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

Abstract

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 μ_B 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.