Accepted Manuscript A comparative study on the supercapacitive behaviour of solvothermally prepared metal ferrite (MFe2O4, M฀=฀Fe, Co, Ni, Mn, Cu, Zn) nanoassemblies M.L. Aparna, A. Nirmala Grace, P. Sathyanarayanan, Niroj Kumar Sahu PII: S0925-8388(18)30572-3 DOI: 10.1016/j.jallcom.2018.02.127 Reference: JALCOM 45003 To appear in: Journal of Alloys and Compounds Received Date: 10 October 2017 Revised Date: 15 January 2018 Accepted Date: 11 February 2018 Please cite this article as: M.L. Aparna, A.N. Grace, P. Sathyanarayanan, N.K. Sahu, A comparative study on the supercapacitive behaviour of solvothermally prepared metal ferrite (MFe2O4, M฀=฀Fe, Co, Ni, Mn, Cu, Zn) nanoassemblies, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.02.127. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. 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Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. AC C EP TE D M AN US C RI PT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT A comparative study on the supercapacitive behaviour of solvothermally prepared metal ferrite (MFe2O4, M=Fe, Co, Ni, Mn, Cu, Zn) nanoassemblies M L Aparna, A Nirmala Grace, Sathyanarayanan P*, Niroj Kumar Sahu* RI PT Centre for Nanotechnology Research, VIT University, Vellore, India Emails: nirojs@vit.ac.in (orcid id: 0000-0002-0499-4108), sathya.punniyakoti@vit.ac.in *To whom correspondence to should be addressed • M AN US C Highlights Ferrite nanoassemblies (NAs) were prepared by PEG-600 assisted solvothermal method. • Reaction mechanism of formation of NAs with various distribution is explicated. • Capacitance vary on surface area, distribution, morphology and composition of NAs. • CoFe2O4 NAs exhibit superior charge storage behaviour and specific capacitance. • Internal series resistance and potential window in CV vary with composition of NAs. D Abstract: Metal ferrites show excellent electrochemical properties owing to the multiple TE oxidation states of the metal ions which make them more suitable for electrode materials in supercapacitor applications. Here, we report the synthesis of mesoporous metal ferrites EP (MFe2O4, M=Fe, Co, Ni, Mn Cu, Zn) by solvothermal method in ethylene glycol solvent with AC C the assistance of PEG 600 as co-solvent. The metal ferrites found to crystallize in either spinel or inverse spinel or mixed spinel structure with crystallite size varying from 20 to 35 nm and average particle size of 50-140 nm determined from XRD pattern and FESEM images respectively. The supercapacitive performances were studied by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS) in 3M KOH solution as electrolyte and the performance of various ferrites were compared. The specific capacitance of metal ferrites (MFe2O4, M=Fe, Co, Ni, Mn, Cu, Zn) were calculated to be 101 F g-1, 444.78 F g-1, 109.26 F g-1, 190 F g-1, 250 F g-1, 138.95 F g-1 respectively at a scan rate 1 ACCEPTED MANUSCRIPT of 2 mV s-1. Highest specific capacitance was found for CoFe2O4 as compared to others metal ferrites. Keywords: cyclic voltammogram, chronopotentiometry, supercapacitors, spinel structure, RI PT electrochemical impedance spectroscopy, internal series resistance 1. Introduction The excessive usage of non-renewable natural resources such as fossil fuels and petroleum M AN US C has multiplied the need of alternate energy sources to meet the global demand. Furthermore, excessive usage of such resources causes depletion of resources and also causes serious environmental issues which are hazardous to the nature and human life. Energy from renewable sources is an alternative for this but climatic conditions play a vital role in the production of energy. Over the past few decades this issue is addressed by development of new energy storing devices and systems. Supercapacitors, batteries, solar cells and fuel cells D fall under the category of energy storing devices. Supercapacitors is especially attractive and TE has a clear advantage in terms of faster charging and discharging rates, reduction in weight and size, long cycle life, higher power density, easy operation and packaging and higher EP energy efficiency over batteries and fuel cells[1-3]. They are widely used in flexible transistors, portable electronics, pacemakers, memory back-ups, hybrid electric vehicles, AC C military devices etc.