Enhancing quantum capacitance of iron sulfide supercapacitor through defect-engineering: A first-principles calculation

Quantum capacitance (CQ) has recently emerged as a solution for the saturated growth of supercapacitor performances. The CQ depends on the density of states of the electrode materials, which could be tuned by several approaches (i.e., introducing dopants). In this case, a transition metal sulfide material with a narrow band gap, such as hexagonal iron sulfide (h-FeS), has enormous advantages. In this study, we predict p-type defect importance in the h-FeS structure on capacitance enhancement through the quantum capacitance mechanism. From the first-principle calculation, the Cr-doped h-FeS (FeS:Cr) anode results in integrated quantum capacitance (CQint) of 1043 F/g, or 27 times more than pristine h-FeS. From the formation energy calculation, the Cr-doped structure has a lot of stable charged states enabling additional redox-related capacitance. As dopant concentration increased, the CQint is found to increase but then saturate due to delocalized state appearances. These findings imply that a first-principles understanding is essential to predict the enhanced capacitance properties on h-FeS and TMS supercapacitors.