raman peaks are present for both MnO2 and carbon. The incorporation of MnO2 in the carbon film shows multiple peaks (shown by the red



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line in Figure 7b) along with carbon D (1352 cm−1) and G (1595 cm−1) peaks in which 305 (Eg), 497 (Eg), 578 (A1g), and 654 (A1g), cm-1 are associated with Mn3O4.38-41 The Raman spectrum in Figure 7b (shown by the pink curve) shows that the cTiO2/MnO2 has both rutile TiO2 and Mn3O4, based on the characteristics peaks and the D, G, and 2D signature for pristine carbon films. The intensity ratio of D band over G band is frequently used to evaluate the level of disorder in carbon materials. The ID/IG ratios for carbon and carbon metal-oxide films calculated from the Raman spectra are 1.004 and 1.04, respectively. The crystallite size of the samples was quantified using an empirical formula ID/IG = 4.4 nm/La proposed by Knight and White.42 Using this equation, in-plane graphitic crystallite sizes (La) are found to be 4.37 and 4.23 nm for the carbon and carbon metal-oxide composite films, respectively. The observed decrease in the ID/IG ratio as well as the in-plane graphitic crystallite sizes indicate that metal-oxide loading into the carbon matrix increases the number of defects in the carbon films.43 3.4 XRD analysis. To obtain more evidence about the crystallinity and phase of the synthesized porous hybrid films, wide-angle powder XRD analysis was performed at a slow scan rate as shown in Figure 7c. The XRD data of the films has been analyzed using the Pearson crystal data (PCD) and the Joint Committee on Powder Diffraction Standards (JCPDS) databases. The XRD pattern of the porous carbon film in Figure S3 shows a distinct broad peak around 2θ = 22·4°, which corresponds to the (002) characteristic lattice planes for glassy carbon (PCD reference number 820505).34 This characteristic XRD peak for carbon was observed in all three types of composite films (Figure 7c). For the cTiO2 film (shown by the red line in Figure 7c), the main characteristic XRD peak for rutile titanium dioxide found at 2θ = 27·5° corresponds to the (110) lattice plane (JCPDS No. 761940).44 Other distinct peaks are found at 36.1, 39·2, 41·2, 44, 54·5, 56.7, 62, 64, 68.8, and 70 18 ACS Paragon Plus Environment

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degrees; these were indexed as (101), (200), (111), (210), (211), (220), (002), (310), (301), and (112), respectively. All diffraction peaks matched well with the standard data for rutile TiO2, confirming the formation of this single phase. The intense diffraction peaks at (110) and (101) demonstrate the high crystallinity of the titania particles in the porous carbon films. Figure 7c shows the XRD patterns of the cMnO2 composite film (shown by the green line). All the diffraction peaks in the XRD pattern are indexed to pure Mn3O4 with tetragonal unit cells and with a space group I41/amd (141).41, 45 All the diffraction peaks in Figure 7c (marked by the green line) for cMnO2 can be indexed as tetragonal Mn3O4 and MnO2 [JCPDS No. 24-0734].39, 45 The sharp XRD peaks at 36°, 39.5°, and 58.8° along with the characteristic (002) peak for carbon suggest that the manganese particles in cMnO2 composite film are highly crystalline in nature. The three most intense peaks correspond to the (211), (004), and (321) lattice planes of manganese dioxide, respectively. The phase morphology of MnO2 is significantly influenced by high temperature heat treatment. It can be seen that high temperature carbonization of the cMnO2 produces a pure product of Mn3O4 and MnO2 phases. It is reported that at about 500 °C and above, MnO2 undergoes a chemical transformation by loss of oxygen and consequently becomes Mn2O3.46 This phase persists up to 800 °C along with MnO2 phase and then upon further heating around 1000 °C it transforms completely into Mn3O4. The other characteristic XRD peaks are observed at 18, 28.8, 32.3, 36.5, 60, 64.4, 68.1, 70.1, and 73.9 degrees; and can be indexed as (101), (112), (103), (202), (224), (314), (402), (305), and (420), respectively for the mixed phase of MnO2 and Mn3O4 (PCD File No. 150289). Interestingly, the XRD peaks for cTiO2/MnO2 composite film (shown by the blue curve in Figure 7c) show all the characteristics for titanium dioxide and manganese dioxide along with the main characteristic carbon peak, which is shown in Figure 7c by a blue line. This indicates that both