[1] [4-6]. Supercapacitors not only bridge the gap between conventional capacitors and batteries but also can be used in combination with fuel cells or batteries in many storage systems. [7] Electric double layer capacitors (EDLCs) and pseudocapacitors are the two class of supercapacitors where charge storing mechanism is by formation of double layer of charges at the electrode-electrolyte interface and by fast redox reactions respectively[8, 9]. The surface area and porous nature of carbon based materials make it suitable for charge storage 2 ACCEPTED MANUSCRIPT and ternary metal oxides [10] possess variable oxidation states making it apt for energy storage thereby giving better energy density and specific capacitance. Generally carbon based materials or its derivatives store charge by EDLC mechanism and metal oxides or conducting polymers store charge by pseudocapacitive mechanism [11]. Carbon based materials , metals RI PT oxides and conducting polymers can be used combining either two to enhance their capacitive nature[12]. However recent studies show that pseudocapacitors (basically transition metal oxides - TMOs) exhibit better capacitive behaviour than EDLCs [13] due to M AN US C the multiple oxidation states available for the transition metals resulting in fast redox reactions and unique crystal structure [6]. Among metal oxides, amorphous RuO2 is widely studied as it shows highest specific capacitance of 720 F/g in H2SO4 electrolyte. However, the cost and toxicity limits its practical application. TMOs based nanomaterials are widely investigated for energy related applications as they are earth abundant, low price, they are eco-friendly and also exhibit interesting electrochemical D property like high specific capacitance [13, 14]. Binary TMOs on the other hand has more TE advantages like synergistic effect and co-existence of two metal ions, which enhances its capacitive nature, electronic conductivity and stabilization [4, 14-16]. Binary TMOs can also EP be dopped with different transition metals which changes the composition of the material and AC C hence can vary the capacitive behaviour [17-19]. Among binary TMOs, metal ferrites MFe2O4 (M=Fe, Co, Ni, Mn, Cu, Zn) has gained lot of research interest because of their good conductivity, redox chemistry, ease of synthesis, abundancy, eco-friendliness and 3D diffusion pathways [20, 21] and therefore, find applications in various fields which includes energy devices[17], electrochemical applications[22], environmental applications[23] and biological applications[24]. MFe2O4 found to crystallize either in normal spinel or in inverse spinel cubic structure. In spinel structure, oxygen divalent anions (O2-) forms the face centred cubic (fcc) unit cell with M2+ and Fe3+ occupying the tetrahedral and octahedral sites 3 ACCEPTED MANUSCRIPT respectively. In inverse spinel structure, unit cell remains the same but tetrahedral sites are completely occupied by the trivalent (Fe3+) cations and octahedral sites are evenly occupied by trivalent (Fe3+) cations and divalent (M2+) cations [13, 25]. This interesting structure enables the hopping of electrons between the valence states available for metals in oxygen RI PT sites and thereby enhances their electrical conductivity [20, 26-28]. The major problem concerned with TMO nanomaterials is their limited conductivity, however this can be improved by using conducting substrates with considerably large surface area for material M AN US C loading purpose and use of this substrate lowers contact resistance and enhances the conductivity and electrochemical property [29, 30]. Liu et al. reported highest specific capacitance of 208.6 F g-1 and 275 F g-1 for Fe3O4 and carbon coated Fe3O4 nanorods respectively in 1M Na2SO3 [31]. The carbon coating acted as network for electrical conduction and resulted in synergistic effect between the layers of carbon giving better supercapacitive performance. Maqbool et al. developed the simplest D synthesis method using non-ionic surfactants namely tween 20 and tween 80 in ethylene TE glycol (EG) medium to prepare uniform sub-200 nm spherical magnetite nanoassemblies and reported a specific capacitance of 75 F g-1[32]. Wang and his co-workers synthesized hollow EP shell controllable CoFe2O4 microspheres from a simple one-step strategy and reported a AC C specific capacitance of 1450 F g-1 for triple shelled structure[4] . The effect of nanorods, nanowires and nanoparticles morphology on capacitive behaviour where Fe3O4 nanoassemblies have been investigated and reported highest specific capacitance of 106 F g-1 for