release by hybrids of poly(3,4-ethylenedioxypyrrole) with Fe3O4 nanostructures or Co3O4 nanorods

Accepted Manuscript Effective Pseudocapacitive Charge Storage/Release by Hybrids of Poly(3,4ethylenedioxypyrrole) with Fe3O4 Nanostructures or Co3O4 Nanorods B Narsimha Reddy, Sathish Deshagani, Melepurath Deepa, Partha Ghosal PII: DOI: Reference:

S1385-8947(17)31980-0 https://doi.org/10.1016/j.cej.2017.11.068 CEJ 18039

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

1 August 2017 11 November 2017 13 November 2017

Please cite this article as: B.N. Reddy, S. Deshagani, M. Deepa, P. Ghosal, Effective Pseudocapacitive Charge Storage/Release by Hybrids of Poly(3,4-ethylenedioxypyrrole) with Fe3O4 Nanostructures or Co3O4 Nanorods, Chemical Engineering Journal (2017), doi: https://doi.org/10.1016/j.cej.2017.11.068

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Effective Pseudocapacitive Charge Storage/Release by Hybrids of Poly(3,4ethylenedioxypyrrole) with Fe3O4 Nanostructures or Co3O4 Nanorods B Narsimha Reddya,b, Sathish Deshagania, Melepurath Deepaa,*, Partha Ghosalc a

Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi-502285, Sangareddy, Telangana (India) b

Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi -110016 (India)

c

Defence Metallurgical Research Laboratory, Defence Research & Development Organisation (DRDO), Hyderabad-500058, Telangana (India).

Abstract The synergy between two redox materials, the high electrical conductivity and the chemically robust durable structure of poly(3,4-ethylenedioxypyrrole) (PEDOP) combined with the high theoretical capacities, cost-effectiveness and abundance of M3O4 (M: Fe or Co) type oxides is exploited. Hybrid films of PEDOP@Co3O4 nanorods (NRs) and PEDOP@Fe3O4 nanostructures (NSs) are fabricated over flexible carbon (C)-fabric substrates for the very first time. A symmetric supercapacitor cell based on the PEDOP@Fe3O4 NSs hybrid outperforms the remaining cells based on pristine oxides, polymer and even PEDOP@Co3O4 NRs. The PEDOP@Fe3O4 NSs hybrid based cell delivers a high specific capacitance of 673 F g-1 at 1 A g-1 with 83% retention after 5000 cycles. Additionally, it exhibits a wide voltage window of 1 V and an extraordinarily high energy density of 93 Wh kg-1 at a power density of 0.5 kW kg-1. Conducting atomic force microscopy studies reveal that the nanoscale (high) current flowing domains are uniform and almost seamless across the film of PEDOP@Fe3O4 NSs. In contrast, the insulating or low current flowing domains predominate the surfaces of PEDOP and 1

PEDOP@Co3O4 NRs. The nano-level electrical conductivity for the PEDOP@Fe3O4 NSs film is also seven-fold times higher than that of PEDOP and PEDOP@Co3O4 NRs films. The reasons for enhanced electrochemical responses of the hybrids compared to the pristine oxides are explained. The porous morphology, enhanced electrical conduction, and lower ion-diffusion resistance offered by the PEDOP@Fe3O4 NSs film enables improved charge transfer and transport, thus manifesting in a good rate response, high energy density, and acceptable endurance parameters. A real time application of an illumination of green light emitting diode with three such cells also highlights the practical viability of this hybrid. Keywords: Conducting polymer; Poly(3,4-ethylenedioxypyrrole); Metal oxide; Supercapacitor; Specific capacitance *Corresponding author. M. Deepa: Email: [email protected]; Tel: +91-40-23016024 Note: B. N. Reddy. aWork done at IIT Hyderabad, and bpresent address.

Introduction Supercapacitors based on conducting polymer hybrids or conducting polymer nanostructures rely on storing and releasing charge by undergoing redox reactions [1,2]. In such cells, the nanostructured morphology of the polymer provides (i) an enhanced effective surface area (for the same geometric area), and (ii) an increased electrical conductivity as well in comparison to the bulk polymer particles, which increases the ion-uptake capability of the electrodes and thus improves the specific capacitance (SC) of the cell. In hybrids of conducting polymers with transition metal oxides like MnO2, both the components contribute to ion storage via Faradaic charge transfer reactions [2,3]. Similarly for hybrids of the polymer with carbon nanostructures like reduced graphene oxide (RGO) [4], RGO accumulates charge by an electrical double layer formation. As a consequence, the SC for the hybrid is enhanced relative to either that achieved 2

for the pristine conducting polymer or the oxide or the carbonaceous entity. Typically, for such cells, specific capacitances (SCs) of about 200 F g-1 or more are generally reported at low scan rates of the order of 1-5 mV s-1 or at low current densities (less than 1 A g-1) [5-7]. However, by the use of some novel electrode architectures, many groups have reported high capacitances at high current densities. They have also shown the retention of such high capacities upon longterm cycling. Some notable examples from literature are enumerated here. PANI-modified oriented graphene hydrogel (OGH) films were employed as freestanding electrodes in flexible solid-state supercapacitors [8]. The cell exhibited a SC of 530 F g-1, keeping 80% of its original value up to 10,000 charge-discharge cycles at a current density of 10 A g-1. The energy density of the cell was 11.3 Wh kg-1 at a power density of 9.6 kW kg-1. The voltage window was 0.8 V for a single cell, and for three cells connected in series, it was 2.4 V [8]. A combination of vertical PANI nanowire arrays and nitrogen plasma etched carbon fiber cloths (eCFC) yielded 3D nanostructured PANI/eCFC hybrids [9]. A two-electrode flexible supercapacitor based on PANI/eCFC delivered a high SC (1035 F g-1 at a current density of 1 A g-1), good rate capability (88% capacity retention at 8 A g-1), and long-term cycle life (10% capacity loss after 5000 cycles). [9]. New type of 3D porous and thin graphite foams (GF) were used as substrates for the growth of metal oxide core/shell nanowire arrays to form integrated electrodes [10]. The nanowire core was Co3O4, and the shell was a hybrid of a conducting polymer (poly(3,4-ethylenedioxythiophene), PEDOT) and a metal oxide (MnO2). The resulting supercapacitor exhibited a SC of 400 F g-1 at a high current density of 5 A g-1, an energy density of ~10 Wh kg-1, and a power density of 20 W kg-1, which is 2-3 times greater than the values generally delivered by commercial carbon based supercapacitors (3-4 Wh kg-1) [10]. A graphene oxide/bacterial cellulose hybrid coated with poly(pyrrole) exhibited a high volumetric

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capacitance (278 F cm-3) along with 95.2% retention of 556 F g-1 capacitance over 5000 recycling tests [11]. Composites of carbon or sole carbon nanostructures, such as NiCo2O4/graphene [12], nitrogen-doped porous carbon prepared by the carbonization of poly(ophenylenediamine) [13] and MnOx nanowires wrapped by nitrogen-doped carbon layers [14] have also delivered outstanding capacitances and good cycling stabilities in the past. The above described brief literature survey clearly shows that by use of innovative electrode architectures based on hybrid materials, supercapacitors capable of delivering specific capacitances in the range of 500 to 1000 F g-1, at nominal scan rates or current densities can be developed. Considering the ever-increasing usage of portable electronic devices, such power sources are the need of the hour. The charging/discharging times are typically of the order of a few tens of seconds or a few minutes, which is acceptable for supercapacitors. Other advantages of conducting polymers are: low cost and the fact that their synthesis requires no elevated temperature, or inert atmosphere or any capital equipment. While PEDOT has been extensively used in the past for supercapacitors [1-4,7,10], but its’ less used pyrrole analogue, namely, poly(3,4-ethylenedioxypyrrole) or PEDOP has rarely been used for electrochemical storage, except for a few reports by our group [15-17]. Asymmetric supercapacitors were constructed using PEDOP or PEDOP/Bi2S3 hybrid and graphite (Gr) as working and counter electrodes [15]. The SC of the hybrid based cell (201 F g-1) was found to be 3.58 times greater than that of the pristine polymer (56 F g-1), at the same current density of 1 A g-1. Similarly, hybrids of PEDOP enwrapped Sb2S3 nanorods delivered a SC of 1008 F g-1 at 1 A g-1, but the cycling stability was monitored only for 1000 cycles [16]. PEDOP/V2O5 nanobelts hybrid-Gr cell gave a SC of 224 F g-1 in comparison to a SC of ~37 F g-1 achieved for the pristine V2O5 based cell [17].

