Co₉S₈/NC@FeCoS₂/NC composites with hollow yolk shell structure as the counter electrode for Pt-free dye-sensitized solar cells

Carbon-based materials derived from metal-organic framework compounds have tunable chemical composition, morphology, and electrochemically active surface combined with good electrical conductivity and stability. In this work, Fe(OH)₃ grows on a zeolitic imidazolate framework, 2-Methylimidazole cobalt salt (ZIF-67), through oxidation and deposition. The Co₉S₈/NC@FeCoS₂/NC composites with hollow yolk shell structure (HYS-Co₉S₈/NC@FeCoS₂/NC) is prepared through solvothermal sulfidation combined with high-temperature carbonization using ZIF-67@Fe(OH)₃ as the precursor. A dye-sensitized solar cell assembled with HYS-Co₉S₈/NC@FeCoS₂/NC as a counter electrode achieves a photovoltaic conversion efficiency of 7.07% (PCE_Pt = 7.36%). The reason for this phenomenon is that the hollow yolk shell structure of Co₉S₈/NC@FeCoS₂/NC material has a high specific surface area, which affords a good deal of active sites and promotes the contact between electrolyte and catalytic sites. The Fe-Co bimetal center synergies with each other to catalyze the reduction of iodide ion, showing a high catalytic activity. Besides, the material has good electrochemical stability in iodide-based electrolysis.