Fe3O4 nanowires on account of its mesoporosity, high pore volume and surface area [2]. Yu et al. reported a specific capacitance of 1135.5 F g-1 in 1M H2SO4 for NiFe2O4 nanoparticles grown on carbon cloth substrate prepared in a hydrothermal route [33]. Zate and his co-workers explored manganese ferrite thin films as a suitable electrode material for supercapacitor and have reported a specific capacitance of 313 F g-1 in 1M KOH at a scan 4 ACCEPTED MANUSCRIPT rate of 5 mV s-1 [34]. Zhu et al. synthesized porous CuFe2O4 microspheres via solvothermal route using polyvinyl pyrrole (PVP) as the directing agent, demonstrating the nanostructure formation by the aggregation of nanocrystals and showed a specific capacitance of 334 F g-1 [35]. Vadiyar et al investigated on mechanochemical technique for synthesizing ZnFe2O4 thin RI PT film nanoflakes on a stainless steel substrate and this architecture resulted in a specific capacitance of 768 F g-1 at 5 mA cm-2 with 88% retention capacity [36]. J.S.Sagu et al. [37] has explained the pseudocapacitive nature of CoFe2O4 thin films and the reason for this is due M AN US C to the presence of FeOOH and CoOOH surface groups. They have reported a specific capacitance of 540 µFcm-2 with a time constant of 174 ms and retention of 80% after 7000 cycles, where 1M NaOH was used as the electrolyte. Over the past few decades, extensive experimental and theoretical studies have been made to understand the electrochemical behaviour of various electrode materials. The above carried out works clearly indicate that the electrochemical behaviour is strongly influenced by D the factors such as the surface area and porous nature of the material, surfactant used, TE synthesis method, morphology of the material, electrolyte used and concentration of electrolyte. Hence in this direction, to compare and predict the best binary transition metal EP oxide suitable for energy storage application, we demonstrate a facile, simple and AC C reproducible poly-ethylene (PEG) 600 assisted solvothermal method for the synthesis of MFe2O4 (M=Fe,Co,Ni,Mn,Cu,Zn) and test the electrochemical behaviour keeping all the conditions and parameters identical. Finally, we determine the best transition metal oxide which can provide better performance among the above mentioned six and the mechanism involved in it. 2. Materials and Methods 2.1 Chemicals and Reagents 5 ACCEPTED MANUSCRIPT Ferrous chloride tetrahydrate (FeCl2.4H2O, ACS reagent ≥ 99%), cobaltous chloride hexahydrate (CoCl2.6H2O, ACS reagent ≥ 97%), nickel chloride hexahydrate (NiCl2.6H2O, Bioreagent), manganese chloride tetrahydrate (MnCl2.4H2O, ACS reagent ≥ 98%), cuprous chloride dihydrate (CuCl2.2H2O, ACS reagent ≥ 99%), zinc chloride (ZnCl2, ACS reagent ≥ RI PT 97%), ferric chloride hexachloride (FeCl3.6H2O, ACS reagent ≥ 97%), hydrazine hydrate (N2H4, 50-60%) and 5 wt% perfluorinated nafion resin solution of reagent grade were purchased from Sigma Aldrich. Ethylene glycol (EG, C2H6O2, 99%) and polyethylene glycol M AN US C 600 were bought from SD fine Chemical Ltd. All these were used without any further purification. 2.2 Preparation of MFe2O4 nanoassemblies Metal ferrites were synthesized using EG as solvent and PEG 600 as co-solvent by solvothermal method. In a typical process of preparing Fe3O4 nanospheres, 1g of FeCl3.6H2O D and FeCl2.4H2O were taken in 2:1 molar ratio and dissolved in 50 ml of EG in a 100 ml beaker under magnetic stirring followed by subsequent addition of 5 ml PEG 600 . EG and TE PEG 600 played the role of surfactant and co-solvent. Post 30 minutes, 2 ml of hydrazine EP hydrate which acts as a reducing agent giving a basic atmosphere, was added in a drop wise manner to it. The whole procedure was carried out under N2 ambient and the above solution AC C is transferred to a 100 ml Teflon lined stainless steel autoclave when the solution turned black. N2 gas is purged to autoclave for 5-10 minutes to remove the oxygen residue from the prepared solution and autoclave was kept in a hot air oven at 200°C for 24 h. After cooling the autoclave to room temperature, the precipitates formed were extracted by washing with water and ethanol several times under magnet and the obtained particles were dried at 70°C for 12 h to get Fe3O4 nanoparticles. All the other binary TMOs (MFe2O4= Co,Ni,Mn,Cu,Zn) are prepared from the above same procedure by taking 1g of trivalent salt (FeCl3. 6H2O) and divalent salt (MCl2.nH2O) in 2:1 molar ratio under identical conditions. 