4

PEDOP compared to PEDOT, offers a lower oxidation potential, is extremely stable in the doped state, and is also characterized by a high electrical conductivity in the oxidized form, thus rendering it to be an ideal material as an electrode for supercapacitors [18,19]. Cobalt oxide (Co3O4) and iron oxide (Fe3O4) have been used in the past as electrodes in supercapacitors and batteries, due to their low cost, natural abundance, high theoretical capacitances (3650 and 2299 F g-1 respectively) and their environmentally benign nature [20-22]. However, their SCs are limited by the redox performance of the oxide. Therefore, by combining these oxides with a chemically robust, electrochemically active, electrically conducting, and a highly durable conducting polymer like PEDOP, the ensuing hybrids are expected to deliver stable, high-rate pseudocapacitive responses. Until now, no reports exist on the preparation and characterization of PEDOP@Co3O4 and PEDOP@Fe3O4 hybrids. To bridge this gap, and also with the objective to develop easily processable high performance psuedocapacitors, here we present the fabrication of hybrid films of PEDOP@Co3O4 nanorods (NRs) and PEDOP@Fe3O4 nanostructures (NSs) over light-weight flexible substrates (Carbon (C)-fabric). Symmetric supercapacitor cells are assembled with the pristine oxides, sole polymer and hybrids for a meaningful comparison of their electrochemical properties. The greatly enhanced performance parameters of the hybrid films, in comparison to the pristine polymer or oxide films have been assessed by electrochemical methods and elucidated by conducting atomic force microscopy (C-AFM) studies. This study shows that PEDOP hybrid based cells, and particularly the one based on PEDOP/Fe3O4 NSs offers high specific energy, low internal resistance, long operational lifetimes, and a wide voltage window, thereby rendering it suitable as a power source for small scale electronic devices. Experimental

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Chemicals Cobalt

nitrate (Co(NO3)2.6H2O), urea, sodium thiosulfate (Na2S2O3.5H2O), cetyl

trimethylammonium bromide (CTAB), lithium perchlorate (LiClO4), 3,4-ethylenedioxypyrrole (EDOP) (2% w/v in tetrahydrofuran), poly(methyl methacrylate) (PMMA, average MW: 996000), poly(vinylidene fluoride) (PVdF, average MW: 534000), ferrous chloride (FeCl2.4H2O) were procured

from Sigma-Aldrich and

used

directly.

Propylene carbonate (PC),

poly(ethyleneglycol) (PEG, average Mw: 4000), sodium hydroxide (NaOH), N-methyl pyrrolidone (NMP) were purchased from Merck. . Ultrapure water (resistivity ∼18.2 MΩ cm) was obtained through Millipore Direct-Q3 UV system. Carbon (C)-fabric of 0.2 mm thickness was purchased from Alibaba Pvt. Ltd. Synthesis of Co3O4 nanorods (NRs), Fe3O4 nanostructures (NSs) and NRs Co3O4 NRs were hydrothermally synthesized using a previously reported method [23]. In a typical procedure, 20 mmol Co(NO3)2.6H2O, 10 mmol urea, and 1 g of CTAB were dissolved in deionized water (100 mL) at room temperature by magnetic stirring. The mixture was stirred vigorously for 10 min., until a transparent red solution was formed. It was transferred into a Teflon-lined autoclave, which was filled up to 80% of its’ total volume, sealed, and heated at 120 o

C for 24 h. The precipitate obtained was allowed to cool down to room temperature. The

precipitate was washed with distilled water and ethanol several times, and then dried at 50 oC under vacuum. Co3O4 NRs, were obtained by thermal treatment of the above reaction mixture at 200 oC for 3 h in a muffle furnace. For Fe3O4 NSs [24], 1.6 g of FeCl2.4H2O and 1.0 g Na2S2O3.5H2O were dissolved in 20 mL of deionized water in a bottle. Then, 5.6 g of PEG and 1 g of NaOH (~1.25 M) were added successively into the solution under stirring, resulting in a dark-green slurry. The bottle was then 6

sealed and heated to 120 oC in an electric oven for 20 h. The resulting precipitate of Fe3O4 NSs was isolated by centrifugation and washed thoroughly with water and dried at 80 oC. The same procedure was slightly modified and Fe3O4 NRs were also prepared. For Fe3O4 NRs, except for the amounts of PEG and NaOH, all other precursor quantities and methodology were used as stated above. Fe3O4 NRs were prepared from a solution (volume: 20 mL) containing 6 g of PEG and 4.8 g of NaOH (~6 M). Fabrication of electrodes and cells EDOP (0.1 M) and LiClO4 (0.1 M) were dissolved in acetonitrile (30 mL). To this monomer bath, Co3O4 NRs (1.5 g) or Fe3O4 NSs (1.5 g) or Fe3O4 NRs (1.5 g) were added to the monomer bath and the solution was sonicated for 30 min. Carbon fabric (C-fabric) with an active area of 1 cm2, was employed as the working electrode. A Pt sheet was used as the counter electrode and an Ag/AgCl/KCl electrode was used as the reference electrode. Hybrid films of PEDOP@Co3O4 NRs or PEDOP@Fe3O4 NSs or PEDOP@Fe3 O4 NRs were obtained under potentiostatic conditions at room temperature in chronoamperometric mode by application of +1.0 V to the working electrode for 500 s. Pristine PEDOP films were also prepared by using the same monomer-salt bath without the NRs or the NSs. Co3O4 NRs films were obtained by drop casting on a C-fabric, a slurry of 10 mg of Co3O4 NRs powder, 100 µL of NMP, 5% PVdF which was sonicated for 20 min. The film was dried at 80 oC for 2 h. By replacing the Co3O4 NRs powder with the Fe3O4 NSs powder, and by using the same procedure, films of Fe3O4 NSs were also obtained. A quasi-solid polymer electrolyte was synthesized by dissolving PMMA (0.6 g) in the 1 M LiClO4/PC (4 g) by continuous stirring at 80 oC for 5-6 h until a transparent viscous gel was obtained which had little flow properties when cooled to room temperature. A borosilicate glass fiber membrane was wetted with the gel electrolyte. It also functioned as a separator. It was