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TiO2 and MnO2 are present in the porous cTiO2/MnO2 composite carbon film. The average crystallite sizes of TiO2 and MnO2 were calculated from the XRD pattern by the Debye-Scherrer equation: D = 0.90 × λ/β × cos(θ) where β is the full peak width at half height (FWHM) corresponding to the diffraction angle (θ), λ is the wavelength of the X-ray, and 0.90 is Scherrer’s constant.35 The average crystallite size of the TiO2 and MnO2 nanoparticles was found to be ~65 and 40 nm, for the (111) and (321) reflection planes, respectively, which is in good agreement with the FESEM and TEM results. 3.5 XPS analysis Figure 8(a) shows the deconvoluted XPS spectra of Ti, Mn, C and O spectrum. The peaks identified at 458.5 and 464.2 eV corresponds to 2p3/2 and 2p1/2) of Ti 2p peak. Also the peak separation (2p3/2 and 2p1/2 ) of 5.7 eV suggests that the presence of TiO2 phase.47 Figure 8(b) shows the xps of Mn 2p orbital. The peak present at 642.5 eV corresponds to the Mn4+ oxidation state of Birnessite phase of MnO2. The peaks centered at 642 and 653.4eV corresponds to the Mn2p3/2 and Mn2p1/2 transitions respectively. Also, small deconvoluted peak at 640.8 corresponds to the Mn3+ oxidation states of Mn3O4 phase while the peak obtained at 645.5 attributed to Mn (+7) oxidation state.48 The deconvoluted xps spectra of C 1s are shown in figure 8c. The peaks identified at 284.8, 286.2 and 289.2 eV correspond to C-C, C-O and C=O bond, respectively. Figure 8 (d) shows the deconvoluted O1s XPS spectra. The peak identified at 531.2eV corresponds to the Mn–OH hydroxyl bond while the peak present at 533 eV is attributed to organic C=O bond.49


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Figure 8: Deconvoluted XPS spectra of (a) Ti 2p (b) Mn 2p (c) C 1s and (d) O 1s. 3.5 Electrochemical supercapacitor properties of the porous films. The specific capacitance of the as-prepared porous carbon electrodes was determined by cyclic voltammetry in the voltage range from 0 V to 0.8 V. Figure 9a shows the CV curve at a 50 mV/s scan rate for the different thicknesses of porous carbon electrodes in 1 M Na2SO4 aqueous electrolyte at 25 °C. It was observed that the optimized porous carbon film (CF2) shows a larger number density of pores than porous carbon films CF1 and CF3. The thickness of CF2 is lower than CF1 but higher than CF3. Therefore, the specific capacitance per gram for porous carbon film CF2 prepared under optimized conditions is higher. This result is consistent with the idea that the formation of the porous structure is affected by using different rotation speeds of the spinner during the spincoating process. The FESEM analysis shows a more porous structure in the CF2 film, while CF1 and CF2 have fewer pores.



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Figure 9. (a) CV at 50mV s-1 for porous carbon electrodes CF1, CF2 and CF3 prepared at different spin coating speed and polymer concentration, (b) CV for CF2 porous carbon electrode at scan rate from 5-200mVs-1. The decrease in porosity (or pore density) of CF1 and CF3 films prepared at low and very high spin-coating speed and optimized polymer concentration show less penetration of electrolytes and limited utilization of active carbon materials, which could cause a decrease in the specific capacitance. The BET analysis of thin films shows that CF2 thin film possess larger total pore volume and mesopore volume as compared to CF1 and CF3 thin films, which suggests that the better penetration of electrolyte ions in to the electrode, thereby causing higher specific capacitance. Figure 9b shows the CV curves for porous carbon electrode (CF2) at 5 mVs-1 to 200 mVs-1 scan rates. It can be observed that all the CV curves obtained at lower and higher scan rates have nearly rectangular shapes and exhibit symmetrical current voltage characteristics.50 The mirror image of the CV curves shows the capacitor behaviour for the as-prepared electrode.


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Scane rate, 50 mVs-1

(a)

20

cMnO2 electrode

(b)

10

Current (A/g)

Current Density (A/g)

15

5 0

cTiO2

-5

cMnO2 cTiO2/MnO2_2

-10

0.0

0.2

0.4

0.6

10 0 5 mV/s 10 mV/s 20 mV/s 50 mV/s 100 mV/s 200 mV/s

-10 -20

cTiO2/MnO2_1

-15

0.8

0.0

0.2

Voltage(V)

0.4

0.6

0.8

Potential (V)

20

Specific Capacitance F/g

30

Current Density (A/g)

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


cTiO2/MnO2 electrode

(c)

10 0 5 mV/s 10 mV/s 20 mV/s 50 mV/s 100 mV/s 200 mV/s

-10 -20 -30 0.0

0.2

0.4

Voltage (V)

0.6

0.8

300

CF1 CF2 CF3 cTiO2

(d)

250

cMnO2

200

cTiO2/MnO2_1 cTiO2/MnO2_2

150 100 50 0 0

50

100

150

200

Scan Rate (mV/s)

Figure 10. (a) CV curve of different electrodes at a fixed scan rate for (b) CV curve of cMnO2, (c) CV curve of cTiO2/MnO2 film, and (d) Specific capacitance of all samples.

