Supporting Information for

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1 Supporting Information for High-Performance Supercapacitor Electrode based on Cobalt Oxide - Manganese Dioxide -Nickel Oxide Ternary 1D Hybrid Nanotubes Ashutosh K. Singh, 1,2, Debasish Sarkar, 3,,*, Keshab Karmakar, 2 Kalyan Mandal, 2 and Gobinda Gopal Khan 4,* 1 Large Area Device Laboratory, Centre for Nano and Soft Matter Sciences, Jalahalli, Bengaluru , India 2 Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake City, Kolkata , India 3 Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru , India 4 Centre for Research in Nanoscience and Nanotechnology, University of Calcutta, Technology Campus, Block JD2, Sector III, Salt Lake City, Kolkata , India ( Authors contributed equally to this work.) * Corresponding authors: deb.sarkar1985@gmail.com and gobinda.gk@gmail.com S-1

2 Experimental Methods Reagents Aluminium foil (99.99+% pure, 0.2 mm thick), Oxalic acid (99.9+% pure), Cobalt sulphate heptahydrate (CoSO 4.7H 2 O, % pure), Nickel sulphate hexahydrate (NiSO 4.6H 2 O, 99.9+% pure), Manganese sulphate monohydrate (MnSO 4.H 2 O, 99.9+% pure), Boric acid (H 3 BO 3, 99.9+% pure), Sodium hydroxide (NaOH, 99.9+% pure) and Potassium hydroxide (KOH, 99.9+% pure) were purchased from SIGMA-ALDRICH. All chemicals were of analytical grade and were used without further purification. Synthesis of Co 3 O 4 -MnO 2 -NiO ternary hybrid nanotubes Highly ordered nanoporous anodic aluminium oxide (AAO) templates were fabricated by the controlled two-stage electrochemical anodization of high-purity aluminium foil in oxalic acid solution as described elsewhere. 1-3 The software controlled three electrode electrodeposition unit (potentiostat AutoLab-30) equipped with power supply was used to synthesise high-density arrays of Co-Mn-Ni alloy nanotubes with the help of AAO template with one side coated with a conductive gold (Au) layer grown by the thermal evaporation technique. A high-purity Pt wire and an Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The arrays of Co-Mn-Ni alloy nanotubes were grown in the pores of AAO using the aqueous solution of 40 g L -1 CoSO 4.7H 2 O, 45 g L -1 MnSO 4.H 2 O 50 g L -1 NiSO 4.6H 2 O and 30 g L -1 H 3 BO 3 as the electrolyte at room temperature. Here, boric acid and NH 4 OH were used as a buffer to maintain the ph of the electrolyte around 3.5 and also to control the electrodeposition process. The deposition of the arrays of Co-Mn-Ni alloy nanotubes was conducted for 30 minutes by using a dc voltage of V, following the linear sweep voltammetry (LSV) results. After the growth of the Co-Mn-Ni alloy nanotubes the template was removed by dissolving it in 2 M NaOH aqueous solution. The open arrays of Co-Mn-Ni alloy nanotubes grown on Au layer were finally oxidized to form Co 3 O 4 - MnO 2 -NiO ternary hybrid nanotubes by annealing at 600 C for 5 hours in air atmosphere. The mass of the active electrode material was measured by using a microbalance by subtracting the mass of the equal area Au foil layer, on which the hybrid nanotubes were grown, from the total mass of the cathode (hybrid nanotubes material and the Au layer). The loading density of the Co 3 O 4 - MnO 2 -NiO ternary hybrid nanotubes was found to be mg cm -2. S-2

3 Structural Characterization The crystal structures of the as prepared 1D Co 3 O 4 - MnO 2 -NiO ternary hybrid nanotubes were analyzed by X-ray diffraction (XRD, Panalytical X'Pert Pro diffractometer). Morphology of the arrays of Co-Mn-Ni alloy nanotubes and Co 3 O 4 - MnO 2 -NiO ternary hybrid nanotubes were studied using field emission scanning electron microscope (FESEM, FEI Quanta-200 Mark-2), transmission electron microscope (TEM, FEI TECNAI G2 TF20ST) and energy filtered transmission electron microscope (EFTEM). The crystalline structure of the hybrid nanotubes was further investigated by high-resolution TEM (HRTEM). Moreover, the elemental composition and chemical states of different ionic species of Co 3 O 4 -MnO 2 -NiO ternary hybrid nanotubes were further investigated by energy dispersive X-ray (EDAX) and X-ray photo-electron spectroscopy (XPS, AXIS ULTRA). Electrochemical Measurements The electrochemical properties of the samples were investigated by cyclic voltammetry (CV) and galvanostatic (GV) charge/discharge tests by using a software controlled conventional three-electrode electrochemical cell (potentiostat AutoLab-30) consisting of the as-prepared samples as the working electrode, Ag/AgCl as the reference electrode, a Pt wire as the counter electrode and 1 M KOH solution as the electrolyte, at room temperature. A potential window in the range from -0.2 V to +0.6 V was used in all the measurements. The CV measurements were performed at different scan rates varying from 2 to 100 mv/s. Galvanostatic charge/discharge measurements were conducted at various current densities varying from 12.2 to 18.4 A/g to evaluate the specific capacitance, areal capacitance, power density and energy density. Electrochemical impedance spectroscopy (EIS) was carried out to prove the capacitive performance in 1 M KOH within the frequency range of Hz at charging condition (at 0.55V) of the electrode with an ac field amplitude of 5 mv. Growth mechanism of 1D ternary NTs in AAO Based on the results of this present synthesis process and our previous works on the AAO template synthesis of transition metal and transition metal oxides, 4-7 the growth mechanism of the 1D ternary NTs could be demonstrated. It is found that the electro-deposition voltage/current and the electro-deposition time are the two major parameters to control the shape/morphology of the deposited nanostructures within the nanotubes of AAO. At a given S-3

