Pinning ultrasmall greigite nanoparticles on graphene for effective transition-metal-sulfide supercapacitors in an ionic liquid electrolyte

To meet the future demands for off-grid power, high-performance electrochemical energy storage based on earth-abundant materials is essential. Supercapacitors are attractive in this sense due to their sustainable carbon-based architecture, rapid charging/discharging, and long cycle-life in comparison to battery chemistries. However, hybridizing carbon electrodes with inorganic phases is intensively explored in supercapacitor research to mitigate their low energy content. Iron sulfides are attractive because they are non-toxic and composed of earth-abundant elements, but, despite their hydrophobic nature, they have only been studied in aqueous electrolytes, limiting the energy content due to the narrow voltage stability window of water. Here, exploiting a rapid growth method and a highly functionalized graphene support, we strongly immobilized greigite (Fe3S4) nanoparticles with an ultrasmall size which could not be attained in the absence of graphene. The respective supercapacitor cell was found significantly more electroactive in the ionic liquid electrolyte than in water, boosting the energy content. Furthermore, greigite has high conductivity and fast surface faradaic reactions due to the enzyme-mimicking triple redox state of its thiocubane basic structural unit. Thus, fully reversible and fast redox processes in the expanded voltage-window of the ionic liquid also endowed excellent rate capability, cycling stability, and power. The work demonstrates a pathway, not previously explored, whereby greigite/graphene hybrids can surpass in these aspects top-rated supercapacitor materials.