7

sandwiched between two identical electrodes (e.g. PEDOP@Fe3O4 NSs/C-fabric) and the whole configuration was held together using binder clips and placed in an online vacuum oven for 1 h, and then used for characterization. Instrumentation techniques X-ray diffraction patterns were recorded on a PANalytical, X’PertPRO instrument with CuKα (λ = 1.5406 Å) radiation. Raman spectra were recorded for the active materials on a Bruker Senterra Dispersive Raman Microscope spectrometer. A laser excitation wavelength of 532 nm was used. Surface morphology analysis was performed using a field emission scanning electron microscope (Carl Zeiss Supra 40 FE-SEM). Atomic force microscopy (AFM), and conductingAFM (C-AFM) measurements were performed on the electrodes using a Bruker (Veeco) Multimode 8 with ScanAsyst (Nanoscope 8.10 software) microscope. The conductive probes were coated with Pt/Ir on front and back sides. The probe tip had a radius of 10 nm, spring constant of 0.2 N cm-2, a current sensitivity of 1 nA V-1 and a load force of 50 nN was maintained between the tip and the sample. The sample was deposited as a thin layer over a stainless steel (SS) foil, which in turn was affixed on a SS disk with conducting carbon tape. A strip of pin-hole free silver paste was used for taking contacts. The C-AFM cantilever scans the surface while in contact and both the topography and the current flowing through the sample are imaged at the same time. Contact topography image is produced by using the feedback loop to maintain a constant tip deflection and the current image is generated by measuring the current flow. A 50 mV bias was applied to the tip during imaging. Transmission electron microscopy (TEM) images were obtained on a JEOL 2100 microscope operating at an accelerating voltage of 200 kV by using samples deposited over carbon coated copper grids. Brunauer-Emmett-Teller (BET) surface area analysis and Barrett-Joyner-Halenda (BJH) pore size and volume analysis 8

were performed using Micromeritics ASAP 2020 Quantachrome instrument under nitrogen at 77.3 K, after degassing at 300 °C for 4 h. Galvanostatic charge-discharge measurements were performed on a battery testing unit (Arbin Instruments, BT 2000) at different current densities in different voltage ranges for two-electrode cells. Specific capacitance (F g-1) was determined from the galvanostatic charge/discharge curves. Electropolymerization, cyclic voltammetry and electrochemical impedance spectroscopic (EIS) studies were performed on an Autolab PGSTAT 302N potentiostat-galvanostat-frequency response analyzer. Results and discussion Structural studies The XRD patterns of Co3O4 NRs and Fe3O4 NSs are shown in Figure 1(a and b). The pattern of Co3O4 NRs reveals prominent peaks at 2θ = 19o, 31.3o, 36.9o, 44.9o, 59.5o and 65.3o, which concur with inter-planar spacings (d) of 4.67, 2.86, 2.44, 2.02, 1.55, and 1.43 Å. These d-values correspond to hkl planes of (111), (220), (311), (400), (511) and (440) of Co3O4 with a face centered cubic (fcc) lattice, as per powder diffraction file (PDF): 781970. The diffractogram of Fe3O4 NSs shows multiple intense peaks at 2θ = 31o, 35.6o, 49.4o, 54o, 62.6o, 64.1o, and 71o, that align with d values of 2.95, 2.51, 1.9, 1.7, 1.48, 1.4 and 1.32 Å. The peaks correspond to the (220), (311), (331), (422), (440), (531) and (620) planes of the fcc crystal structure of Fe3O4 (PDF: 653107). The current versus time transients for the electropolymerization of pristine polymer (PEDOP), and its’ co-electrodeposition with Co3O4 NRs and Fe3O4 NSs are shown in Figure 1c. The medium is composed of 0.1 M EDOP and 0.1 M LiClO4 dissolved in acetonitrile, with additional oxides, in case of the hybrids. C-fabric serves as the working electrode and has an

9

active electrode area of 1 cm2. In all the three cases, at t = 0 s, upon application of a constant dc voltage of +1 V to the C-fabric, the current shows a spike. This high initial current is a double layer charging current due to the accumulation of dopant ions (triflate or CF3SO3-) from the supporting electrolyte at the C-fabric/solution interface. It is followed by an exponential decay, and thereafter the current saturates, and acquires an almost plateau like behavior for the remainder of the deposition period (till 500 s). This current is a Faradaic current corresponding to the oxidation of the monomer EDOP, followed by its oligomerization, and polymerization onto the C-fabric. The average currents in this regime are 0.025, 0.083 and 0.11 mA for PEDOP, PEDOP@Co3O4 NRs and PEDOP@Fe3O4 NSs respectively. The currents are 3.3 and 4.4 times greater for the hybrid films of PEDOP@Co3O4 NRs and PEDOP@Fe3O4 NSs, relative to the pristine polymer film. During electrodeposition, the applied electric bias acts as a driving force and the Co3O4 NRs and Fe3O4 NSs are dragged alongwith the EDOP oligomers to the C-fabric electrode. The oxide and the polymer are co-deposited on the electrode and the schematics in Scheme 1(a and b) illustrate the processes. Raman spectra of Co3O4 NRs, Fe3O4 NSs and PEDOP, obtained at an excitation wavelength of 532 nm are presented in Figure 2(a-c). The Raman spectrum of Co3O4 NRs (Figure 2a) shows distinct peaks at 193, 466, 514 and 615 cm-1, followed by a strong intense peak at 670.5 cm-1. These peaks are assigned to the F2g, Eg, F2g, F2g and A1g modes of Co3O4. In a previous report, the Raman modes for a single crystal of Co3O4 were observed at 194 (F2g), 488 (Eg), 522 (F2g), 618 (F2g) and 691 (A1g) cm-1 respectively, when an excitation wavelength of 514.5 nm was employed [25]. In another study, for Co3O4 nanoparticles prepared by the thermal decomposition of a Co-complex, Raman active modes were observed at 468, 510, 605 and 670 cm-1 [26]. Authors attributed the downshift of the modes to lower wavenumbers by nearly 20 cm-1,

10

compared to bulk Co3O4, to the optical phonon confinement effect in nanostructured materials, which induces uncertainty in the phonon vectors and is responsible for the downshift of the Raman peaks. In yet another report, the fact that Raman scattering is sensitive to the microstructure of nanocrystalline materials was confirmed. For hollow Co3O4 spheres, five Raman peaks at 184, 465, 510, 602, and 669 cm-1 were obtained [27]. Similarly, for Co3O4 nanorods prepared by a hydrothermal route, five unique Raman peaks were located at 194, 475, 516, 613, and 680 cm-1 respectively [28]. In another study on mesoporous Co3O4 nanoflakes, Raman peaks were observed at 194, 475, 518, 617 and 682 cm-1 [29]. The peaks in all the aforementioned studies [26-29] were downshifted compared to bulk Co3O4, and these downshifts were attributed to the size and morphology of the nanostructure. This corroborates with our observations. The Raman spectrum of Fe3O4 NSs (Figure 2b) reveals two weak peaks at 303 and 540 cm-1 and one prominent peak at 669 cm-1, which originate from the Eg, T2g and A1g modes respectively. Previously, for natural magnetite (Fe3O4), Raman bands were observed at 193, 306, 538, 666 cm-1 [30]. For Fe3O4 NSs prepared herein, except for the 193 cm-1 mode, which was not observed, the remaining Raman peak positions concur reasonably well with the peaks observed by the authors, thus confirming the formation of Fe3O4. The Raman spectrum of PEDOP (Figure 2c) shows multiple strong peaks at: 540, 1135 and 1200 cm-1, which are attributed to the oxyethylene ring deformation, the C-O-C deformation mode and a combination of the Cα-Cα′ inter-ring stretching and Cβ-H bending modes respectively. A doublet like peak is observed at 1405/1443 cm-1, followed by a strong peak at 1650 cm-1. They are attributed to the symmetric Cα=Cβ(-O) stretching vibration and C-NH deformation (from the pyrrole ring of PEDOP) mode respectively. These spectral assignments confirm the successful formation of PEDOP. The above assignments were made by matching our 11