The specific capacitance was calculated for all carbon composites by using the equation Cs= (1/𝑚ʋ∆𝑉)∫ 𝐼(𝑉)𝑑𝑉, Where Cs is the specific capacitance, m is the mass of active material, ʋ is the scan rate, ΔV is the scanned potential window, and ∫ 𝐼(𝑉)𝑑𝑉 is the integral area under the CV curve. The specific capacitances for porous C film CF2 were 44, 37, 33, 26, 18.7, and 15 Fg-1 at 5, 10, 20, 50, 100, and 200 mVs-1, respectively. However, the specific capacitances calculated for porous films CF1 and CF3 were 37 and 21 Fg-1, respectively at 5 mVs-1. This decrease in specific capacitance is probably due to a decrease in the porosity of the carbon films.51 Thus, the higher specific capacitance has been obtained for porous carbon electrode CF2 where carbon matrix acts as a 3D structure for metal oxide incorporation. We have further calculated the areal and volumetric capacitance of the carbon thin films and their composites with TiO2 and MnO2. The areal capacitance of CF1 is calculated to be 10 mF cm-2, which is higher than CF2 (7.04 mF cm-2) and CF3 (2.52 mF cm-2) thin films. The obtained areal 23 ACS Paragon Plus Environment


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capacitance of as-synthesized material is higher than polyacrylonitrile-based carbon nanofibre reported in the literature (67µF cm-2).52 The volumetric capacitance of CF1, CF2 and CF3 thin films are 6.66, 20.1 and 25.2 F cm-3, respectively at 5 mVs-1 scan rate. Figure 10a presents a comparative study of the CV curves for cTiO2, cMnO2, and cTiO2/MnO2 composite thin films at a scan rate of 50 mVs-1. The increase in the specific capacitance is due to the addition of TiO2 nanoparticles with their favourable redox and pseudo-capacitive behaviour. Furthermore, the incorporation of MnO2 into the porous carbon matrix enhances the specific capacitance compared to the cTiO2 electrodes (Figure 10b). This is because the MnO2-containing electrodes exhibited substantially larger current density than the TiO2 electrodes because MnO2 is more electrochemically active than TiO2. The specific capacitance values obtained for the cMnO2 porous thin film were 237, 202, 178, 146, 118, and 74 Fg-1 at 5, 10, 20, 50, 100, and 200 mVs-1, respectively. The specific capacitance values for the cTiO2 composite thin film were 178,142, 110, 79, 53, and 45 Fg-1 at 5, 10, 20, 50, 100, and 200 mVs-1 respectively (Figure 10c). Furthermore, the addition of both TiO2 and MnO2 to porous carbon electrodes increases the specific capacitance to 297 Fg-1 at a 5 mVs-1 scan rate. This significant increase in specific capacitance is due to the increase in current density of cTiO2/MnO2 thin films compared to cTiO2 and cMnO2 (Figure 10d). This can be attributed to the increased surface area and improved charge transport of MnO2 materials, compared to the porous carbon films compared to the porous carbon films containing only TiO2 nanoparticles.53 The areal and volumetric capacitance of c-TiO2 thin film are 56.9 mFcm-2 and 162.7 F cm-3, respectively, whereas for c-MnO2 thin films the values are 73.4 mF cm-2 and 210 F cm-3, respectively at 5 mVs-1 scan rate. Moreover, it was found that the areal and volumetric capacitance of cTiO2/MnO2 thin film the values are 95 mF cm-2 and 271.5 F cm-3, respectively at 5 mVs-1 scan rate. The volumetric capacitance value of


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cTiO2/MnO2 thin film is higher than the porous carbon thin films (220 F cm-3, 200 nm thin film)54 while, the gravimetric capacitance is higher than carbon nanofibres web electrode synthesized by electrospinning technique (175 Fg-1).55 The areal and volumetric capacitances of all as-synthesized samples at 5mVs-1 scan rate are shown in Table S1. Figure 11a shows the charge/discharge curves obtained for different thicknesses of carbon films. The longer discharge time obtained for the moderately thin film (as compared to the thicker and thinnest film) causes the higher capacitance. This is due to the formation of an optimized pore structure and better penetration of electrolytes into the carbon film electrode CF2. Figure 11b shows the charge/discharge behaviour of the thinner film at different current densities. After loading different metal oxides into the carbon film, the charge/discharge behaviour is shown in Figure 11c. The increase in discharge time for metal-oxide-impregnated thin carbon films as compared to porous thin carbon films is attributed to the redox behaviour and faradic nature of the metal oxides.



Figure 11. Charge/discharge curve for (a) different thicknesses of carbon films (CF1, CF2 and CF3 electrodes) at 3 Ag-1. (b) For thinner film at different current density (c) for different metal oxide loading in thin carbon film (d) for cTiO2/MnO2 film.

The longer discharge time of the cMnO2 thin film as compared to the cTiO2 thin film is primarily due to better electrochemical behavior and higher specific capacitance. Furthermore, the addition of both metal oxides results in longer discharge times, which corresponds to the improved charge transport of MnO2 nanoparticles within a porous carbon matrix containing TiO2 nanoparticles. Figure 11d shows the charge/discharge behavior over a single cycle for a cTiO2/MnO2 thin film electrode at various current densities. The long-term cycling behavior at a 100 mVs-1 scan rate for the cTiO2/MnO2 thin film electrode is shown in Figure 12; there is 95 % retention of the specific capacitance after 1000 cycles for cTiO2/MnO2 thin film electrodes. However, during the first 400 cycles, the capacitance loss is greater than that during the last 500 cycles; this is attributed to the fact that the electrolyte ions are not fully penetrating into the porous structure of the electrodes. As the electrolytes penetrate fully into the pores of the carbon film electrode, the specific capacitance regains up to 95 % of its initial value after 1000 cycles.

