Small polaron hopping conduction in NiMnO3/NiMn2O4 nano-cotton and its emerging energy application with MWCNT

Abstract Self-assembled ilmenite/spinel NiMnO3/NiMn2O4 nano-cotton and it's MWCNT blended composites were synthesized via one-step sol-gel auto combustion and ultra-sonication techniques with a perspective to investigate its dielectric, conduction mechanism and enhanced electrochemical performance to explore its applications as supercapacitor materials and high-temperature energy storage devices. The as-prepared double phase oxides were characterized initially by X-ray diffractometry, FTIR and Raman techniques to ascertain its phase formation and structural information. The FESEM and TEM micrographs reveal the growth of nanoparticles homogeneously and cotton-like morphology over observed space. The EDS elemental analysis and mappings exposed the true stoichiometrical ratios of elemental particles Ni, Mn, O and C and their uniform distribution. The coercivity (264 Oe) and saturation magnetization (1.2 emu/gm.) of MWCNT blended NiMnO3/NiMn2O4 oxides curtailed significantly than NiMnO3/NiMn2O4 which recommends its approach towards the superparamagnetic region. The fitted Nyquist plot ( Z ' v s . Z ' ' ) distinguished the individual contribution of grain and grain boundary and activation energy for grain (0.87 eV) and grain boundary (1.2 eV) was estimated from liner plot of extracted data. The frequency dispersing conductivity spectra was fitted with a modified equation of Jonscher's Power Law proposed by us, σ a c ( ω ) = σ 0 ( d c ) + A ( ω − b ) n and temperature dependent frequency exponent (n) and Ln (A) suggests non-overlapping small polaron tunneling (NSPT) model for its conduction mechanism. Displacement of both Ni2+ and Ni3+ ions through the channel like cavities of ilmenite/spinel structure along a, b and c axis provides strong support of the NSPT model. The electrochemical supercapacitor properties of NiMnO3/NiMn2O4 and MWCNT blended NiMnO3/NiMn2O4 were examined in 2 M aqueous KOH solution and excellent capacitive properties were observed for both compounds. Higher capacitance value (1347 F-g−1) and energy density 140 W h kg−1 were obtained at a current density of 5 A g−1 for MWCNT blended NiMnO3/NiMn2O4 with compared to its parent counterparts.