FeS₂ Nanoparticles in S-Doped Carbon: Ageing Effects on Performance as a Supercapacitor Electrode

Although transition metal sulfides have prodigious potential for use as electrode materials because of their low electronegativities, their large volume changes inhibit broad application. Moreover, there is only limited knowledge of the ageing processes of these materials at the nanoscale. Herein, nano-C/FeS2 materials were prepared via one-pot syntheses from green biodegradable carbon precursors, followed by activation and sulfidation. The increased activation/sulfidation time led to an increase in the size of the nanoparticles (7 to 17 nm) and their aggregation, as well as in an increase in the specific surface area. The materials were then used as electrodes in 2-electrode symmetric supercapacitors with 2 M KOH. The activation process resulted in improved capacitance (60 F g(-1) at 0.1 A g(-1)) and rate capability (36%) depending on the composite porosity, conductivity, and size of the FeS2 particles. The ageing of the FeS2 nanoparticles was investigated under air, and a progressive transformation of the nano-FeS2 into hydrated iron hydroxy sulfate with a significant morphological modification was observed, resulting in drastic decreases in the capacitance (70%) and retention. In contrast, the ageing of nano-FeS2 during cycling led to the formation of a supplementary iron oxyhydroxide phase, which contributed to the enhanced capacitance (57%) and long-term cycling (132% up to 10,000 cycles) of the device.