100

Capactance retaintion (%)

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80 cTiO2/MnO2 electrode

60 40 20 0 0

200

400

600

800

1000

Number of cycle

Figure 12. Capacitance retention vs. number of cycles for cTiO2/MnO2 thin film at 100 mVs-1 for 1000 cycle. 26 ACS Paragon Plus Environment

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The electrochemical impedance spectroscopy (EIS analysis) was performed on carbon thin films to check the charge transfer and diffusion of electrolytes in the pores (shown in Figure S4). The small series resistance (Rs) and charge transfer resistance of CF2 as compared to CF1 and CF3 films confirm that enhanced diffusion and better transport of electrolyte ions in the supercapacitor cells. The capacitance obtained for the porous metal oxide/carbon thin films by spin coating here compares favourably or surpasses the existing benchmarks. For example, the capacitance values for thin films of carbon, cTiO2, cMnO2 and cTiO2/MnO2 based electrodes in literature and even some binder-free metal-oxide based electrodes are compared (shown in the Table 2).56, 57 Moreover, we have compared our results with literature data obtained for PAN polymer based supercapacitors which indicates these as-obtained porous films have huge potential.52, 55, 58 Interestingly, the composite porous films prepared using spin coating method showed better performances compared to other reported works as shown in Table 2.

Table 2. Comparison of specific capacitances of the reported thin films of porous carbon, TiO2, MnO2, and carbon/TiO2/MnO2-based electrodes. All the results reported here use neutral aqueous electrolytes in the three-electrode electrochemical system. Supercapacitor cell configuration

Activated porous carbon film/0.5 M Na2SO4/Pt Porous carbon film/1 M

Deposition method and conditions Carbonization of commercial aromatic polyimide films and activation Spin coating of PAN polymer

Film properties

Film thickness or specific mass/cm-2

7 µm thick film and shrink 200 nm after carbonization

Surface area 880 m2/g after 10 min activation

Multilayered 3D porous

~350-nm thin film

Specific capacitance (F g-1)

Reference

20

Lausevic et al.54

44

Present work 27

ACS Paragon Plus Environment


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Na2SO4/Pt

TiO2 thin film/1 M Na2SO4 /Pt

cTiO2/1 M Na2SO4/Pt Agarose gel/MnO2 /SS)/1 M/ Na2SO4 /Pt αMnO2/graphe ne /1M Na2SO4 /Pt

TiO2/ MnO2/1 M Na2SO4 /Pt

Simple and low cost successive ionic layer adsorption and reaction (SILAR) method Spin coating of PAN/TiO2 precursor

Dip coating

Hydrothermal method

Hydrothermal method

MnO2/graphen Redox reaction e/carbon in aqueous nanotubes/ 1 M Na2SO4 Na2SO4 /Pt

MnO2/TiO2/1 M Na2SO4/Pt

Coating based on hydrolysis

carbon thin films with average pore size 2 µm cracked and smooth morphology of film on which agglomerated small globular particles are overgrown Porous carbon film decorated with 60–70 nm TiO2 nanoparticles

Thickness of 0.66 µm/ 0.23 mg.cm-

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16

Deshmukh et al.59

178

Present work

100

Park et al.60

116

Chen et al.61

120

Zhang et al.62

2

~350-nm thin film

Porous MnO2 film containing macro and micropores

Specific surface area 255.33 m2 g

20-30 nm MnO2 nanotubes wraped by graphene sheet Mesoporous MnO2 nanosheets decorated on 1D TiO2 nanowires Sheet of graphene/multi wall carbon nanotubes and MnO2 nanoparticles Hollow diatom SiO2, TiO2 nanospheres and MnO2 mesoporous nanosheets

Specific mass 0.4 mg cm-2

−1

Specific surface area 57.8 m2 g−1

2.5 mg cm-2

Surface area of 29.96 m2 g−1

126

Deng et al.63

231.2

Guo et al.64


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cTiO2/MnO2/1 M Na2SO4/Pt

Spin coating of PAN/TiO2/Mn O2 precursor

TiO2 and MnO2 nanoparticles distributed throughout the porous films.