6 ACCEPTED MANUSCRIPT 2.3 Preparation of hexagonal CoFe2O4 Nanoparticles (NPs) For a comparative study on the charge storage/capacitance behaviour of nanomaterials with same composition and different morphology, we prepared CoFe2O4 NPs of hexagonal morphology. For this, 1g of FeCl3.6H2O and CoCl2.6H2O in 2:1 molar ratio was dissolved in RI PT 10 ml of DI water followed by addition of 45 ml PEG 600 under magnetic stirring. Then similar autoclave method has been adopted as described in the section 2.2 to obtained NPs of M AN US C hexagonal shape. 2.4 Material Characterisation The morphology of the as prepared samples was examined from the FESEM images recorded using Carl Zeiss Neon 40 Crossbeam. The size distributions of the nanoparticles were calculated by standardising the FESEM image using Image-J software, where 80-100 particles were considered. The histograms were plotted and fitted to a Gaussian curve. From D the Gaussian curve their average size and standard deviation were calculated. XRD patterns TE are obtained from D8 Advanced Bruker powder X-ray Diffractometer with a CuKα radiation (λ= 1.5406Å) operating at 30 kV and 40mA. Fourier Transform Infrared (FTIR) spectrum of EP the samples are collected for wavenumber range of 400-4500 cm-1 using a IR Affinity-1 Spectrophotometer, operated in KBr disc mode. Branauer-Emmett-Teller (BET) theory and AC C Barrett-Joyner-Halenda (BJH) theory are used to estimate the surface area and pore size distribution of the samples, this is performed by the adsorption and desorption of N2 gas onto the samples at 77.3 K in the Quantachrome Nova Station 1000 instrument after degassing the samples at 350°C for 12 h. 2.5 Electrochemical Characterisation Electrochemical measurements are performed in a 3-electrode system using electrochemical workstation (CHI 660C USA). 3M KOH was used as the electrolyte. 7 ACCEPTED MANUSCRIPT Ag/AgCl electrode, Pt wire and (4cm×1cm) carbon paper are used as the reference, counter and working electrode respectively. 3 mg of the active material is dispersed completely in a 240 µl of ethanol. 6 µl of this prepared solution was drop casted to 1cm2 area of the working electrode. 3 µl of 5 wt% perfluorinated nafion resin solution was drop casted which act as a RI PT binder. The active material loading on the working electrode was approximately 0.3 mg cm-2 for every sample. The CV is recorded for different scan rates in a particular potential range which varied for each metal ferrite. Chronopotentiometry is recorded at different current M AN US C densities and EIS is done at their respective open circuit potential for a frequency range of 1105 Hz at an excitation sinusoidal voltage of 5 mV. Results and Discussions 3.1 Mechanism of formation of metal ferrite nanoassemblies The divalent and trivalent metal salts are dissolved in 50 ml of EG and 5 ml of PEG 600 D and stirred for 30 minutes to mix up well in the solvent. Upon addition of hydrazine hydrate TE the divalent and trivalent cations forms complex with hydrazine hydrate. Hydrazine hydrate reacts with water exothermically forming ammonium hydroxide, a strong alkaline ambient, EP with the evolution of H2 and N2 gas. Hydrazine hydrate also forms intermediate complex with metal ions (M3+ and M2+). The evolved hydrogen and nitrogen gas molecules assist the AC C formation of pores in the ferrite nanoassemblies. PEG-600 surfactant molecules acts as a cross linking polymer which leads the formation of metal ferrite nanossemblies (MFNs) at 200 ºC and at high pressure (autoclave system)[24] . The probable reaction mechanism is given below: 2FeCl3.6H2O + MCl2.4H2O + yN2H4 (FeCl3)2(MCl2)(N2H4)y yN2H4 + yH2O EG EG EG (FeCl3)2(MCl2)(N2H4)y M2+ + 2Fe3+ + yN2H4 + yH2O yNH4OH (pH = 9) + (y/2)H2 ( ) + (y/2)N2 ( ) 8 ACCEPTED MANUSCRIPT Fe3+ + OH- EG M2+ +OHFe(OH)3 + M(OH)2 yNH4+ + yOH- EG EG Fe(OH)3 M(OH)2 PEG-600 MFe2O4 + H2O 3.2 Microstructural Analysis RI PT yNH4OH Figure 1a shows the XRD patterns of the ferrite nanoassemblies, confirming the presence M AN US C of ferrite phase (normal spinel or inverse spinel cubic structure) and also reveal that the metal ferrites formed are crystalline in nature. The characteristic peaks at around 2θ = 30.0°, 35.5°, 42.8°, 56.7° and 62° corresponds to (220), (311), (400), (511) and (440) crystal planes respectively with a maximum intensity peak at 35.5° corresponding to (311) plane in all the samples. The patterns obtained are in well