peak positions with the experimental Raman data of PEDOT performed by Garreau et al [31]. In their work, Raman peaks were observed at 571, 1111, 1226 and 1431 cm-1. These peaks were ascribed to the δ(-O-CH2-CH2-O-)ring, δ(C-O-C), ν(Cα-Cα′) + δ(Cβ-H) and ν(Cα=Cβ(-O)) modes in PEDOT respectively. The differences in the peak positions between PEDOT and PEDOP arise from the difference in the electronegativity of the hetero-atom, sulfur in the case of PEDOT, and –NH in PEDOP. Figure 2(d and e) shows the TEM images of PEDOP@Co3O4 NRs and PEDOP@Fe3O4 NSs. The micrograph of the PEDOP@Co3O4 NRs hybrid shows juxtaposed polymer particles of irregular shapes and elongated rod like structures of Co3O4, indicating a good mixing of the two components. The corresponding selected area electron diffraction (SAED) pattern (Figure 2d′) shows bright spots superimposed over diffuse concentric rings, indicating the polycrystalline nature of the composite. The spots are indexed to the (400) and (331) planes of the fcc lattice of Co3O4, which match with the XRD findings. The image of PEDOP@Fe3O4 NSs hybrid shows hexagonal shapes of Fe3O4 embedded in layers of polymer, which is relatively featureless. The SAED pattern (Figure 2e′) is characterized by spots which are assigned to the (220), (331) and (422) reflections of Fe3O4 with a fcc structure. The FE-SEM images of Fe3O4 NSs, Co3O4 NRs, PEDOP and hybrid films are displayed in Figure 3. The images of Fe3O4 NSs (Figure 3a and b) show clusters of hexagonal and pyramidal shaped particles and some irregular shaped particles as well. The corresponding topography image (Figure 3c) also shows that Fe3O4 particles are characterized by well-defined geometric shapes.

In solution phase, ferrous hydroxide or Fe(OH)2 is first formed under alkaline

conditions, which decomposes in the presence of thiosulfate to Fe3+ and Fe2+ species containing hydrated Fe3O4, and upon thermal treatment, Fe3O4 results. Here, the surfactant PEG present in 12

the precursor bath controls the shape of the particles formed via the micellar shapes the surfactant adopts in solution [24]. The average size of the particles ranges between 50-500 nm. The images of Co3O4 (Figure 3(d and e)) show monodisperse entwined nanorods with no specific orientation, 10-30 nm in width, and with lengths varying from few tens of nanometers to several microns. The topographical image (Figure 3f) also mirrors the same crisscross arrangement of nanorods. In the case of Co3O4 NRs, initially, under the hydrothermal conditions, Co(NO3)2 decomposes in the presence of urea to yield a precipitate of cobalt hydroxyl carbonate (Co2(OH)2CO3). Upon further thermal treatment in air, this complex undergoes oxidation to yield Co3O4 NRs. The surfactant, CTAB, by the virtue of its ability to form chain like structures in solution phase, steers the formation of the rod like shapes [23]. The micrograph of the pristine polymer PEDOP shows a granular continuous morphology with no striking feature (Figure 3g). The PEDOP@Fe3O4 NSs hybrid’s image (Figure 3h) shows the polymer co-existing with the Fe3O4 NSs, implying a uniform mixing of the oxide with PEDOP. Similarly, the image of the PEDOP@Co3O4 NRs (Figure 3i) hybrid shows the oxide nanorods to be surrounded by globular shaped PEDOP grains. The structural integrity of the Fe3O4 NSs and Co3O4 NRs is preserved in their hybrids respectively. The Brunauer-Emmett-Teller (BET) surface area analysis and Barrett-Joyner-Halenda (BJH) pore sizes and volume analysis were carried out under an inert atmosphere (nitrogen) at 77.3 K, after degassing at 300 °C for 4 h. Figure S1 (supporting information) shows the BET plots, and from the analysis it is deduced that the PEDOP@Fe3O4 NSs hybrid has the highest surface area of 120 m2 g-1, followed by the PEDOP@Co3O4 NRs hybrid (94 m2 g-1), Fe3O4 NSs (61 m2 g-1) and Co3O4 NRs (53 m2 g-1), in that order. The corresponding pore volumes are 0.4, 0.27, 0.18 and 0.21 cm3 g-1. The higher surface area and pore volume of the PEDOP@Fe3O4 NSs hybrid

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indicate that more number of electroactive sites are available for electrochemical reaction with ions and electrons, thus ensuing in a large capacitance. Electrochemical characteristics Symmetric cells with the following electrodes: Co3O4 NRs, Fe3O4 NSs, PEDOP@Co3O4 NRs and PEDOP@Fe3O4 NSs were assembled. In the cells, these materials were deposited over C-fabric, which served as the current collector. The electrolyte was a 1 M LiClO4/PC/15 wt% PMMA based gel. The cyclic voltammograms of the four cells recorded at different scan rates of 5, 10, 20, 30, 40, 50 and 100 mV s-1 are shown in Figure 4(a-d). The galvanostatic chargedischarge characteristics, recorded at different current densities varying from 0.5 to 3.5 A g-1 are shown in Figure 4(e-h). CV plots are recorded for the Co3O4 NRs and PEDOP@Co3O4 NRs based cells over a voltage range of 0 to 0.6 V, which was found to be the operational voltage window for charging and discharging these cells. The working voltage range for the cells based on Fe3O4 NSs and PEDOP@Fe3O4 NSs is wider, and spans from 0 to 1 V. Therefore the CV plots are presented in this range for these cells. The cathodic and anodic current density responses for the Co3O4 NRs based cell are poor, as can be judged from the areas enclosed in the voltammograms. Even at the highest scan rate of 100 mV s-1, the current density tends to rise only at the limiting voltage of +0.6 V, and that too is perhaps due to electrolyte decomposition. This implies a low pseudocapacitive response. The corresponding hybrid of PEDOP@Co3O4 NRs shows good pseudocapacitive behavior, which is reflected in the enlarged bounded voltammogram areas, and also in the increasing area as a function of scan rate. The plots are not perfectly rectangular, as expected for double layer capacitive materials like CNTs. Nonetheless, the redox response is much superior to that shown either by pristine PEDOP or Co3O4 NRs.

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To delineate the contribution of PEDOP and Co3O4 NRs to the CV response of the hybrid film, the CV plots of pristine PEDOP based cells were recorded (Figure 5a) over a voltage range of 0 to 0.6 V. The shape of the polymer’s voltammograms are different from those of the hybrid, and the current densities are higher for the hybrid compared to the polymer. This indicates that the PEDOP@Co3O4 NRs hybrid is electrochemically more active than pristine PEDOP or Co3O4 NRs. The CV plots of PEDOP@Fe3O4 NSs (Figure 4d) show enhanced redox storage capacity compared to Fe3O4 NSs, which is evident from the larger areas enclosed in the voltammograms for the hybrid. While PEDOP undergoes charge-discharge in a limited voltage range of 0 to 0.6 V, in the presence of Fe3O4 NSs, the upper voltage limit for the PEDOP@Fe3O4 NSs hybrid extended to 1 V. The redox reactions of M3O4 (M = Co or Fe), where in the anodic branch, a fraction of M2+ oxidize to M3+, and in the reverse sweep, M3+ reduce to M2+ are shown below (as forward and backward reactions). (M(x+y)2+O. M23+O3) (x = y = 0.5)
(Mx2+O(1-y). M(2+y)3+O(3 + (3/2)y)) + ye- (x ≠ y) [M: Fe or Co] (1) Similarly, PEDOP undergoes reduction to form neutral polymer (discharged state), and upon oxidation, the anion-doped polymer (charged state) is obtained. (PEDOP)m+:(ClO4)m- + n(ClO4)-
(PEDOP)(m+n)+:(ClO4)(m+n)- + ne-

(2)