~350-nm thin film

297

Present work

4. CONCLUSIONS A simple, scalable, fast, and economical two-step method similar to the roll-to-roll process for the synthesis of multilayered macroporous polymers and carbon, well as the synthesis of carbon/TiO2, carbon/MnO2, and carbon/TiO2/MnO2 composite thin films has been demonstrated by employing a spin-coating technique. The process involves a polyacrylonitrile polymeric precursor followed by carbonization at 900 °C. Macroporosity in the film arises by virtue of selforganization after spin coating. The porosity is controllable and can be tailored by optimizing the parameters like film thickness, viscosity and solvent evaporation of the polymeric blend, which support the growth of instabilities and de-wetting of the sub-micron spin-coated polymer films. The technique proposed herein is a top-down approach for the synthesis of nanostructured materials, is superior to the conventional hydrothermal method, and is particularly useful for applications requiring thin films. The porous film allows the in-situ synthesis of carbon/TiO2/MnO2 nanocomposites; and the unique, interconnected, multilayered pores and pore walls facilitate higher metal-oxide nanoparticle loading. The synthesized TiO2 and MnO2 nanoparticles were transformed to the chemically and thermally most stable pure rutile and Mn3O4 phases, respectively. The as-synthesized porous carbon thin films were tested for their supercapacitor activity and found to be dependent on the thickness and porosity of the films. A maximum specific capacitance of ~44 Fg-1 was achieved with a 350-nm thick film with an average pore size around 2 µm. The as-prepared cTiO2, cMnO2, and cTiO2/MnO2 electrodes 29 ACS Paragon Plus Environment


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exhibit high specific capacitance of 178, 237, and 297 Fg−1, respectively at 5 mVs-1 which is significantly higher compared to the earlier reported thin film electrodes. MnO2 loading was also optimized and it was observed that the specific capacitance increases with increases in loading. This enhancement of the capacitance occurs because of the high surface area, unique porous nanostructures, and highly crystalline impregnated nanoparticles. This work provides a fundamental basis for the design and fabrication of high performance porous carbon composite electrode materials for a wide variety of thin film applications requiring loading of metal oxides and other functional nanomaterials. The coating process is compatible with the high speed rollto-roll processes for the manufacture of the thin film electrochemical and other devices. Supporting Information TGA analysis for the composite film, electrochemical impedance spectroscopy (EIS) and the XRD analysis of the pristine porous carbon film, and the calculation of areal and volumetric capacitances of all as-synthesized samples.This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. #

Both the authors contributed equally to this work.

Acknowledgements This work was supported by the DST Unit of Excellence on Soft Nanofabrication at Indian Institute of Technology Kanpur from the Department of Science and Technology (DST), New Delhi, India.


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REFERENCES (1)

Zhang, X.; Wang, X.; Su, J.; Wang, X.; Jiang, L.; Wu, H.; Wu, C., The effects of surfactant template concentration on the supercapacitive behaviors of hierarchically porous carbons. J. Power Sources 2012, 199, 402-408.

(2)

Jiang, H.; Lee, P. S.; Li, C., 3D carbon based nanostructures for advanced supercapacitors. Energy & Environ. Sci. 2013, 6, 41-53.

(3)

Yan, Y.; Cheng, Q.; Zhu, Z.; Pavlinek, V.; Saha, P.; Li, C., Controlled synthesis of hierarchical polyaniline nanowires/ordered bimodal mesoporous carbon nanocomposites with high surface area for supercapacitor electrodes. J. Power Sources 2013, 240, 544550.

(4)

Simon, P.; Gogotsi, Y., Materials for electrochemical capacitors. Nature Mater 2008, 7, 845-854.

(5)

Tao, J.; Liu, N.; Ma, W.; Ding, L.; Li, L.; Su, J.; Gao, Y., Solid-state high performance flexible supercapacitors based on polypyrrole-MnO2-carbon fiber hybrid Structure. Sci Rep 2013, 3, Artile No. 2286.

(6)

Whittingham, M. S., Materials challenges facing electrical energy storage. MRS Bulletin 2008, 33, 411-419.

(7)

Zhanga, L. L.; Zhao, X. S., Carbon-based materials as supercapacitor electrodes. Chem. Soc. Rev. 2009, 38, 2520-2531.

(8)

Cheng, Q.; Ma, J.; Zhang, H.; Shinya, N.; Qin, L.-C.; Tang, J., Electrodeposition of MnO2 on carbon nanotube thin films as flexible electrodes for supercapacitors. Trans Mater Res Soc Japan 2010, 35, 369-372.

(9)

Kim, D.-H.; Lu, N.; Ma, R.; Kim, Y.-S.; Kim, R.-H.; Wang, S.; Wu, J.; Won, S. M.; Tao, H.; Islam, A.; Yu, K. J.; Kim, T.-i.; Chowdhury, R.; Ying, M.; Xu, L.; Li, M.; Chung, H.J.; Keum, H.; McCormick, M.; Liu, P.; Zhang, Y.-W.; Omenetto, F. G.; Huang, Y.; Coleman, T.; Rogers, J. A., Epidermal electronics. Science 2011, 333, 838-843

(10)

Rogers, J. A.; Huang, Y., A curvy, stretchy future for electronics. PNAS 2009, 7, 10875– 10876.