4 voltage the metal ions start depositing at the interface or junction of AAO and the Au substrate (see Scheme 1). The interface actually acts as the preferential nucleation site for the first electrodeposition. 8 Afterwards, metal ions deposit on the Au substrate mainly and cover up the whole Au substrate within few seconds. After covering the Au substrate the metal ions start to deposit following the inner surface of the AAO tube wall. 8 Now, when the applied voltage of perfect (as obtained from LSV) the tubular type nanoarchitecture of the metal is formed if the deposition time is considerably small, as in this stage the metal ions deposit on the inner wall of the porous AAO tubes (as shown in Scheme 1). However, if the electrodeposition is carried out for a long time the metal ions gradually cover up the whole space within the tube wall and the morphology of the fabricated nanostructures becomes nanowires like. However, it is found that if the applied voltage is considerably high enough, even the short deposition tome could produce solid nanowires instead of nanotubes. Scheme 1: Growth mechanism of the 1D ternary NTs/NRs in AAO. The performance of the 1D ternary NTs electrode There are few literatures available regarding ternary hybrid nanostructures investigated for supercapacitor applications, though, mostly based on metal oxide-carbon nanocomposities, like MnO 2 /CNT/conducting polymer ternay nanocomposite, 9 γ- MnO 2 /CNT/graphene nanosheet composite, 10 SWNT/WO 3 /PANI composite thin films 11 (Journal of Alloys and Compounds, 658, 2016, ), MWCNT/PANI/MnO 2 ternary coaxial nanowires 12 (Journal of Power Sources, 196, 11, ), and Graphene/SnO 2 /PPy (GSP) nanocomposite 13 (RSC Advances, 2012, 2, ) etc. However, here for the first time we have combined three metal oxides together in a single unit with no organic components and investigated their electrochemical properties. Interestingly, our results including capacitance and cycle life, remain superior as compared to most of the other ternary composites mentioned above. S-4

5 Moreover, we have also investigated binary nanostructured materials based on pristine metal oxides, like Fe 2 O 3 /MnO 2 core-shell nanowires, 5 Co-Ni/Co 3 O 4 -NiO core-shell binary nanowires 7 though, we have observed significantly improved electrochemical performance in case of this ternary nanotube electrode. The performance of this ternary hybrid nanotube electrode as compared to other binaryternary composites is summarized below in Table S1: Table S1 System under investigation Co 3 O 4 -MnO 2 -NiO ternary hybrid 1D nanotube arrays [Present work] α-fe 2 O 3 /MnO 2 core-shell nanowires 5 Ni/NiO core-shell nanowires 6 Co-Ni/Co 3 O 4 -NiO core-shell nanowires 7 Spinel Ni 2 CoO 4 aerogel 14 MnO 2 /CNT/conducting polymer ternary nanocomposite 9 Capacitance 2525 F/g; 1125 mf/cm F/g 717 F/g 2013 F/g 1400 F/g 427 F/g SWNT/WO 3 /PANI composite thin films mf/cm 2 Graphene/SnO 2 /PPy (GSP) nanocomposite F/g MWCNT/PANI/MnO 2 ternary coaxial nanowires F/g S-5

6 Figures Figure S1. FESEM micrograph of the as prepared arrays of Co-Mn-Ni alloy nanotubes. Figure S2. Side view of FESEM micrograph of the as-prepared Co 3 O 4 -MnO 2 -NiO ternary hybrid NTs. S-6

7 Figure S3. SAED pattern micrograph of the as prepared arrays of Co 3 O 4 - MnO 2 -NiO ternary hybrid nanotubes. Figure S4. XRD pattern of the Co 3 O 4 - MnO 2 -NiO ternary hybrid nanotubes. S-7