The specific capacitance (SC), power density (P) and energy density (E) of the cells were calculated by using the following equations. The masses of both electrodes were taken into consideration. SC = I (current density, A g-1) × ∆t (discharge time, s) / ∆V (voltage window, V) E (Wh kg-1) = C × ∆V2 × 1000 / 2 × 3600

(3)

(4) 15

P (W kg-1) = 3600 × E / ∆t

(5)

The charge-discharge characteristics of a Co3O4 NRs symmetric cell (Figure 4e) shows that the discharge capacitance increases progressively from ~14 to 64.6 F g-1, as a function of current density, when the current density is decreased from 3.5 to 0.5 A g-1, in steps of 0.5 A g-1. Similarly, for the Fe3O4 NSs symmetric cell, the SC increased from 15 to 52 F g-1, over the same current density range. The SC of the Co3O4 NRs cell is slightly higher than that of the Fe3O4 NSs cell, at a low current density of 0.5 A g-1. This could be because of the difference in their voltage windows. It is 1.6 times larger for the cell with Fe3O4 NSs. The pristine PEDOP based cell delivers SCs of 70, 110 and 95 F g-1 at 0.5, 1 and 2 A g-1 current densities. The values are not very high perhaps due to the compact granular morphology of PEDOP, which does not allow fast ion intercalation and reaction. In the two hybrids, the synergy between the two components: the polymer (PEDOP) and the oxide (Co3O4 NRs or Fe3O4 NSs) comes to the fore, yielding high pseudocapacitances. The good electrical conductivity of the doped polymer (PEDOP-ClO4 -) enables facile electron transport across the cross-section of the hybrid (PEDOP@Co3O4 NRs or PEDOP@Fe3O4 NSs), which improves the ion-uptake from the electrolyte. The morphologies of Co3O4

and Fe3O4, the

nanorod-like shapes, and the nano-hexagons/other geometric shapes, and their ability to store charge by undergoing redox reactions at the electrode/electrolyte interface, via reversible M2+ to M3+ (M: Co or Fe) transitions, allows maximum utilization of the electrochemically accessible sites, thus leading to improved SC performance. In the pristine materials: the polymer (PEDOP) or the oxide (Co3O4 NRs or Fe3O4 NSs) species tend to aggregate which decreases the availability of electroactive sites, thus adversely impacting their ion-storage capacities. In the hybrids, the polymer prevents the Co3O4 NRs or Fe3O4 NSs from aggregating, and at the same 16

time, the oxides inhibit the polymer particles from coalescing. The PEDOP@Co3O4 NRs hybrid based cell delivers a SC of 582 F g-1 (at 0.5 A g-1) which systematically decreases to 67 F g-1 (at 3.5 A g-1), and the PEDOP@Fe3O4 NSs hybrid based cell yields a capacitance of 632 F g-1 at 0.5 A g-1, which decreases to 267 F g-1 at 3.5 A g-1. The retention of a reasonably good capacitance at a high current density of 3.5 A g-1, indicates that this hybrid reacts rapidly at high current densities, which can be credited to the ease of ion-penetrability in the PEDOP@Fe3O4 NSs architecture. The PEDOP@Fe3O4 NSs hybrid based cell, however, shows the highest SC at 1 A g-1, the value being 673 F g-1. In a previous report [20], at 1 A g-1, an asymmetric cell with Co3O4 nanowires (NWs) deposited over a Ni foam-, and carbon aerogel- electrodes and containing a KOH-poly(vinyl alcohol) (PVA) gel as the electrolyte yielded a SC of 57.5 F g-1. A symmetric cell with Co3O4 NWs@Ni foam electrodes, gave a capacitance of 278 F g-1 [20]. In another study of note, a cell based on graphite foam loaded with Co3O4 core/PEDOT shell nanowires gave a SC of ~300 F g-1 at 5 A g-1, and when MnO2 was also included in the shell, the SC increased to ~400 F g-1 [10]. For nitrogen and Fe3O4 doped in-situ carbon nanosheets, a high SC of 586 F g-1 at 0.5 A g-1 and 340 F g-1 capacitance retention at 10 A g-1 in a three-electrode system was achieved [32]. In another study, cells with Fe3O4 microcubes and its composites with CNTs and RGO gave SCs of ~65, 110 and 220 F g-1 respectively at 0.5 A g-1, over a voltage window of −1 to 0 V [33]. The variation of specific capacitance as a function of current density (also known as rate capability), is shown in Figure 5b. Over the current density range of 0.5 to 3.5 A g-1, the SCs are the highest for the hybrid based cells, and the capacitances are comparable for the two oxide based cells, though much lower in magnitudes compared to the hybrids. This study shows the practical utility of the hybrid based cells and particularly that of the PEDOP@Fe3O4 NSs hybrid 17

based cell. Since the PEDOP@Fe3O4 NSs hybrid composite gave the highest SC at 1 A g-1, which is the maximum SC achieved among all cells, the cycling stabilities for all the four cells were monitored at 1 A g-1 (Figure 5c). The SC for the PEDOP@Fe3O4 NSs hybrid based cell is the highest, and remains so over 5000 charge-discharge cycles. From an initial capacitance of 673 F g-1, it drops to 561 F g-1. Approximately 83% of its’ original capacitance is retained, at the end of 5000 cycles. The capacitance retention is slightly lower for the PEDOP@Co3O4 NRs hybrid based cell. It drops from 407 F g-1 to 317 F g-1, and the retention is ~78%. The Co3O4 NRs and Fe3O4 NSs have inferior cyclabilities compared their respective hybrids, both in terms of initial capacitances, and capacitance preservation with cycling. They retain only 55 and 57% of their original capacitances after 5000 cycles. The energy density versus power density profiles (Figure 5d) for the four cells show that higher energy densities are achieved by the two hybrids, compared to the pristine oxides. The highest energy density obtained here is 93 Wh kg-1, and it is achieved at a power density of 0.5 kW kg-1, for the PEDOP@Fe3O4 NSs hybrid based cell. The operational voltage window is 1 V. The highest energy density achieved for the PEDOP@Co3O4 NRs hybrid based cell is 29.9 Wh kg-1 at 0.15 kW kg-1. This is much lower, mainly due to a narrow voltage window of 0.6 V. In a previous study, for a cell with Co3O4/vertically aligned graphene nanosheets/C-fabric electrodes, a maximum energy density of 80 Wh kg-1 was achieved at a power density of 0.5 kW kg-1, while the highest power density was of 20 kW kg-1 at the energy density of 27 Wh kg-1 [34]. Yet another study reported an energy density of ~ 18 Wh kg-1 at 1 A g-1, over an operational window of 1.5 V, for an asymmetric Co3O4 NWs/Ni foam-carbon aerogel supercapacitor [20]. Notably, a Co3O4 core/PEDOT-MnO2 shell nanowires cell gave an energy density of ∼9.8 Wh kg-1 at a high power density of 20 kW kg-1 [10]. Here, at a power density of 1.75 kW kg-1, the energy density is