(11)

Lipomi, D. J.; Vosgueritchian, M.; BenjaminC-K.Tee; Hellstrom, S. L.; Lee, J. A.; CourtneyH.Fox; Bao, Z., Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes. Nature Nantech. 2011, 6, 788-792. 31 ACS Paragon Plus Environment


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

(12)

Page 32 of 37

Someya, T.; Kato, Y.; Sekitani, T.; Iba, S.; Noguchi, Y.; Murase, Y.; Kawaguchi, H.; Sakurai, T., Conformable, flexible, large-area networks of pressure and thermal sensors with organic transistor active matrixes. PNAS 2005, 102, 12321-12325

(13)

Mondal, K.; Sharma, A., Recent advances in electrospun metal-oxide nanofiber based interfaces for electrochemical biosensing. RSC Advances 2016, 6, 94595-94616.

(14)

Li, D.; Zhou, H.; Honma, I., Design and synthesis of self-ordered mesoporous nanocomposite through controlled in-situ crystallization. Nature Mater 2004, 3, 65-72.

(15)

Raimondi, F.; Scherer, G. G.; Kotz, R.; Wokaun, A., Nanoparticles in energy technology: Examples from electrochemistry and catalysis. Angew. Chem. Int. Ed. 2005, 44, 21902209.

(16)

Jung, H. Y.; Karimi, M. B.; Hahm, M. G.; Ajayan, P. M.; Jung, Y. J., Transparent, flexible supercapacitors from nano-engineered carbon films. Sci Rep 2012, 2, (Article No. 773).

(17)

El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B., Laser scribing of high-performance and flexible graphene-based electrochemical capacitors. Science 2012, 335, 1326-1330

(18)

McCreery, R. L., Advanced carbon electrode materials for molecular electrochemistry. Chem. Rev. 2008, 108, 2646–2687.

(19)

Shchukin, D. G.; Caruso, R. A., Inorganic macroporous films from preformed nanoparticles and membrane templates: Synthesis and investigation of photocatalytic and photoelectrochemical properties. Adv. Funct. Mater. 2003, 13, 789–794.

(20)

Xie, X.; Zhang, C.; Wu, M.-B.; Tao, Y.; Lvac, W.; Yang, Q.-H., Porous MnO2 for use in a high performance supercapacitor: replication of a 3D graphene network as a reactive template. Chem. Commun. 2013, 49, 11092-11094.

(21)

Ghodbane, O.; Pascal, J.-L.; Favier, F., Microstructural effects on charge-storage properties in MnO2-based electrochemical supercapacitors. ACS Appl. Mater. Interfaces 2009, 1, 1130–1139.

(22)

Li, G.-R.; Feng, Z.-P.; Ou, Y.-N.; Wu, D.; Fu, R.; Tong, Y.-X., Mesoporous MnO2/carbon aerogel composites as promising electrode materials for high-performance supercapacitors. Langmuir 2010, 26, 2209–2213.

(23)

Devaraj, S.; Munichandraiah, N., Effect of crystallographic structure of MnO2 on its electrochemical capacitance properties. J. Phys. Chem. C 2008, 112, 4406–4417. 32 ACS Paragon Plus Environment

Page 33 of 37

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


(24)

Hou, Y.; Cheng, Y.; Hobson, T.; Liu, J., Design and synthesis of hierarchical MnO2 nanospheres/carbon

nanotubes/conducting polymer

ternary composite

for

high

performance electrochemical electrodes. Nano Lett. 2010, 10, 2727–2733. (25)

Huang, H.; Wang, X., Graphene nanoplate-MnO2 composites for supercapacitors: a controllable oxidation approach Nanoscale 2011, 3, 3185-3191.

(26)

SainanYang; Cheng, K.; Huang, J.; Ye, K.; Xu, Y.; Cao, D.; Wang, X. Z. G., Highcapacitance MnO2 nanoflakes on preformed C/TiO2 shell/core nanowire arrays for electrochemical energy storage Electrochimia Acta 2014, 120, 416-422.

(27)

Yang, P.; Xiao, X.; Li, Y.; Ding, Y.; Qiang, P.; Tan, X.; Mai, W.; Lin, Z.; Wu, W.; Li, T.; Jin, H.; Liu, P.; Zhou, J.; Wong, C. P.; Wang, Z. L., Hydrogenated ZnO core–shell nanocables for flexible supercapacitors and self-powered systems. ACS Nano 2013, 7, 2617–2626.

(28)

Lei, Z.; Shi, F.; Lu, L., Incorporation of MnO2-coated carbon nanotubes between graphene sheets as supercapacitor electrode. ACS Appl. Mater. Interfaces 2012, 4, 1058– 1064.

(29)

Jiang, H.; Ma, J.; Li, C., Mesoporous carbon Incorporated metal oxide nanomaterials as supercapacitor electrodes. Adv. Mater. 2012, 24, 4197–4202.

(30)

Huang, Y.-G.; Zhang, X.-H.; Chen, X.-B.; Wang, H.-Q.; Chen, J.-R.; Zhong, X.-X.; Li, Q.-Y., Electrochemical properties of MnO2-deposited TiO2 nanotube arrays 3D composite electrode for supercapacitors. Int. J. Hydrogen Energy 2015, 40, 14331-14337.

