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Spinel type manganese oxide (Mn 3 O 4) nanoparticles were synthesized by ultrasonic irradiation assisted co-precipitation method using PEG and NaOH. The formation of tetragonal hausmannite phase (Mn 3 O 4) with spinel structure was confirmed from XRD and FTIR studies. The average crystallite size was determined from Scherrer formula (39.9 nm) and Williamson-Hall relation (36.5 nm). The presence of elements, the overall oxidation state of manganese (Mn 2+ , Mn 3+ and Mn 4+) and chemical composition of these nanoparticles were determined using XPS. The morphology, chemical composition and electrochemical performance of Mn 3 O 4 nanoparticles were analysed and studied by SEM, EDS, CV, CP and AC impedance analysis. The maximum specific capacitance of Mn 3 O 4 nanoparticles was found to be 296 F/g at a current density of 1 A/g 2. Hence, the use of surfactant and the presence of ultrasonic irradiation in the synthesis process plays a dominant role in the electrochemical performance of Mn 3 O 4 supercapacitors.

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To analyze the impact of preparation routes on the electrochemical behavior of nanoparticles, manganese cobaltite (MnCo 2 O 4 ) has been synthesized by combustion (MnC-C) and hydrothermal route (MnC-H). The structural properties of synthesized nanoparticles were characterized by X-ray diffractometer studies which confirm the formation of cubic spinel phase with average crystallite size of 26 nm for combustion route prepared and 24 nm for hydrothermal route. The FT-IR spectrum shows two strong bands observed at 651 and 559 cm -1 that are characteristic to stretching vibrations of tetrahedral and octahedral sites of spinel MnCo 2 O 4 compounds. Elemental analysis, oxidation state, and chemical composition of these nanoparticles were examined using X-ray photoelectron spectroscopy. The morphology of synthesized nanoparticles was analyzed by SEM images. Loosely packed flake-like morphology was observed for MnC-H and typical spongy network structure with voids or pores was seen for MnC-C samples. BET analysis reveals the presence of mesopores and micropores in the prepared compounds. Influence of preparation route on capacitor behavior was evaluated by performing electrochemical characterizations such as cyclic voltammetry, chronopotentiometry, and AC impedance analysis. MnC-H exhibits a higher specific capacitance of 671 F g -1 at 5 mV s -1 scan rate compared to 510 F g -1 exhibited by MnC-C electrode material. Excellent capacitance retention of 92 % was demonstrated by MnC-H over 1000 continuous cycling. Results indicate that MnCo 2 O 4 prepared via controlled synthesis conditions (hydrothermal) shows better performance than combustion prepared.

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Cobalt oxide (Co 3 O 4 )-graphene nanosheets (GNS) composite was prepared using a novel route. Room temperature prepared graphite oxide was exfoliated at low temperature and subsequently reduced to GNS by chemical method. Successful compound formation was confirmed and structural details were obtained from XRD studies. Cobalt oxide was found to crystallize in spinel fcc structure with average crystallite size of 9 nm in the composite. FTIR and XPS study confirms the removal of oxygen containing functional group in reduced graphene and spinel formation of cobalt oxide in the composite. Raman spectra depict the reestablishment of sp 2 conjugated network of carbon atoms, on reduction to graphene. FESEM images reveal the nanosheet like morphology of the graphene being retained in the composite and promoting ion diffusion channels. Electrochemical characterization discloses the pseudocapacitive behaviour of the composite material. Higher specific capacitance of 650 F/g was exhibited by GNS-Co 3 O 4 at 5 mV/s scan rate. Symmetrical supercapacitor fabricated using GNS-Co 3 O 4 demonstrated superior power characteristics. Graphene in the composite has substantially increased the electron and ion transport in the electrode material leading to enhanced performance.

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