18

37 Wh kg-1 for the cell with PEDOP@Fe3O4 NSs hybrid. Similarly at 1.05 kW kg-1, the energy density is 3.35 Wh kg-1 for the PEDOP@Co3O4 NRs cell. For a Fe3O4@carbon nanosheets composite cell, an E of 18.3 Wh kg-1 was achieved at P = 351 W kg-1 [32]. The energy densities of the cells with Co3O4 NRs are in the range of 3.2-0.7 Wh kg-1 over a power density range of 0.15-1.05 kW kg-1. For the cells with Fe3O4 NSs, energy density varies from 7.2-2.2 Wh kg-1 over power densities of 0.25 - 1.75 kW kg-1. The cells here, especially the hybrid based ones, are better performers at low current or power densities. In future, this limitation needs to be overcome. C-AFM analysis To explain the enhanced electrochemical performance of the hybrids, and especially the PEDOP@Fe3O4 NSs hybrid, the electron conduction properties were compared. For this, concurrent topography (Figure 6(a-c)) and current (Figure 6(d-f)) images were recorded for films of PEDOP, PEDOP@Co3O4 NRs and PEDOP@Fe3O4 NSs. The scanned area was 1 µm × 1 µm for each film. A cartoon illustrating the method of measurement is shown in Figure 7a. The corresponding cross-section profiles recorded along the middle of the images are provided below the images. The topography of PEDOP (Figure 6a) shows interlinked globular cauliflower shaped structures, typical of conducting polymers. The surface of PEDOP@Co3O4 NRs is composed of elongated fiber like structures of Co3O4 juxtaposed with polymer particles (Figure 6b). The PEDOP@Fe3O4 NSs hybrid (Figure 6c) is made up of pyramidal shapes and irregular shaped grains. The current images are acquired when a Pt/Ir tip scans the film surface and a small bias voltage of 50 mV was applied between the tip and the film surface. The bright domains in the current map represent the high current regions and the dark regions signify low current carrying capabilities. The current is color-scaled on the right side of the images. The 19

maximum current magnitudes are 1.1, 3.5 and 4.2 nA for the films of PEDOP, PEDOP@Co3O4 NRs and PEDOP@Fe3O4 NSs respectively. PEDOP surface is characterized by uniformly distributed small bright spots but well-separated by insulating regions. The bright regions carry currents of about 0.8 nA. The surface of PEDOP@Co3O4 NRs is made of randomly distributed domains of high currents, separated by large insulating or low current regions. The bright regions here carry currents of ~2.6 nA. PEDOP@Fe3O4 NSs’ surface shows uniform, and closely spaced high current domains, with very few insulating domains. The bright domains here conduct an average current of 3.2 nA. The homogeneity and the almost seamless distribution of the high current regions, indicates that this hybrid has a high electrical conductivity, which promotes iontransfer at the electrode/electrolyte interface during charge-discharge and leads to a high specific capacitance and energy density for the cell based on this film. The correlation between electrical conductivity of the electroactive surface and the iontransfer rate is the following: greater the number of electrons that the electroactive material (bulk and surface) can conduct, higher will be the number of ions it can react with, for the material is electrically neutral. On the other hand, the presence of large low current carrying domains in the PEDOP@Co3O4 NRs film is responsible for its’ lower specific capacitance. The reason for the more uniform and closely spaced distribution of the conducting regimes on the PEDOP@Fe3O4 NSs’ surface compared to of PEDOP and PEDOP@Co3O4 NRs is the differences in their inherent compositions. The pristine polymer, PEDOP is doped by the insulating perchlorate ions which are the dopant ions that balance the positive charges on the PEDOP backbone during film formation, and they tend to protrude outwards, thus resulting in poor surface conduction properties. Similar observations have been made for untreated PEDOT:poly(styrene sulfonate) (PSS) films [35]. In both PEDOP@Fe3O4 NSs and PEDOP@Co3O4 NRs, these dopant ions are

20

obscured by the presence of Fe3O4 NSs or Co3O4 NRs, and therefore conduction properties are improved. Since both Fe3O4 NSs and Co3O4 NRs are composed of metal in bi-valent and trivalent states, this valency difference, coupled with the fact that they have partially filled dorbitals contribute to improving the electrical conductivity. Besides this factor, the morphology of the oxide, also contributes to the electrical conduction behavior, which accounts for the difference in the current maps of PEDOP@Fe3O4 NSs and PEDOP@Co3O4 NRs. Point contact I-V curves were recorded at six equidistant points on each current image, and the average I-V profile for each film is displayed in Figure 7b. A quasi-linear current-voltage dependence is observed over potential ranges of −1.25 to 0.7 V, −1.9 to 1.2 V, and 0.6 to 1.6 V for the PEDOP, PEDOP@Co3O4 NRs and PEDOP@Fe3O4 NSs films. The nanoscale electrical conductivity is calculated using the relation: σ = (I/V) or (slope) × (d/πr2), where r is the radius of the conducting tip and d is the thickness of the film. The values are 9.2 and 8.6 mS cm-1 for the PEDOP, and PEDOP@Co3O4 NRs, and 66.8 mS cm-1 for the PEDOP@Fe3O4 NSs film. The conductivity is ~ 7 times greater for the PEDOP@Fe3O4 NSs film, which promotes electron propagation, and therefore indirectly improves ion storage. The overall current densities in the applied bias range of −3 to +3 V, even in regions where the behavior is non-Ohmic, are greater in magnitude for the PEDOP@Fe3O4 NSs, followed by the PEDOP@Co3O4 NRs and PEDOP films, in that order. This also signifies the enhanced electrical conduction of the hybrids. Impedance studies EIS spectra of symmetric cells, recorded at an ac amplitude of 20 mV, over a frequency range of 1 MHz to 0.01 Hz are shown in Figure 5(e and f). The Co3O4 NRs and Fe3O4 NSs cells show inclined line behavior, indicative of charge storage largely via accumulation, and less by a

21

Faradaic process. The PEDOP@Co3O4 NRs and PEDOP@Fe3O4 NSs hybrids based cells show a skewed arc in the high frequency to intermediate frequency domain, which is followed by an inclined line. The high frequency intercept is the electrolyte resistance (RS). The arc-like response in the hybrids stems from the presence of PEDOP, which stores charge mainly by charge transfer or redox reactions, unlike the pristine oxides which prefer a double layer charging mechanism. This is confirmed from the Nyquist plot of a symmetric cell based on pristine PEDOP (inset of Figure 5e), which comprises of a semi-circle followed by a slanting line. The ion-diffusion resistance (RΣ) is calculated from the following equation: RS + RΣ/3 = Rintercept (6) The ion-diffusion resistance is found to be lower for the PEDOP@Fe3O4 NSs hybrid (RΣ ~ 264 Ω) compared to that obtained for the PEDOP@Co3O4 NRs hybrid cell (RΣ ~561 Ω) and the PEDOP cell (RΣ ~5130 Ω). In comparison to the pristine polymer, in the hybrids, the movement of perchlorate ions is assisted by the oxide NSs and NRs. The more porous morphology of the hybrids compared to the compact morphology of pristine PEDOP film, allows easy penetration of the ions, and as a consequence, ion-diffusion is facile. The post-cycling impedance spectra for cells with PEDOP@Co3O4 NRs and PEDOP@Fe3 O4 NSs hybrids are shown in Figure 7c. After 5000 cycles, the impedance profile is almost linear for the PEDOP@Fe3O4 NSs hybrid, whereas, a skewed arc before the capacitive line is observed for the PEDOP@Co3O4 NRs based cell. The cycled PEDOP@Co3O4 NRs shows a RΣ of 270 Ω, which is half of the original RΣ obtained for the same cell, prior to cycling. The loss of active material with repetitive cycling, for repeated ion intercalation and deintercalation, can cause pulverization and detachment of the material from the current collector. This thinning of the active material lowers the ion-diffusion