(31)

Moon, G. D.; Joo, J. B.; Dahl, M.; Jung, H.; Yin, Y., Nitridation and Layered Assembly of Hollow TiO2 Shells for Electrochemical Energy Storage. Advanced Functional Materials 2013, 24, 848-856.

(32)

Cheng, Q.; Tang, J.; Ma, J.; Zhang, H.; Shinya, N.; Qin, L.-C., Graphene and nanostructured MnO2 composite electrodes for supercapacitors. Carbon 2011, 49, 2917– 2925.

(33)

Lee, J.; Jung, Y. S.; Warren, S. C.; Kamperman, M.; Oh, S. M.; DiSalvo, F. J.; Wiesner, U., Direct access to mesoporous crystalline TiO2/carbon composites with large and uniform pores for use as anode materials in lithium ion batteries. Macromol. Chem. Phys. 2011, 212, 383–390.



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

(34)

Page 34 of 37

Mondal, K.; Kumar, J.; Sharma, A., Self-organized macroporous thin carbon films for supported metal catalysis. Colloids and Surfaces A: Physicochem. Eng. Aspects 2013, 427, 83-94.

(35)

Mondal, K.; Kumar, J.; Sharma, A., TiO2-nanoparticles-impregnated photocatalytic macroporous carbon films by spin coating. Nanomater. Energy 2013, 2, 121 –133.

(36)

Li, L.; Chen, C.; Xie, J.; Shao, Z.; Yang, F., The preparation of carbon nanotube/MnO2 composite fiber and its application to flexible micro-supercapacitor. J. Nanomater. 2013, Volume 2013, (Article ID: 821071 ), 1-5.

(37)

Bezrodna, T.; Gavrilko, T.; Puchkovska, G.; Shimanovska, V.; Baran, J.; Marchewka, M., Spectroscopic study of TiO2 (rutile)–benzophenone heterogeneous systems. J. Molecular Structure 2002, 614, 315–324.

(38)

Gao, T.; Fjellvåg, H.; Norby, P., A comparison study on Raman scattering properties of α and β-MnO2. Anal. Chimica Acta 2009, 648, 235-239.

(39)

Tian, Z.-Y.; Kouotou, P. M.; Bahlawane, N.; Hervé, P.; Ngamou, T., Synthesis of the catalytically active Mn3O4 spinel and its thermal properties. J.Phys.Chem.C 2013, 117, 6218−6224.

(40)

Hou, C.; Lang, X.-Y.; Han, G.-F.; Li, Y.-Q.; Zhao, L.; Wen, Z.; Zhu, Y.-F.; Zhao, M.; Li, J.-C.; Lian, J.-S.; Jiang, Q., Integrated solid/nanoporous copper/oxide hybrid bulk electrodes for high-performance lithium-ion batteries. Sci. Rep. 2013, 3, (Article No. 2878), 1-7.

(41)

Bussamara, R.; Melo, W. W. M.; Scholten, J. D.; Migowski, P.; Marin, G.; Zapata, M. J. M.; Machado, G.; Teixeira, S. R.; Novak, M. A.; Dupont, J., Controlled synthesis of Mn3O4 nanoparticles in ionic liquids. Dalton Trans. 2013, 42, 14473–14479.

(42)

Ferrari, A. C.; Robertson, J., Resonant Raman spectroscopy of disordered, amorphous, and diamondlike carbon. Phys. Rev. B 2001, 64, Article No. 075414.

(43)

Ha, T. A.; Tran, V. M.; Le, M. L. P., Nanostructured composite electrode based on manganese dioxide and carbon vulcan–carbon nanotubes for an electrochemical supercapacitor Adv. Nat. Sci: Nanosci. Nanotechnol. 2013, 4, No. 035004.

(44)

Yan, J.; Wu, G.; Guan, N.; Li, L.; Li, Z.; Cao, X., Understanding the effect of surface/bulk defects on the photocatalytic activity of TiO2: anatase versus rutile. Phys.Chem.Chem.Phys. 2013, 15, 10978-10988. 34 ACS Paragon Plus Environment

Page 35 of 37

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


(45)

Cui, X.-L.; Li, Y.-L.; Li, S.-Y.; Sun, G.-C.; Ma, J.-X.; Zhang, L.; Li, T.-M.; Ma, R.-B., Mn3O4 nano-sized crystals: Rapid synthesis and extension to preparation of nanosized LiMn2O4 materials. J. Chem. Sci. 2014, 126, 561–567.

(46)

Devaraj,

S.;

Munichandraiah,

N.,

Electrochemical

supercapacitor

studies

of

nanostructured α-MnO2 synthesized by microemulsion method and the effect of annealing. J. Electrochem. Soc. 2007, 154, A80-A88. (47)

Ghazzal, M. N.; Chaoui, N.; Genet, M.; Gaigneaux, E. M.; Robert, D., Effect of compressive stress inducing a band gap narrowing on the photoinduced activities of solgel TiO2 films. Thin Solid Films 2011, 520, 1147-1154.