agreement with the JCPDS card number 19-0629, 22-1086, 74-2081, 73-1964, 77-0010 and 82-1049 respectively. Observed peak shifts is D because of the change in crystallite size where, left shift is observed for smaller crystallites TE and right shift for larger crystallites[38]. Lattice strain calculated from the slope of the EP Williamson-Hall (W-H) plot is shown in figure 1b. This strain at the grain boundaries could also cause a small shift in peak position. ZnFe2O4 experiences tensile stress (+ve strain) on AC C the account of its lowest divalent cationic (Zn2+) radius. MnFe2O4 has the largest lattice constant as shown in figure 1c, which results in left shift of the major intense peak compared to XRD patterns of other metal ferrites. The crystallite size, inter-planar spacing and lattice constant is calculated for the major three intense peaks (311) at 35.5°, (220) at 30° and (440) at 62° using the Debye Sherrer formula and the obtained results are tabulated as in table SI1. Crystallite size (nm), = 0.9 (1) 9 ACCEPTED MANUSCRIPT where, λ is the wavelength of the radiation used, β is the full width half maximum. Crystallite size and strain factor calculated from the W-H plot is tabulated in table SI2. In NiFe2O4 the peak at 44.49° corresponds to the (200) plane indicating the formation of NiO as reported by Jahromi et. al. [39]. In CuFe2O4 the growth of crystals is found to be more in the RI PT direction of (400) plane at 43.23° obtained from the I(400)/IS (400) ratio of 4.04 [40], where Is is the standard value of the intensity corresponding to (400) plane. The calculated crystallite sizes are 23.2 nm, 30.83 nm, 30.08 nm, 29.05 nm, 31.32 nm, and 34.44 nm for MFe2O4 M AN US C (M=Fe, Co, Ni, Mn, Cu and Zn) respectively. Although the metal ferrites were synthesized under identical conditions, the particle size is observed to vary for all the TMOs. The reasons for different dimension and different morphology of the nanoparticles may be (a) the reaction kinetics for the formation of metal oxides by the reduction of metal chloride precursor through hydrazine hydrate for various metals are different and (b) each metal cation can have different affinity towards the polyol used, forming intermediate glycolate complex (-O-CH2- D CH2-O-M)n [41-43] which control the size of nanoparticles during nucleation and growth TE process. Absence of any extra peak shows that the samples are free from contamination. Table SI2 shows a comparison between the crystallite size and particle size which reveals the EP formation of spherical nanoassemblies. It also includes the ionic radius of the divalent and trivalent metal cations as reported by Shannon [44] considering its normal spinel/inverse AC C spinel cubic structure. Since the XRD patterns of the four samples, namely Fe3O4, CoFe2O4, ZnFe2O4 and MnFe2O4 are almost similar, EDX spectra (figure SI3) are recorded for these four samples. The elemental composition in the different samples confirms the formation of their respective ferrites and the atomic % obtained are well matched with their corresponding stoichiometric ratios. Figure 2 shows the FTIR spectrum of the as-prepared metal ferrite samples. The peak at around 400 cm-1 and 600 cm-1 corresponds to the M-O stretching at the octahedral and 10 ACCEPTED MANUSCRIPT tetrahedral sites further validating the formation of ferrite phase. The difference in bond strength between each metal atom and oxygen atom leads to different bond length which can be observed in small variation of the peak position of each metal ferrite [45]. Vibrational bands at 1365 cm-1 and 1210 cm-1 correspond to C-H bending and C-O stretching RI PT respectively. Broad peak at 3300 cm-1 refers to O-H stretching indicating coating of PEG-600 in the metal ferrite nanomaterial. Figure 3 shows the FESEM images of the metal ferrites exhibiting nearly spherical M AN US C morphology. The average particle size of MFe2O4 (M=Fe,Co,Ni,Mn,Cu and Zn) is calculated to be 36.1±0.8 nm, 51.3±0.4 nm, 41.9±2.2 nm, 37.6±2.8 nm, 135.1±5.2 nm and 81.1±2.3 nm respectively. The cluster formation is clearly visible in figure 3 showing larger particles formed by the assembling of smaller particles. The most uniform particle size distribution is seen for CoFe2O4, proved from the least standard deviation. It has to be pointed out that the change in solvent during the synthesis tends to produce particles with different morphology D (see SI information). Figure SI2 shows the