22

resistance. For the PEDOP@Fe3O4 NSs hybrid, the behavior appears to be more resistive than capacitive, especially in the lower frequency range. Morphology effect on cell performance From all the above studies, it is evident that the PEDOP@Fe3O4 NSs hybrid based cell outperforms the PEDOP@Co3O4 NRs hybrid based cell. To analyze the effect of the active material’s morphology on the electrochemical performance of the supercapacitor, symmetric cells of PEDOP@Fe3O4 NRs were fabricated and studied. Here, nanorods instead of nanostructures of Fe3O4 were used. The TEM image of Fe3O4 NRs is shown in Figure 8a. The image shows overlapping elongated rod like structures of Fe3O4 with lengths of the order of 0.2 to 1.5 µm and widths are roughly in the range of 10 to 60 nm. The TEM image of the PEDOP@Fe3O4 NRs hybrid (Figure 8b), is composed of large polymer particles with ill-defined shapes which are juxtaposed with the Fe3O4 NRs. A symmetric cell with PEDOP@Fe3O4 NRs hybrid based electrodes, and a 1 M LiClO4/PC/15 wt% PMMA based gel was assembled. The cyclic voltammograms of the cell were recorded at different scan rates of 5, 10, 20, 30, 40, 50 and 100 mV s-1, and they are presented in Figure 8c. The galvanostatic charge-discharge characteristics, recorded at different current densities varying from 0.5 to 3.5 A g-1 are also shown (Figure 8d). The voltammograms are characterized by leaf like shapes, and the area under the curve increases as a function of scan rate. The charge-discharge characteristics are different from that observed for PEDOP@Fe3O4 NSs and PEDOP@Co3O4 NRs based cells. While in these hybrids, the discharge times were found to be longer than charging times (particularly at low current densities of 1 and 0.5 A g-1), on the other hand, for the PEDOP@Fe3O4 NRs based cell, the discharge times were found to be 2.5 and 2.4 times lower than the corresponding charging times at 1 and 0.5 A g-1. This obviously resulted in lower specific capacitances (SCs) 23

for the PEDOP@Fe3O4 NRs based cell. Rate performance of the cell is shown in Figure 8e. The SC varies from ~145 F g-1 (at 3.5 A g-1) to 109 F g-1 (at 0.5 A g-1), with a maximum SC of 378 F g-1 (at 2 A g-1). In contrast, the PEDOP@Fe3O4 NSs hybrid based cell yields a very high SC of 632 F g-1 at 0.5 A g-1, which decreases to 267 F g-1 at 3.5 A g-1. Discharge capacitance is a measure of the electrical energy (in the form of charge) available per unit mass for use. Therefore, the shorter discharge times observed for the PEDOP@Fe3O4 NRs based cell render it to be inferior to the PEDOP@Fe3O4 NSs based cell. Nyquist plot for the PEDOP@Fe3O4 NRs based cell (Figure 8f) shows an inclined line, indicative of fast electrochemical reactions, and charge transfer and transport processes appear to be more capacitive than resistive. Elucidation of the performance of hybrids versus oxides An ideal supercapacitor is expected to store high electrical energy per unit mass, and at the same time should be able to deliver this electrical energy (in the form of charge) at a very fast rate (unlike batteries). Nanostructured metal oxides like Co3O4 NRs and Fe3O4 NSs compared to bulk oxides are more suitable for supercapacitor electrodes due to the following features. (a) The higher effective surface areas of the oxide nanostructures in comparison to the bulk oxides for the same geometric area allows more number of electrolyte ions to participate in the Faradaic reactions with the oxide species, thus leading to higher capacitances compared to bulk oxides. (b) Compared to bulk materials, in nanostructured oxides, the path lengths for electron and ion transport are considerably reduced, which results in fast charge/discharge rates. However, these two advantages are offset by two major inadequacies. (a) Metal oxides are poor electrical conductors. Every electron which is transported by the oxide from the current collector, is compensated by a cation from the electrolyte. Thus, the capacitance is indirectly governed by the electrical conductivity of the active material. It is therefore adversely affected by the poor 24

electrical conductivity of the oxide (be it Fe3O4 or Co3O4). (b) Secondly, repetitive chargedischarge is accompanied by ion extraction and insertion, which leads to severe volume changes. Pristine oxides are unable to accommodate this volume change easily. This results in the pulverization of the original particle morphology and causes the breakdown of the electrical or Ohmic connections from C-fabric, the current collector, thus resulting in dismal cycling performances. It must be recalled that Co3O4 NRs and Fe3O4 NSs retain only 55 and 57% of their initial capacitances after 5000 cycles. Use of hybrid materials is therefore an effective strategy to overcome these shortcomings. In a conducting polymer (PEDOP)-metal oxide hybrid, firstly, the free volume or the voids that exist between the entangled chains of the conducting polymer can accommodate the volume expansion which the electroactive material undergoes during ion-insertion. Secondly, the polymer (PEDOP) also reduces the direct exposure of Fe3O4 NSs or Co3O4 NRs to the electrolyte and conserves the structural integrity of the oxide nanostructures. PEDOP also improves the interfacial properties and prevents the aggregation of the oxide nanostructures, which results in an increased capacitance for the hybrid relative to the pristine oxides. Thirdly, the higher electrical conductivity of PEDOP compared to the pristine oxides, increases the specific capacitance, particularly at high current densities. At the same current density of 3.5 A g-1, the SCs of the hybrids is greatly enhanced compared to the SCs of the corresponding oxides. All the above factors cumulatively result in significantly improved cycling performances, rate responses and specific capacitances for the hybrids compared to the oxides. The difference in performances of the two hybrids: PEDOP@Fe3O4 NSs and PEDOP@Co3O4 NRs however, is controlled by the intrinsic properties of the two oxides and their morphologies. The effect of morphology on performance was assessed by the following experiments and analysis. We also prepared Fe3O4

25

nanorods (NRs) and hybrid films of PEDOP@Fe3O4 NRs. We found that electrochemical performance of the PEDOP@Fe3O4 NRs hybrid is not as good as that of the PEDOP@Fe3O4 NSs hybrid. The inferior performance of the Fe3 O4 NRs hybrid in comparison with that of the Fe3O4 NSs hybrid clearly shows that Fe3O4, when it is made up of the pyramidal and hexagonal 3D structures, is electrochemically more active than is’ counterpart composed of nanorods. Application of a PEDOP@Fe3O4 NSs hybrid based cell Figure 9 displays a demonstration of a real-time application of the symmetric cell based on the PEDOP@Fe3O4 NSs hybrid films. The photograph of the uncoated pristine C-fabric (Figure 9a) shows it be composed of intertwined carbon fibers which give it a mesh-like appearance. The porous mesh like structure of C-fabric also enables a higher loading of the electroactive material, compared to what can be achieved with the widely used current collectors like planar SS or Cu foil. The PEDOP@Fe3O4 NSs hybrid is electrodeposited over the C-fabric, and a uniform coating is obtained, as can be judged from Figure 9b. A single symmetric cell made up of two electrodes of PEDOP@Fe3O4 NSs coated C-fabric, and separated by an electrolyte loaded separator is charged using a potentiostat/galvanostat and then immediately connected to a multimeter, which clearly displays 1 V (Figure 9c), when it is flat. An equivalent cell when completely bent into a “U” - shape and held in that position using a clip, delivers a maximum voltage of 0.56 V, showing the ability of the cell to perform reasonably well, despite its curved physical state (Figure 9d). The flexibility of the cell is showcased via this photograph. Figure 9e shows three such symmetric cells in their discharged states connected in series and to a green LED. After charging the individual cells to ~1 V using a potentiostat/galvanostat, the same cells (in charged states, and in series) illuminate the LED (Figure 9f). This demonstration shows that