8 Figure S5. EDS spectrum of the Co 3 O 4 - MnO 2 -NiO ternary hybrid nanotubes. 2p 3/2 Co 2p (a) Mn 2p (b) 2p 3/2 Intensity (a.u.) 2p 1/2 Intensity (a.u.) 2p 1/ Binding energy (ev) Binding energy (ev) 2p Ni 2p (c) 3/2 2p 1/2 O II O 1s (d) Intensity (a.u.) Intensity (a.u.) O I Binding energy (ev) Binding energy (ev) Figure S6. XPS spectrum of (a) Co 2p, (b) Mn 2p, (c) Ni 2p and (d) O 1s, respectively, in Co 3 O 4 - MnO 2 -NiO ternary hybrid nanotubes. S-8

9 Specific capacitance (F/g) Areal capacitance (mf/cm2) Scan rate (mv/s) Figure S7. Variation of areal and specific capacitances for the ternary hybrid nanotube electrode as a function of potential scan rate. Figure S8. FESEM micrograph of the Co3O4-MnO2-NiO ternary hybrid NTs after 5700 charge-discharge cycles. S-9

10 References (1) Sarkar, J.; Khan, G.G.; Basumallick, A. Nanowires: Properties, Applications and Synthesis via Porous Anodic Aluminium Oxide Template. Bull. Mats. Sci. 2007, 30, (2) Khan, G. G.; Mukherjee, N.; Mondal, A.; Bandyopadhyay, N. R.; Basumallick, A. Optical and Field Emission Characteristics of Anodic Aluminium Oxide/ZnO Hybrid Nanostructure. Mats. Chem. Phy. 2010, 122, (3) Khan, G. G.; Singh, A. K.; Mandal, K. Structure Dependent Photoluminescence of Nanoporous Amorphous Anodic Aluminium Oxide Membranes: Role of F + Center Defects, J. Lumin. 2013, 134, (4) Sarkar, D.; Khan, G. G.; Singh A. K.; Mandal, K. Enhanced Electrical, Optical and Magnetic Properties in Multifunctional ZnO/α-Fe 2 O 3 Semiconductor Nanoheterostructures by Heterojunction Engineering. J. Phys. Chem. C, 2012,116, (5) Sarkar, D.; Khan, G. G.; Singh A. K.; Mandal, K. High-Performance Pseudocapacitor Electrodes Based on α-fe 2 O 3 /MnO 2 Core Shell Nanowire Heterostructure Arrays. J. Phys. Chem. C, 2013,117, (6) Sarkar, D.; Khan, G. G.; Singh A. K.; Mandal, K. Unique Hydrogenated Ni-NiO Core-Shell 1D Nano-heterostructures with Superior Electrochemical Performance as Supercapacitor. J. Mater. Chem. A, 2013, 1, (7) Singh A. K.; Sarkar, D.; Khan, G. G.; Mandal, K. Designing One Dimensional Co- Ni/Co 3 O 4 -NiO Core/shell Nano-heterostructure Electrodes for High-performance Pseudocapacitor, App. Phys. Lett. 2014, 104, (8) Li, X.; Wang, Y.; Song, G.; Peng, Z.; Yu, Y.; She, X.; Li, J.; Synthesis and Growth Mechanism of Ni Nanotubes and Nanowires, Nanoscale Res. Lett. 2009, 4, (9) Hou, Y.; Cheng, Y.; Hobson, T.; Liu, J. Design and Synthesis of Hierarchical MnO 2 Nanospheres/Carbon Nanotubes/Conducting Polymer Ternary Composite for High Performance Electrochemical Electrodes. Nano. Lett., 2010, 10, (10) Rakhi, R. B.; Chen, W.; Cha, D.; Alshareef, H. N. Nanostructured Ternary Electrodes for Energy-Storage Applications, Adv. Energy. Mater., 2012, 2, (11) Yuksel, R.; Durucan, C.; Unalan, H. E. Ternary nanocomposite SWNT/WO 3 /PANI Thin Film Electrodes for Supercapacitors. J. Alloys Compd., 658, 2016, (12) Li, Q.; Liu, J.; Zou, J.; Chunder, A.; Chen, Y.; Zhai, L. Synthesis and Electrochemical Performance of Multi-walled Carbon Nanotube/polyaniline/MnO 2 Ternary Coaxial Nanostructures for Supercapacitors. J. Power Sources, 196, 11, (13) Wang, W.; Hao, Q.; Lei, W.; Xia, X.; Wang, X.; Graphene/SnO 2 /polypyrrole Ternary Nanocomposites as Supercapacitor Electrode Materials. RSC Adv., 2012, 2, S-10

11 (14) Wei, T.; Chen, C.; Chien, H.; Lu,S.; Hu, C. A Cost-Effective Supercapacitor Material of Ultrahigh Specific Capacitances: Spinel Nickel Cobaltite Aerogels from an Epoxide-Driven Sol Gel Process. Adv. Mater., 2010, 22, S-11