Comparative electrochemical energy storage performance of cobalt sulfide and cobalt oxide nanosheets: experimental and theoretical insights from density functional theory simulations.

In this study, we have carried out studies on supercapacitor performance comparing cobalt oxide (Co3O4) with cobalt sulfide (Co3S4) nanosheets grown using a facile electrodeposition approach. We have investigated the origin of enhanced energy storage performance of Co3S4 as compared to Co3O4 both by supported experiments and density functional theory investigations. Cobalt oxide exhibits a specific capacitance of 200 F g-1 at a current density of 2 A g-1, whereas a high specific capacitance of 558 F g-1 was achieved in the case of the Co3S4 nanosheets. The enhanced supercapacitor performance of Co3S4 is due to the high surface area, better wettability and high conductivity of the nanosheets. The asymmetric device exhibited a maximum energy density of 47.3 W h kg-1 with a power density of 2388.4 W kg-1 for Co3S4//MWCNT. The electrochemical impedance spectroscopic analysis revealed that Co3O4 has a substantially bigger semicircle as compared to Co3S4, confirming inferior charge-transfer characteristics in Co3O4. Density functional theory (DFT) simulations revealed that bulk structures of both Co3S4 and Co3O4 have an anti-ferromagnetic (AFM) configuration with Co atoms at the tetrahedral site having an opposite spin (∼2.55 µB each) and those at the octahedral sites being non-magnetic. Co3S4 nanosheets are found to be more conducting due to the presence of higher density of states near the Fermi level and a smaller bandgap compared to Co3O4 which support the observed experimental data on enhanced energy storage performance of Co3S4.

Urea-Assisted Room Temperature Stabilized Metastable ß-NiMoO4: Experimental and Theoretical Insights into its Unique Bifunctional Activity toward Oxygen Evolution and Supercapacitor.

Room-temperature stabilization of metastable ß-NiMoO4 is achieved through urea-assisted hydrothermal synthesis technique. Structural and morphological studies provided significant insights for the metastable phase. Furthermore, detailed electrochemical investigations showcased its activity toward energy storage and conversion, yielding intriguing results. Comparison with the stable polymorph, α-NiMoO4, has also been borne out to support the enhanced electrochemical activities of the as-obtained ß-NiMoO4. A specific capacitance of ∼4188 F g-1 (at a current density of 5 A g-1) has been observed showing its exceptional faradic capacitance. We qualitatively and extensively demonstrate through the analysis of density of states (DOS) obtained from first-principles calculations that, enhanced DOS near top of the valence band and empty 4d orbital of Mo near Fermi level make ß-NiMoO4 better energy storage and conversion material compared to α-NiMoO4. Likewise, from the oxygen evolution reaction experiment, it is found that the state of art current density of 10 mA cm-2 is achieved at overpotential of 300 mV, which is much lower than that of IrO2/C. First-principles calculations also confirm a lower overpotential of 350 mV for ß-NiMoO4.