26

these cells can be scaled-up by coating the hybrid over larger areas, and applied to devices, which require high power for short time spans. Conclusions Hybrid films of PEDOP coated Co3O4 NRs or Fe3O4 NSs were fabricated over light-weight flexible C-fabric substrates. Symmetric cells were constructed using a gel polymeric electrolyte. The beneficial effects of PEDOP such as large conductivity, electrochemical and chemical stability and low cost, and the attributes of M3O4 (M: Fe or Co), such as natural abundance, high theoretical capacity, and ecologically friendly nature, come to the fore in the hybrids thus resulting in improved charge storage responses. The PEDOP@Fe3O4 NSs hybrid based cell delivers a SC of 673 F g-1 at 1 A g-1, which reduces to 561 F g-1, at the end of 5000 chargedischarge cycles. It also exhibits a broad voltage window of 1 V, a spectacularly high energy density of 93 Wh kg-1, which is achieved at a power density of 0.5 kW kg-1, and a reasonably good rate performance. Nanoscale current maps of the PEDOP@Fe3O4 NSs surface show that this hybrid is composed of uniformly distributed and well-connected domains of high currents, as opposed to pristine PEDOP and PEDOP@Co3O4 NRs films. The latter films are predominantly constituted by low current or insulating domains. The propensity of the surface of the PEDOP@Fe3O4 NSs hybrid to conduct high currents allows a high ion uptake at the electrode/electrolyte interface, and the high electrical conductivity of the PEDOP@Fe3O4 NSs film (7-times greater than that of PEDOP and PEDOP@Co3O4 NRs) also facilitates ion transport across the cross-section of the film, thus resulting in an increased SC. The effect of morphology on cell performance was also analyzed by comparing the electrochemical characteristics of cells with PEDOP@Fe3O4 NSs and PEDOP@Fe3O4 NRs. The cells with the Fe3O4 NSs show a significantly increased electroactivity compared to the cell with the Fe3O4 NRs. This revealed 27

that the mixed morphology of pyramidal and hexagonal 3D structures is more effective than sole nanorods in storing and releasing ions during charge-discharge. The electrochemical charge storage characteristics of PEDOP@Fe3O4 NSs based cells show that they can be applied as power sources for low power requiring electronic devices. Acknowledgements Financial support from the Department of Science & Technology (Project: India-UK Center for education and research in clean energy (IUCERCE), Grant no. DST/RCUK/JVCCE/2015/04 (1) (G)) is gratefully acknowledged. B.N.R. is thankful to SERB for national-post doctoral fellowship (N-PDF). D.S. is thankful to University Grants Commission (UGC) for the grant of junior research fellowship. References [1] H. Yang, H. Xu, M. Li, L. Zhang, Y. Huang, and X. Hu, Assembly of NiO/Ni(OH)2/PEDOT nanocomposites on contra wires for fiber-shaped flexible asymmetric supercapacitors, ACS Appl. Mater. Interfaces 8 (2016) 1774–1779. [2] R. Liu, J. Duay, S. B. Lee, Electrochemical formation mechanism for the controlled synthesis of heterogeneous MnO2/poly(3,4-ethylenedioxythiophene) nanowires, ACS Nano 5 (2011) 5608–5619. [3] R. Liu, J. Duay, S. B. Lee, Redox exchange induced MnO2 nanoparticle enrichment in poly(3,4-ethylenedioxythiophene) nanowires for electrochemical energy storage, ACS Nano 4 (2010) 4299–4307.

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Scheme and Figures

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Scheme 1 Schematics for preparation of (a) PEDOP@Co3O4 NRs and (b) (i) PEDOP@Fe3O4 NSs and (ii) PEDOP@Fe3O4 NRs films.

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Figure 1 XRD patterns of (a) Co3O4 NRs and (b) Fe3O4 NSs. (c) Current versus time curves corresponding to the deposition in chronoamperometric mode of PEDOP, PEDOP@Co3O4 NRs, and PEDOP@Fe3O4 NSs films.

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Figure 2 Raman spectra of (a) Co3O4 NRs, (b) Fe3O4 NSs and (c) PEDOP, all obtained using a laser excitation wavelength of 532 nm. TEM images of (d) PEDOP@Co3O4 NRs and (e) PEDOP@Fe3O4 NSs and their SAED patterns are shown in (d′) and (e′).

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Figure 3 FE-SEM images of (a,b) Fe3O4 NSs, (d,e) Co3O4 NRs, (g) PEDOP, (h) PEDOP@Fe3O4 NSs and (i) PEDOP@Co3O4 NRs. AFM-topography images of (c) Fe3O4 NSs and (f) Co3O4 NRs.

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Figure 4 (a-d) Cyclic voltammograms recorded at different scan rates of 5, 10, 20, 30, 40, 50 and 100 mV s-1, and (e-h) galvanostatic charge-discharge curves recorded at different current densities of 0.5, 1, 1.5, 2, 2.5, 3 and 3.5 A g-1 for symmetric cells of Co3O4 NRs, PEDOP@Co3O4 NRs, Fe3O4 NSs, and PEDOP@Fe3O4 NSs.

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Figure 5 (a) Cyclic voltammograms of a symmetric cell of PEDOP recorded at 10, 50 and 100 mV s-1, and inset shows the galvanostatic charge-discharge curves recorded at 0.5, 1, and 2 A g-1. (b) Rate capability curves, (c) cycling stability performances and (d) energy density versus power density plots for symmetric cells of Co3O4 NRs, PEDOP@Co3O4 NRs, Fe3O4 NSs, and PEDOP@Fe3O4 NSs. Nyquist plots of symmetric cells of (e) Fe3O4 NSs and PEDOP@Fe3O4 NSs and (f) Co3O4 NRs and PEDOP@Co3O4 NRs, top insets of (e) and (f) are enlarged views and the bottom inset in (e) is the Z″ versus Z′ plot for the pristine PEDOP based symmetric cell.

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Figure 6 Concurrent topography and current images of (a,d) PEDOP, (b,e) PEDOP@Co3O4 NRs, and (c,f) PEDOP@Fe3O4 NSs. Topographical and current section profiles of (a′,d′) PEDOP, (b′,e′) PEDOP@Co3O4 NRs, and (c′,f′) PEDOP@Fe3O4 NSs are shown underneath the respective images.

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Figure 7 (a) Cartoon showing the working of C-AFM, (b) averaged point contact I-V curves, extracted from the regions shown in the current maps of PEDOP, PEDOP@Co3O4 NRs, PEDOP@Fe3O4 NSs in Figure 5(d, e and f); the dotted lines represent the linear fits. (c) Nyquist plots of symmetric cells of PEDOP@Fe3O4 NSs and PEDOP@Co3O4 NRs, after 5000 chargedischarge cycles (at 1 A g-1).

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Figure 8 TEM images of (a) Fe3O4 NRs and (b) PEDOP@Fe3O4 NRs. Electrochemical characteristics of a symmetric cell of PEDOP@Fe3O4 NRs: (c) cyclic voltammetry plots recorded at different scan rates, (d) charge-discharge characteristics at different current densities, (e) SC variation with current density and (f) a Nyquist plot recorded over a frequency range of 1 MHz to 0.01 Hz.

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Figure 9 Photographs of (a) C-fabric, (b) PEDOP@Fe3O4 NSs coated C-fabric, charged symmetric cell of PEDOP@Fe3O4 NSs giving voltages of (c) ~ 1 V (when flat) and (d) ~ 0.56 V (when bent fully), upon connecting to a multimeter, (e) three symmetric cells (in discharged states), connected in series and to a green LED and (f) the same cells (in charged states) illuminate the LED.

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Graphical Abstract

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Highlights: • • • • •

Fe3O4 nanostructures (NSs) and Co3O4 nanorods (NRs) were prepared by hydrothermal routes. Poly(3,4-ethylenedioxypyrrole) (PEDOP)@M3O4 (M: Co or Fe) hybrids were electrodeposited on C-fabric. A PEDOP@Fe3O4 NSs based cell gives a capacitance of 673 F g-1 with 83% capacitance retention after 5000 cycles. The highest energy density of 93 Wh kg-1 is achieved at a power density of 0.5 kW kg-1. Conducting atomic force microscopy studies reveal seamless high current flowing domains in this hybrid.

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