Strategic Design of Vacancy-Enriched Fe_1–xS Nanoparticles Anchored on Fe₃C-Encapsulated and N-Doped Carbon Nanotube Hybrids for High-Efficiency Triiodide Reduction in Dye-Sensitized Solar Cells

A new class of hybrids with the unique electrocatalytic nanoarchitecture of Fe1–xS anchored on Fe3C-encapsulated and N-doped carbon nanotubes (Fe1–xS/Fe3C–NCNTs) is innovatively synthesized through a facile one-step carbonization–sulfurization strategy. The efficient synthetic protocols on phase structure evolution and dynamic decomposition behavior enable the production of the Fe1–xS/Fe3C–NCNT hybrid with advanced structural and electronic properties, in which the Fe vacancy-contained Fe1–xS showed the 3d metallic state electrons and an electroactive Fe in +2/+3 valence, and the electronic structure of the CNT was effectively modulated by the incorporated Fe3C and N, with the work function decreased from 4.85 to 4.63 eV. The meticulous structural, electronic, and compositional control unveils the unusual synergetic catalytic properties for the Fe1–xS/Fe3C–NCNT hybrid when developed as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs), in which the Fe3C- and N-incorporated CNTs with reduced work function and increased charge density provide a highway for electron transport and facilitate the electron migration from Fe3C–NCNTs to ultrahigh active Fe1–xS with the electron-donating effect, and the Fe vacancy-enriched Fe1–xS nanoparticles exhibit ultrahigh I3– adsorption and charge-transfer ability. As a consequence, the DSSC based on the Fe1–xS/Fe3C–NCNT CE delivers a high power conversion efficiency of 8.67% and good long-term stability with a remnant efficiency of 8.00% after 168 h of illumination, superior to those of traditional Pt. Furthermore, the possible catalytic mechanism toward I3– reduction is creatively proposed based on the structure–activity correlation. In this work, the structure engineering, electronic modulation, and composition control opens up new possibilities in constructing the novel electrocatalytic nanoarchitecture for highly efficient CEs in DSSCs.