FESEM images of hexagonal CoFe2O4 NPs TE formed when PEG 600 was used as a solvent/surfactant instead of EG. EP To investigate the porous nature and surface area of the samples, N2 adsorption-desorption measurements are carried out and the obtained results are depicted in figure 4. MFe2O4 AC C (M=Co, Ni, Cu and Zn) exhibited type IV isotherm where as Fe3O4 and MnFe2O4 exhibited type III isotherm of IUPAC classification [46], possessing hysteresis loop due to their porous nature and multilayer kind of adsorption. Hysteresis behaviour is observed as a result of capillary condensation during the desorption process. The observed hysteresis loop is in the range of 0.35-1.0 P/P0 for MFe2O4 (M=Co, Ni, Cu and Zn), 0.8-1.0 P/P0 for Fe3O4 and 0.65-1 P/P0 for MnFe2O4, respectively. Hysteresis position indicates that MFe2O4 (M=Co, Ni, Mn, Cu and Zn) nanoassemblies are mesoporous in nature but Fe3O4 nanoassemblies possess micropores having an average pore diameter of 1.25 nm. The BJH pore size distribution plot 11 ACCEPTED MANUSCRIPT showing the pore size distribution in the range of 3-25 nm for metal ferrite nanoassemblies reveals their mesoporous nature [8]. The pore size distribution is found to be bimodal in nature for M=Fe and Mn ferrite nanoassemblies in accordance with the literature [47] . The BJH reports as 36.5 m2 g-1 and 59.3 m2 g-1 respectively. 3.3 Electrochemical measurements RI PT specific surface area is calculated to be higher for CoFe2O4 nanoassemblies from BET and M AN US C The electrochemical performances of MFe2O4 are investigated by performing CV, Chronopotentiometry (GCD) and EIS experiments. Figure 5 shows the CV of metal ferrites in 3 M KOH for different scan rates from 2-100 mV s-1 in the potential window, which varied for each metal with Ag/AgCl as the reference electrode. One of the major challenges of metal ferrites is its limited conductivity which can be overcome by using substrates like carbon paper, having high conductive surface for material loading. The highly conducting surface of D the carbon paper enhances the conductivity of the metal oxides, which in turn results in better TE electrochemical performance with rectangular voltammograms. Among the Li+, Na+ and K+ based aqueous electrolytes used commonly, K+ based electrolytes have proven to be more EP suitable for electrochemical applications considering its smallest hydrated ionic radius of 3.31 AC C Å [48]. Specific capacitance (F/g) is calculated from the CV curves using the relation: = ∆ ∗ (4) where ∆ is the difference in the peak oxidation and reduction currents in amperes (A), the mass loading in grams (g) and is is the scan rate in mV s-1. Specific capacitance is calculated to be 101 F g-1, 444.78 F g-1, 109.26 F g-1, 190.26 F g-1, 250.83 F g-1and 138.98 F g-1 for MFe2O4 (M=Fe,Co,Ni,Mn,Cu and Zn) respectively at 2 mV s-1. The highest specific capacitance is found in CoFe2O4 MFNs. This is higher than the specific capacitance value 12 ACCEPTED MANUSCRIPT reported by Sankar and group, where they used combustion route for synthesis and a different working electrode (electrode with lesser conductivity compared to the carbon paper) was used for carrying out the electrochemical testing [49]. Present work also demonstrates better performance than the work reported by Wang and his co-workers, where in specific RI PT capacitance was 406.8 F g-1 for single shelled CoFe2O4. Here, our spherical nanoassemblies performed better because of the higher active surface area resulting in more number of active sites for charge storage when compared to the single shelled CoFe2O4 microsphere [4]. Also M AN US C for increased scan rates, the area under the CV curve and the current increases but however it is evident from the figure 7a that the specific capacitance has an inverse relationship with scan rate. At larger scan rates, the ions within the electrolyte might move with higher speed and therefore decreases the accessibility of all the active materials to get involved in the charge storing mechanism. At lower scan rates, the electrolytic ions get enough time to access the working electrode resulting in higher specific capacitance [49]. CuFe2O4 nanoassemblies D gave lesser specific capacitance than porous CuFe2O4 nanospheres [35] because of the larger active sites TE particle size and lesser specific surface area which lowers the number of available for charge storage. Microporous nature of Fe3O4 nanoassemblies lowers the EP accessibility