(48)

Biesinger, M. C.; Payne, B. P.; Grosvenor, A. P.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. C., Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni. Applied Surface Science 2011, 257, 27172730.

(49)

Sharma, R. K.; Rastogi, A. C.; Desu, S. B., Manganese oxide embedded polypyrrole nanocomposites for electrochemical supercapacitor. Electrochimica Acta 2008, 53, 7690-7695.

(50)

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. Electrochem. Acta 2011, 56, 7124–7130.

(51)

Garcia-Gomez, A.; Barranco, V.; Moreno-Fernandez, G.; Ibañez, J.; Centeno, T. A.; Rojo, J. M., Correlation between capacitance and porosity in microporous carbon monoliths. J. Phys. Chem. C 2014, 118, 5134-5141.

(52)

Ra, E. J.; Raymundo-Pinero, E.; Lee, Y. H.; Beguin, F., High power supercapacitors using polyacrylonitrile-based carbon nanofiber paper. Carbon 2009, 47, 2984-2992.

(53)

Lu, X.; Yu, M.; Wang, G.; Zhai, T.; Xie, S.; Ling, Y.; Tong, Y.; Li, Y., HTiO2@MnO2//H-TiO2@C core–shell nanowires for high performance and flexible asymmetric supercapacitors. Adv. Mater. 2013, 25, 267-272.

(54)

Lausevic, Z.; Apel, P. Y.; Krstic, J. B.; Blonskaya, I. V., Porous carbon thin films for electrochemical capacitors. Carbon 2013, 64, 456-463.

(55)

Kim, C.; Yang, K. S., Electrochemical properties of carbon nanofiber web as an electrode for supercapacitor prepared by electrospinning. Appl. Phys. Lett. 2003, 83, 1216-1218. 35 ACS Paragon Plus Environment


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

(56)

Page 36 of 37

Chi, H. Z.; Li, Y.; Xin, Y.; Qin, H., Boron-doped manganese dioxide for supercapacitors. Chem. Commun. 2014, 50, 13349-13352.

(57)

Park, S.; Shim, H.-W.; Lee, C. W.; Song, H. J.; Park, I. J.; Kim, J.-C.; Hong, K. S.; Kim, D.-W., Tailoring uniform γ-MnO2 nanosheets on highly conductive three-dimensional current collectors for high-performance supercapacitor electrodes. Nano Research 2014, 8, 990-1004.

(58)

Liu, T.; Sreekumar, T. V.; Kumar, S.; Hauge, R. H.; Smalley, R. E., SWNT/PAN composite film-based supercapacitors. Carbon 2003, 41, 2440-2442.

(59)

Deshmukh, P. R.; Lokhande, C. D., SILAR deposited TiO2 thin film for supercapacitor application. AIP Conference Proceedings 2013, 1536, 787-788.

(60)

Park, S.; Nam, I.; Kim, G.-P.; Han, J. W.; Yi, J., Hybrid MnO2 Film with Agarose Gel for Enhancing the Structural Integrity of Thin Film Supercapacitor Electrodes. ACS Applied Materials & Interfaces 2013, 5, 9908-9912.

(61)

Chen, C.; Fu, W.; Yu, C., A facile one-step hydrothermal method to produce αMnO2/graphene sheet composites and its electrochemical properties. Materials Letters 2012, 82, 133-136.

(62)

Zhang, Y. X.; Kuang, M.; Hao, X. D.; Liu, Y.; Huang, M.; Guo, X. L.; Yan, J.; Han, G. Q.; Li, J., Rational design of hierarchically porous birnessite-type manganese dioxides nanosheets on different one-dimensional titania-based nanowires for high performance supercapacitors. J. Power Sources 2014, 270, 675-683.

(63)

Deng, L.; Hao, Z.; Wang, J.; Zhu, G.; Kang, L.; Liu, Z.-H.; Yang, Z.; Wang, Z., Preparation and capacitance of graphene/multiwall carbon nanotubes/MnO2 hybrid material for high-performance asymmetrical electrochemical capacitor. Electrochimica Acta 2013, 89, 191-198.

(64)

Guo, X. L.; Kuang, M.; Li, F.; Liu, X. Y.; Zhang, Y. X.; Dong, F.; Losic, D., Engineering of three dimensional

(3-D) diatom@TiO2@MnO2

composites

with

enhanced

supercapacitor performance. Electrochimica Acta 2013, 190, 159-167.


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Graphic for Manuscript

We demonstrate an easy, scalable, two step synthesis method similar to the roll-to-roll process for the synthesis of multilayered of macroporous carbon, carbon/TiO2 (cTiO2), carbon/MnO2 (cMnO2), and carbon/TiO2/MnO2 (cTiO2/MnO2) composite thin films for energy storage applications.


Metal-Oxide Decorated Multilayered Three-Dimensional (3D) Porous

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