of the electrolytic ions into the electrode leading to the least specific capacitance among the metal ferrite nanoassemblies. Our Fe3O4 exhibited better performance as AC C compared to the Fe3O4 nanoassemblies (75 F g-1) synthesised by Liu and his group [32] where long chain hydrocarbon derivative was used as the surfactant. However, Fe3O4 nanowires (190 F g-1) studied by X.Zhao et al. [2] showed a better performance than our Fe3O4 nanoassemblies may be because of different morphology and particle size distribution. MnFe2O4 of our present work gave better specific capacitance when compared to MnFe2O4 colloidal nanoclusters (152.5 F g-1) synthesised by Wang et al. [48], where focus was to study the effect of different electrolytes and their concentration on supercapacitive behaviour. 13 ACCEPTED MANUSCRIPT The capacitive behaviour of hexagonal CoFe2O4 nanoparticles was also investigated by taking electrochemical measurements following the same conditions as used for nanoassemblies. The capacitance value calculated from the CV is found to be 110.8 Fg-1 at a scan rate of 2mV-1. The plausible reason for reduced capacitance compared to that of other RI PT nanoassemblies is the change in morphology. A prominent oxidation and reduction peak is clearly visible in the CV (figure SI2 (a)) which may be due to the presence of FeOOH and CoOOH groups at the surface [37]. The CoFe2O4 nanoassemblies were cluster of spheres M AN US C with porous nature which enhanced the charge storing nature as compared to that of hexagonal shaped particles. The recorded GCD curve is as shown in figure 6 for different current densities from 0.1 A g-1 to 3 A g-1 wherein it was observed that as current density increases, the time taken for charging and discharging decreases. The discharging time is calculated to be 40s, 160s, 80s, 70s, 100s and 50s for MFe2O4 (M=Fe, Co, Ni, Mn, Cu and Zn) respectively at a current D density of 0.2 A g-1. At higher current densities, electrolytic ions raster with high speed TE decreasing their accessibility to the electrode surface in accordance with the literature [49]. EP The long-term cycling stability of the binary TMOs were investigated for 1000 cycles from CV analysis at a scan rate of 10 mV s-1. Figure 7b shows the Coulombic efficiency plot, from AC C which the extent of stability of a material is determined. This study is very much essential for estimating the capacitance retention in a material after a certain number of cycles. CuFe2O4 and ZnFe2O4 exhibited better coulombic efficiency/capacitance retention > 90% after 1000 cycles. Figure 7c shows the Nyquist plot obtained from the EIS studies with the inset showing the high frequency region. This study is important in determining the charge transfer kinetics involved within the 3-electrode system [9]. The interfacial charge transfer resistance is given 14 ACCEPTED MANUSCRIPT by the diameter of the semicircle formed in the EIS spectrum. The equivalent series resistance (Rs) is calculated and found to be least for CoFe2O4 making it more conductive. In this context, in the low frequency region, more vertical line implies more capacitance behaviour. Rct is the charge transfer resistance at the electrode electrolyte interface during the RI PT electrochemical reaction. The calculated data from the electrochemical studies have been tabulated in table 3. The electrical kinetics inside the electrolyte varies with morphology of the electrode materials which modifies the active sites present. For hexagonal CoFe2O4 NPs, M AN US C the internal series resistance is found to be 14.16 Ω from the nyquist plot (figure SI2 (c)), which is much larger than the series resistance experienced by the CoFe2O4 nanoassemblies. This increased resistance lowers the accessibility of electrons within the electrolyte. The dependence of specific capacitance on composition is due to the co-existence of two metal cations in all the MFe2O4 nanoassemblies, so they exhibit synergistic effect. Also depending on the ionic radius of each cation and coating of the polymer, different sizes and D morphology can be achieved. Here all the ferrites (prepared under identical conditions) TE exhibit almost similar morphology with different sizes. When the morphology have more EP active sites on the surface, they exhibit better capacitance. Specific surface area and pore size distribution of the nanoassemblies also plays an important role in the capacitance properties. AC C Uniform sized particles enhances the capacitive behaviour as reported by S.P.Jahromi et al [50]. Here CoFe2O4 nanoassemblies showed the highest specific capacitance owing to their narrow particle size distribution as proved from the least standard deviation and highest specific surface area of 59.3 m2 g-1 as calculated from BJH measurements, so they have more number of active sites available for easy access of ions and their transfer within the electrolyte, therefore they store charge in a more efficient way. Other than these parameters, specific capacitance also depends on how the materials behave within an electrode-electrolyte system, where the internal resistance of material within the electrolyte (see table SI4) has a 15 ACCEPTED MANUSCRIPT major role to play. Increased internal resistance offers hindrance to the movement of ions in the electrolyte and the effectiveness of charge storage reduces. EIS study also shows that the CoFe2O4 nanoassemblies have the least series resistance, enabling better conductivity to the electrolytic ions and therefore improving its charge storing behaviour. The average pore size RI PT of 3.125 nm and hysteresis loop in 0.35-1 P/P0 range gives high degree of mesoporosity to cobalt ferrite nanoassemblies enhancing its charge storing capability. M AN US C 4 Conclusion In summary, we have synthesised nearly spherical nanoassemblies of metal ferrites using solvothermal method and tested for their electrochemical energy storing property. The resulting as-prepared ferrites showed good pseudocapacitive behaviour confirmed by rectangular cyclic voltammograms. We attribute this behaviour due to the presence of two metal ions (Fe3+ and M2+) which allowed the hopping of electrons between the valence states D and also due to the carbon paper used for material loading. Of the six transition metal ferrites, TE CoFe2O4 nanoassemblies exhibited better performance giving a specific capacitance of 444.78 F g-1at a scan rate of 2 mV s-1. This is because of high specific surface area, EP mesoporous surface with the pore diameter of 3.125 nm, low series resistance and uniform size distribution of the CoFe2O4 particles. Morphology of the nanomaterial also plays an AC C important role in the charge storage as proved from the electrochemical behaviour of hexagonal CoFe2O4 NPs. Furthermore, this study can be extended by combining cobalt ferrite with carbon derivative materials for hybrid supercapacitors which may enhance the specific capacitance and cycle stability. Acknowledgements 16 ACCEPTED MANUSCRIPT We are grateful to Science and Engineering Research Board, Department of Science and Technology (SERB-DST), Government of India and Seed Grant, VIT University, India for financial support. [6] [7] [8] [9] [10] [11] [12] [13] [14] M AN US C [5] D [4] S. C. 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(a) XRD patterns (b) Williamson-Hall plot and (c) lattice constant of MFNs. M AN US C RI PT ACCEPTED MANUSCRIPT AC C EP TE D Figure 2. FTIR spectra of different MFNs. AC C EP TE D M AN US C RI PT ACCEPTED MANUSCRIPT Figure 3. FESEM images of (a) Fe3O4, (b) CoFe2O4, (c) NiFe2O4, (d) MnFe2O4, (e) CuFe2O4 and (f) ZnFe2O4 nanoassemblies. AC C EP TE D M AN US C RI PT ACCEPTED MANUSCRIPT Figure 4. N2 adsorption-desorption isotherm of MFNs with inset showing the BJH pore size distribution plot. AC C EP TE D M AN US C RI PT ACCEPTED MANUSCRIPT Figure 5. Cyclic voltammograms of MFNs with increasing scan rates in 3M KOH. AC C EP TE D M AN US C RI PT ACCEPTED MANUSCRIPT Figure 6. Galvanostatic charge-discharge plots of MFNs with decreasing current densities in 3M KOH. ACCEPTED MANUSCRIPT 500 200 CoFe2O4 NiFe2O4 MnFe2O4 300 CuFe2O4 ZnFe2O4 200 100 0 20 40 60 80 CoFe2O4 NiFe2O4 MnFe2O4 100 100 Scan rate (mV/s) Fe3O4 150 CuFe2O4 50 0 0 200 400 600 No of cycles 140 (c) 120 -Zimaginary (ohm) 2 1 80 0 60 2 3 4 Zreal (ohm) Fe3O4 5 CoFe2O4 NiFe2O4 D 40 20 0 10 20 EP 0 TE -Zimaginary (ohms) 100 30 40 MnFe2O4 CuFe2O4 ZnFe2O4 50 60 70 80 ZnFe2O4 800 1000 1200 1400 M AN US C 0 (b) RI PT Specific capacitance (F/g) Fe3O4 400 90 Zreal (ohms) Figure 7. Plot of (a) specific capacitance versus scan rate (b) cyclic stability (specific capacitance versus number of cycles) from cyclic voltammograms at a scan rate of 10 mV/s and (c) metal ferrites at their open circuit potential versus Ag/AgCl as the reference electrode (Nyquist plot) Inset of (c) is the magnified view of the high frequency region. AC C Specific capacitance (F/g) (a)